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An array is a special variable, which can hold more than one value:

Why Use Arrays?

If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?

The solution is an array!

An array can hold many values under a single name, and you can access the values by referring to an index number.

Creating an Array

Using an array literal is the easiest way to create a JavaScript Array.

It is a common practice to declare arrays with the const keyword.

Learn more about const with arrays in the chapter: JS Array Const .

Spaces and line breaks are not important. A declaration can span multiple lines:

You can also create an array, and then provide the elements:

Using the JavaScript Keyword new

The following example also creates an Array, and assigns values to it:

The two examples above do exactly the same.

There is no need to use new Array() .

For simplicity, readability and execution speed, use the array literal method.

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Accessing Array Elements

You access an array element by referring to the index number :

Note: Array indexes start with 0.

[0] is the first element. [1] is the second element.

Changing an Array Element

This statement changes the value of the first element in cars :

Converting an Array to a String

The JavaScript method toString() converts an array to a string of (comma separated) array values.

Access the Full Array

With JavaScript, the full array can be accessed by referring to the array name:

Arrays are Objects

Arrays are a special type of objects. The typeof operator in JavaScript returns "object" for arrays.

But, JavaScript arrays are best described as arrays.

Arrays use numbers to access its "elements". In this example, person[0] returns John:

Objects use names to access its "members". In this example, person.firstName returns John:

Array Elements Can Be Objects

JavaScript variables can be objects. Arrays are special kinds of objects.

Because of this, you can have variables of different types in the same Array.

You can have objects in an Array. You can have functions in an Array. You can have arrays in an Array:

Array Properties and Methods

The real strength of JavaScript arrays are the built-in array properties and methods:

Array methods are covered in the next chapters.

The length Property

The length property of an array returns the length of an array (the number of array elements).

The length property is always one more than the highest array index.

Accessing the First Array Element

Accessing the last array element, looping array elements.

One way to loop through an array, is using a for loop:

You can also use the Array.forEach() function:

Adding Array Elements

The easiest way to add a new element to an array is using the push() method:

New element can also be added to an array using the length property:

Adding elements with high indexes can create undefined "holes" in an array:

Associative Arrays

Many programming languages support arrays with named indexes.

Arrays with named indexes are called associative arrays (or hashes).

JavaScript does not support arrays with named indexes.

In JavaScript, arrays always use numbered indexes .  

WARNING !! If you use named indexes, JavaScript will redefine the array to an object.

After that, some array methods and properties will produce incorrect results .

 Example:

The difference between arrays and objects.

In JavaScript, arrays use numbered indexes .  

In JavaScript, objects use named indexes .

Arrays are a special kind of objects, with numbered indexes.

When to Use Arrays. When to use Objects.

  • JavaScript does not support associative arrays.
  • You should use objects when you want the element names to be strings (text) .
  • You should use arrays when you want the element names to be numbers .

JavaScript new Array()

JavaScript has a built-in array constructor new Array() .

But you can safely use [] instead.

These two different statements both create a new empty array named points:

These two different statements both create a new array containing 6 numbers:

The new keyword can produce some unexpected results:

A Common Error

is not the same as:

How to Recognize an Array

A common question is: How do I know if a variable is an array?

The problem is that the JavaScript operator typeof returns " object ":

The typeof operator returns object because a JavaScript array is an object.

Solution 1:

To solve this problem ECMAScript 5 (JavaScript 2009) defined a new method Array.isArray() :

Solution 2:

The instanceof operator returns true if an object is created by a given constructor:

Complete Array Reference

For a complete Array reference, go to our:

Complete JavaScript Array Reference .

The reference contains descriptions and examples of all Array properties and methods.

Test Yourself With Exercises

Get the value " Volvo " from the cars array.

Start the Exercise

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JavaScript Arrays: Create, Access, Add & Remove Elements

We have learned that a variable can hold only one value. We cannot assign multiple values to a single variable. JavaScript array is a special type of variable, which can store multiple values using a special syntax.

The following declares an array with five numeric values.

In the above array, numArr is the name of an array variable. Multiple values are assigned to it by separating them using a comma inside square brackets as [10, 20, 30, 40, 50] . Thus, the numArr variable stores five numeric values. The numArr array is created using the literal syntax and it is the preferred way of creating arrays.

Another way of creating arrays is using the Array() constructor, as shown below.

Every value is associated with a numeric index starting with 0. The following figure illustrates how an array stores values.

assign values to an array javascript

The following are some more examples of arrays that store different types of data.

It is not required to store the same type of values in an array. It can store values of different types as well.

Get Size of an Array

Use the length property to get the total number of elements in an array. It changes as and when you add or remove elements from the array.

Accessing Array Elements

Array elements (values) can be accessed using an index. Specify an index in square brackets with the array name to access the element at a particular index like arrayName[index] . Note that the index of an array starts from zero.

For the new browsers, you can use the arr.at(pos) method to get the element from the specified index. This is the same as arr[index] except that the at() returns an element from the last element if the specified index is negative.

You can iterate an array using Array.forEach() , for, for-of, and for-in loop, as shown below.

Update Array Elements

You can update the elements of an array at a particular index using arrayName[index] = new_value syntax.

Adding New Elements

You can add new elements using arrayName[index] = new_value syntax. Just make sure that the index is greater than the last index. If you specify an existing index then it will update the value.

In the above example, cities[9] = "Pune" adds "Pune" at 9th index and all other non-declared indexes as undefined.

The recommended way of adding elements at the end is using the push() method. It adds an element at the end of an array.

Use the unshift() method to add an element to the beginning of an array.

Remove Array Elements

The pop() method returns the last element and removes it from the array.

The shift() method returns the first element and removes it from the array.

You cannot remove middle elements from an array. You will have to create a new array from an existing array without the element you do not want, as shown below.

Learn about array methods and properties in the next chapter.

Declaration

There are two syntaxes for creating an empty array:

Almost all the time, the second syntax is used. We can supply initial elements in the brackets:

Array elements are numbered, starting with zero.

We can get an element by its number in square brackets:

We can replace an element:

…Or add a new one to the array:

The total count of the elements in the array is its length :

We can also use alert to show the whole array.

An array can store elements of any type.

For instance:

An array, just like an object, may end with a comma:

The “trailing comma” style makes it easier to insert/remove items, because all lines become alike.

Get last elements with “at”

Let’s say we want the last element of the array.

Some programming languages allow the use of negative indexes for the same purpose, like fruits[-1] .

Although, in JavaScript it won’t work. The result will be undefined , because the index in square brackets is treated literally.

We can explicitly calculate the last element index and then access it: fruits[fruits.length - 1] .

A bit cumbersome, isn’t it? We need to write the variable name twice.

Luckily, there’s a shorter syntax: fruits.at(-1) :

In other words, arr.at(i) :

  • is exactly the same as arr[i] , if i >= 0 .
  • for negative values of i , it steps back from the end of the array.

Methods pop/push, shift/unshift

A queue is one of the most common uses of an array. In computer science, this means an ordered collection of elements which supports two operations:

  • push appends an element to the end.
  • shift get an element from the beginning, advancing the queue, so that the 2nd element becomes the 1st.

Arrays support both operations.

In practice we need it very often. For example, a queue of messages that need to be shown on-screen.

There’s another use case for arrays – the data structure named stack .

It supports two operations:

  • push adds an element to the end.
  • pop takes an element from the end.

So new elements are added or taken always from the “end”.

A stack is usually illustrated as a pack of cards: new cards are added to the top or taken from the top:

For stacks, the latest pushed item is received first, that’s also called LIFO (Last-In-First-Out) principle. For queues, we have FIFO (First-In-First-Out).

Arrays in JavaScript can work both as a queue and as a stack. They allow you to add/remove elements, both to/from the beginning or the end.

In computer science, the data structure that allows this, is called deque .

Methods that work with the end of the array:

Extracts the last element of the array and returns it:

Both fruits.pop() and fruits.at(-1) return the last element of the array, but fruits.pop() also modifies the array by removing it.

Append the element to the end of the array:

The call fruits.push(...) is equal to fruits[fruits.length] = ... .

Methods that work with the beginning of the array:

Extracts the first element of the array and returns it:

Add the element to the beginning of the array:

Methods push and unshift can add multiple elements at once:

An array is a special kind of object. The square brackets used to access a property arr[0] actually come from the object syntax. That’s essentially the same as obj[key] , where arr is the object, while numbers are used as keys.

They extend objects providing special methods to work with ordered collections of data and also the length property. But at the core it’s still an object.

Remember, there are only eight basic data types in JavaScript (see the Data types chapter for more info). Array is an object and thus behaves like an object.

For instance, it is copied by reference:

…But what makes arrays really special is their internal representation. The engine tries to store its elements in the contiguous memory area, one after another, just as depicted on the illustrations in this chapter, and there are other optimizations as well, to make arrays work really fast.

But they all break if we quit working with an array as with an “ordered collection” and start working with it as if it were a regular object.

For instance, technically we can do this:

That’s possible, because arrays are objects at their base. We can add any properties to them.

But the engine will see that we’re working with the array as with a regular object. Array-specific optimizations are not suited for such cases and will be turned off, their benefits disappear.

The ways to misuse an array:

  • Add a non-numeric property like arr.test = 5 .
  • Make holes, like: add arr[0] and then arr[1000] (and nothing between them).
  • Fill the array in the reverse order, like arr[1000] , arr[999] and so on.

Please think of arrays as special structures to work with the ordered data . They provide special methods for that. Arrays are carefully tuned inside JavaScript engines to work with contiguous ordered data, please use them this way. And if you need arbitrary keys, chances are high that you actually require a regular object {} .

Performance

Methods push/pop run fast, while shift/unshift are slow.

Why is it faster to work with the end of an array than with its beginning? Let’s see what happens during the execution:

It’s not enough to take and remove the element with the index 0 . Other elements need to be renumbered as well.

The shift operation must do 3 things:

  • Remove the element with the index 0 .
  • Move all elements to the left, renumber them from the index 1 to 0 , from 2 to 1 and so on.
  • Update the length property.

The more elements in the array, the more time to move them, more in-memory operations.

The similar thing happens with unshift : to add an element to the beginning of the array, we need first to move existing elements to the right, increasing their indexes.

And what’s with push/pop ? They do not need to move anything. To extract an element from the end, the pop method cleans the index and shortens length .

The actions for the pop operation:

The pop method does not need to move anything, because other elements keep their indexes. That’s why it’s blazingly fast.

The similar thing with the push method.

One of the oldest ways to cycle array items is the for loop over indexes:

But for arrays there is another form of loop, for..of :

The for..of doesn’t give access to the number of the current element, just its value, but in most cases that’s enough. And it’s shorter.

Technically, because arrays are objects, it is also possible to use for..in :

But that’s actually a bad idea. There are potential problems with it:

The loop for..in iterates over all properties , not only the numeric ones.

There are so-called “array-like” objects in the browser and in other environments, that look like arrays . That is, they have length and indexes properties, but they may also have other non-numeric properties and methods, which we usually don’t need. The for..in loop will list them though. So if we need to work with array-like objects, then these “extra” properties can become a problem.

The for..in loop is optimized for generic objects, not arrays, and thus is 10-100 times slower. Of course, it’s still very fast. The speedup may only matter in bottlenecks. But still we should be aware of the difference.

Generally, we shouldn’t use for..in for arrays.

A word about “length”

The length property automatically updates when we modify the array. To be precise, it is actually not the count of values in the array, but the greatest numeric index plus one.

For instance, a single element with a large index gives a big length:

Note that we usually don’t use arrays like that.

Another interesting thing about the length property is that it’s writable.

If we increase it manually, nothing interesting happens. But if we decrease it, the array is truncated. The process is irreversible, here’s the example:

So, the simplest way to clear the array is: arr.length = 0; .

new Array()

There is one more syntax to create an array:

It’s rarely used, because square brackets [] are shorter. Also, there’s a tricky feature with it.

If new Array is called with a single argument which is a number, then it creates an array without items, but with the given length .

Let’s see how one can shoot themselves in the foot:

To avoid such surprises, we usually use square brackets, unless we really know what we’re doing.

Multidimensional arrays

Arrays can have items that are also arrays. We can use it for multidimensional arrays, for example to store matrices:

Arrays have their own implementation of toString method that returns a comma-separated list of elements.

Also, let’s try this:

Arrays do not have Symbol.toPrimitive , neither a viable valueOf , they implement only toString conversion, so here [] becomes an empty string, [1] becomes "1" and [1,2] becomes "1,2" .

When the binary plus "+" operator adds something to a string, it converts it to a string as well, so the next step looks like this:

Don’t compare arrays with ==

Arrays in JavaScript, unlike some other programming languages, shouldn’t be compared with operator == .

This operator has no special treatment for arrays, it works with them as with any objects.

Let’s recall the rules:

  • Two objects are equal == only if they’re references to the same object.
  • If one of the arguments of == is an object, and the other one is a primitive, then the object gets converted to primitive, as explained in the chapter Object to primitive conversion .
  • …With an exception of null and undefined that equal == each other and nothing else.

The strict comparison === is even simpler, as it doesn’t convert types.

So, if we compare arrays with == , they are never the same, unless we compare two variables that reference exactly the same array.

For example:

These arrays are technically different objects. So they aren’t equal. The == operator doesn’t do item-by-item comparison.

Comparison with primitives may give seemingly strange results as well:

Here, in both cases, we compare a primitive with an array object. So the array [] gets converted to primitive for the purpose of comparison and becomes an empty string '' .

Then the comparison process goes on with the primitives, as described in the chapter Type Conversions :

So, how to compare arrays?

That’s simple: don’t use the == operator. Instead, compare them item-by-item in a loop or using iteration methods explained in the next chapter.

Array is a special kind of object, suited to storing and managing ordered data items.

The declaration:

The call to new Array(number) creates an array with the given length, but without elements.

  • The length property is the array length or, to be precise, its last numeric index plus one. It is auto-adjusted by array methods.
  • If we shorten length manually, the array is truncated.

Getting the elements:

  • we can get element by its index, like arr[0]
  • also we can use at(i) method that allows negative indexes. For negative values of i , it steps back from the end of the array. If i >= 0 , it works same as arr[i] .

We can use an array as a deque with the following operations:

  • push(...items) adds items to the end.
  • pop() removes the element from the end and returns it.
  • shift() removes the element from the beginning and returns it.
  • unshift(...items) adds items to the beginning.

To loop over the elements of the array:

  • for (let i=0; i<arr.length; i++) – works fastest, old-browser-compatible.
  • for (let item of arr) – the modern syntax for items only,
  • for (let i in arr) – never use.

To compare arrays, don’t use the == operator (as well as > , < and others), as they have no special treatment for arrays. They handle them as any objects, and it’s not what we usually want.

Instead you can use for..of loop to compare arrays item-by-item.

We will continue with arrays and study more methods to add, remove, extract elements and sort arrays in the next chapter Array methods .

Is array copied?

What is this code going to show?

The result is 4 :

That’s because arrays are objects. So both shoppingCart and fruits are the references to the same array.

Array operations.

Let’s try 5 array operations.

  • Create an array styles with items “Jazz” and “Blues”.
  • Append “Rock-n-Roll” to the end.
  • Replace the value in the middle with “Classics”. Your code for finding the middle value should work for any arrays with odd length.
  • Strip off the first value of the array and show it.
  • Prepend Rap and Reggae to the array.

The array in the process:

Calling in an array context

What is the result? Why?

The call arr[2]() is syntactically the good old obj[method]() , in the role of obj we have arr , and in the role of method we have 2 .

So we have a call of the function arr[2] as an object method. Naturally, it receives this referencing the object arr and outputs the array:

The array has 3 values: initially it had two, plus the function.

Sum input numbers

Write the function sumInput() that:

  • Asks the user for values using prompt and stores the values in the array.
  • Finishes asking when the user enters a non-numeric value, an empty string, or presses “Cancel”.
  • Calculates and returns the sum of array items.

P.S. A zero 0 is a valid number, please don’t stop the input on zero.

Run the demo

Please note the subtle, but important detail of the solution. We don’t convert value to number instantly after prompt , because after value = +value we would not be able to tell an empty string (stop sign) from the zero (valid number). We do it later instead.

A maximal subarray

The input is an array of numbers, e.g. arr = [1, -2, 3, 4, -9, 6] .

The task is: find the contiguous subarray of arr with the maximal sum of items.

Write the function getMaxSubSum(arr) that will return that sum.

If all items are negative, it means that we take none (the subarray is empty), so the sum is zero:

Please try to think of a fast solution: O(n 2 ) or even O(n) if you can.

Open a sandbox with tests.

Slow solution

We can calculate all possible subsums.

The simplest way is to take every element and calculate sums of all subarrays starting from it.

For instance, for [-1, 2, 3, -9, 11] :

The code is actually a nested loop: the external loop over array elements, and the internal counts subsums starting with the current element.

The solution has a time complexity of O(n 2 ) . In other words, if we increase the array size 2 times, the algorithm will work 4 times longer.

For big arrays (1000, 10000 or more items) such algorithms can lead to serious sluggishness.

Fast solution

Let’s walk the array and keep the current partial sum of elements in the variable s . If s becomes negative at some point, then assign s=0 . The maximum of all such s will be the answer.

If the description is too vague, please see the code, it’s short enough:

The algorithm requires exactly 1 array pass, so the time complexity is O(n).

You can find more detailed information about the algorithm here: Maximum subarray problem . If it’s still not obvious why that works, then please trace the algorithm on the examples above, see how it works, that’s better than any words.

Open the solution with tests in a sandbox.

  • If you have suggestions what to improve - please submit a GitHub issue or a pull request instead of commenting.
  • If you can't understand something in the article – please elaborate.
  • To insert few words of code, use the <code> tag, for several lines – wrap them in <pre> tag, for more than 10 lines – use a sandbox ( plnkr , jsbin , codepen …)

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  • What is Array in JavaScript?

JavaScript Array is a data structure that allows you to store and organize multiple values within a single variable. It is a versatile and dynamic object. It can hold various data types, including numbers, strings, objects, and even other arrays. Arrays in JavaScript are zero-indexed i.e. the first element is accessed with an index 0, the second element with an index of 1, and so forth.

JavaScript Arrays

JavaScript Array

You can create JavaScript Arrays using the Array constructor or using the shorthand array literal syntax, which employs square brackets. Arrays can dynamically grow or shrink in size as elements are added or removed.

Table of Content

Basic Terminologies of JavaScript Array

Declaration of an array, basic operations on javascript arrays, difference between javascript arrays and objects, when to use javascript arrays and objects, recognizing a javascript array, javascript array complete reference, javascript array examples, javascript cheatsheet.

  • Array: A data structure in JavaScript that allows you to store multiple values in a single variable.
  • Array Element: Each value within an array is called an element. Elements are accessed by their index.
  • Array Index: A numeric representation that indicates the position of an element in the array. JavaScript arrays are zero-indexed, meaning the first element is at index 0.
  • Array Length: The number of elements in an array. It can be retrieved using the length property.

There are basically two ways to declare an array i.e. Array Literal and Array Constructor.

1. Creating an Array using Array Literal

Creating an array using array literal involves using square brackets [] to define and initialize the array. This method is concise and widely preferred for its simplicity.

2. Creating an Array using Array Constructor (JavaScript new Keyword)

The “Array Constructor” refers to a method of creating arrays by invoking the Array constructor function. This approach allows for dynamic initialization and can be used to create arrays with a specified length or elements.

Note: Both the above methods do exactly the same. Use the array literal method for efficiency, readability, and speed.

1. Accessing Elements of an Array

Any element in the array can be accessed using the index number. The index in the arrays starts with 0.

2. Accessing the First Element of an Array

The array indexing starts from 0, so we can access first element of array using the index number.

3. Accessing the Last Element of an Array

We can access the last array element using [array.length – 1] index number.

4. Modifying the Array Elements

Elements in an array can be modified by assigning a new value to their corresponding index.

5. Adding Elements to the Array

Elements can be added to the array using methods like push() and unshift().

6. Removing Elements from an Array

Remove elements using methods like pop(), shift(), or splice().

7. Array Length

Get the length of an array using the length property.

8. Increase and Decrease the Array Length

We can increase and decrease the array length using the JavaScript length property.

9. Iterating Through Array Elements

We can iterate array and access array elements using for and forEach loop.

Example: It is the example of for loop.

Example: It is the example of Array.forEach() loop.

10. Array Concatenation

Combine two or more arrays using the concat() method. Ir returns new array conaining joined arrays elements.

11. Conversion of an Array to String

We have a builtin method toString() to converts an array to a string.

12. Check the Type of an Arrays

The JavaScript typeof operator is used ot check the type of an array. It returns “object” for arrays.

  • JavaScript arrays use indexes as numbers.
  • objects use indexes as names..
  • Arrays are used when we want element names to be numeric.
  • Objects are used when we want element names to be strings.

There are two methods by which we can recognize a JavaScript array:

  • By using Array.isArray() method
  • By using instanceof method  

Below is an example showing both approaches:

Note: A common error is faced while writing the arrays:

The above two statements are not the same.

Output: This statement creates an array with an element ” [5] “.

We have a complete list of Javascript Array, to check those please go through this JavaScript Array Reference article. It contains detailed description and examples of all Array Properties and Methods.

JavaScript Array examples contains list of questions that are majorily asked in interview. Please check this article JavaScript Array Examples for more detail.

We have a Cheat Sheet on Javascript where we have covered all the important topics of Javascript to check those please go through Javascript Cheat Sheet-A Basic guide to JavaScript .

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Reassigning Objects & Arrays using Const

  • Written by Gary Miskimmons
  • Published Mar 24, 2020
  • Read time 5 mins

Reassigning Objects & Arrays using Const featured image

When working with numbers, strings and booleans with const we know through our previous blog post var-vs-const-vs-let that you cannot reassign a const variable. The same goes for any const variables even objects or arrays. But unlike simple variables, objects and arrays have methods and properties that let you modify the object or array.

Const Arrays

The code above has an array variable called numbers holding three values. Even though the numbers array is a const you’re able to update or change the variable. For example, you can add another number to the numbers array by using the push method. Methods are actions you perform on the array or object.

With methods, we can modify our array by adding another value to the end of the array using the push method. Another way you could modify the array is to remove an item from the end by using the pop method. There is to many methods to go over in detail in one blog post but if you visit  The Mozilla Developer Network  you’ll find everything thing you need to know on arrays and everything to do with Javascript.

Const Objects

The modifying principle applies to an object for example.

The code above creates a user object with a name property then it assigns a new age property to object. One thing to remember const does not stop array and objects from being modified it only stops the variable itself from being reassigned or being overwritten for example.

If we attempt to override the user object with another object literal the console will throw an error. That’s because we are trying to reassign a user to a new object literal. However, if you modify the name property directly by assigning it a new value you will not get an error.

In short, you cannot reassign any variables declared with const. However, unlike simple variables ie numbers, strings, and booleans, objects & arrays provided additional properties and methods that let modify their values. Making it the ideal way to declare your structured data with the additional benefit of not being able to be reassigned by stray variables. Thank you for taking the time to read my post stay tuned into our blog for all the latest from Scaffold.

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Functional programming with JavaScript arrays

Learn how javascript’s built-in functions like map() and filter(), as well as compose() and chain(), support a more elegant handling of javascript arrays..

Matthew Tyson

Contributor, InfoWorld |

Functional programming with JavaScript arrays

Traditional JavaScript arrays

Functional programming with arrays, array.map(), array.filter(), array.reduce(), composing functions.

JavaScript arrays are an incredibly flexible way to model collections using techniques from  functional programming . This article introduces you to using tools like forEach() , map() , and reduce() for functional-style arrays.

JavaScript’s arrays can hold heterogeneous types, change size on the fly, and readily insert or remove elements. Traditional methods like slice, splice, and push/pop do this by operating on the array itself, modifying the collection in a “destructive” way:

Although JavaScript’s arrays are very capable out of the box, the functional paradigm improves the clarity and maintainability of array code. In general, functional programming looks to use functions as operators that can be passed into arrays. This allows for operating over the array like the head on a tape, rather than traditional imperative loops that describe in detail what is to occur.

Let's look at a few examples of working with arrays in the functional paradigm.

Array.forEach() is our first example. This lets you pass in a function that performs arbitrary operations on the elements iteratively. It’s a common alternative to the traditional  for loop:

In this example, we are just outputting each element to the console. The equivalent in a for loop would be:

You’ll notice that there are fewer moving parts in the functional version. In particular, we eliminate the iterator ( i ), which is an extraneous variable used to express the logic of the mechanics, rather than a part of the actual intention. Note that I'm not suggesting for loops have no place; they are sometimes the right tool, but often, forEach() is a cleaner approach.

Functional programming as a philosophy promotes “immutability.” That means simply that it likes to avoid modifying variables. Instead, functional programming prefers to take an existing variable and pass it through a “pipeline” (a function or functions) that transforms it into a new variable, leaving the original as-is.

forEach is often used in this way, but it’s also often used “destructively,” as shown here:

This example might not be considered the purest form of functional programming, but it utilizes key functional characteristics such as “first-order functions.” When we refer to a first-order function, we mean that we are using a function like any other reference, in this case, by passing it in as an argument. The long and the short of that story is that functions can act as portable bundles of functionality that are passed around to do jobs in predictable ways.

Note, too, that there are still many cases where an old-fashioned for loop is the best approach. For example, when iterating by a number other than 1, iterating backward, and when handling complex scenarios requiring multiple iterators.

Functions that are non-destructive and avoid any other “side-effects” are said to be “pure functions.” We can use forEach in this way, but the Array.map() function is specifically designed for this purpose. It does not operate on the array itself, but instead runs the function operator and returns the result as a new array:

Array.map() is a very powerful mechanism for transforming arrays. It gives you the ability to do almost anything with an array in a clean fashion. In particular, it avoids complexity in changing the original array, where other code elsewhere might depend on it in unknown or unexpected ways. 

On the other hand, it’s important to bear in mind that Array.map() always makes a copy, which has performance implications. You don’t want to use it on very large arrays. Sometimes, memory considerations dictate that you use another approach.

How this works is that whatever the provided function returns will be kept in the new array. So, we could use the automatically returning version of a function:

This approach can be a lot cleaner for short functions.

Array.map() outputs an array with the same length as the source. If the function doesn’t return something, the output array will be labeled undefined in that position. To create an array with a different length, you can use Array.filter() . In that case, when the functional argument doesn’t return anything, that element will be removed from the target array:

In this example, we take an array of objects holding rock bands and the year they were formed and then use bands.filter() to provide a function that will give us a new array holding only the bands from the 1970s. 

Sometimes, you need to take a whole array and turn it into a single value. For that, you can use Array.reduce :

The function passed to reduce() has two arguments: the “accumulator” and the current element. The accumulator is what will be finally returned, and holds its state across each iteration, allowing you to “collect” everything into a single output.

The reduce function is a very handy tool when you need it. As another quick example, say you wanted a string containing all the band names in a string. You could do this:

The built-in functions you’ve seen so far are fundamental to functional programming (and its programming-in-the-large sibling, reactive programming ). Now, let's consider the idea of linking functions together to achieve some desired functionality.

Two of the most basic and important linking functions are compose() and chain() . Many functional programming and utility libraries include them, but they are also easy to implement. This next example gives you a clear look at how they work:

compose() combines many functions, so that each function's output is fed into the next, from right to left (based on their ordering as passed to the function).  chain() does the same thing, but from left to right.

These functions also give you a look at reduceRight() , the mirror image of  reduce() , which you’ve already seen. The  reduceRight()  function lets you accumulate by going backward across the functional arguments.

The compose() and chain() functions are not specific to arrays, but they can be used with them. Here’s a simple example of using compose() with an array:

Arranging functions is central to both functional and reactive programming. It allows you to reuse and combine functions into new functions. In essence, you can define composite functions that are composed of the capabilities of other, more focused ones. This is similar, conceptually, to how an object-oriented programmer thinks about composing applications out of objects. 

Because functions express their work in such a minimalist way—with input and output being their entire API surface—they provide an exceptionally clean approach. Of course, as you become more sophisticated, you lose some of this clarity. Even the compose() and chain() functions leave behind some of the elegance of straight functions. 

In general, handling array functions like we’ve seen here, using JavaScript’s built-in functions like map() and filter() , is an excellent application of the power of functional programming.    

Next read this:

  • Why companies are leaving the cloud
  • 5 easy ways to run an LLM locally
  • Coding with AI: Tips and best practices from developers
  • Meet Zig: The modern alternative to C
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Matthew Tyson is a founder of Dark Horse Group, Inc. He believes in people-first technology. When not playing guitar, Matt explores the backcountry and the philosophical hinterlands. He has written for JavaWorld and InfoWorld since 2007.

Copyright © 2024 IDG Communications, Inc.

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Destructuring assignment

The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

Description

The object and array literal expressions provide an easy way to create ad hoc packages of data.

The destructuring assignment uses similar syntax but uses it on the left-hand side of the assignment instead. It defines which values to unpack from the sourced variable.

Similarly, you can destructure objects on the left-hand side of the assignment.

This capability is similar to features present in languages such as Perl and Python.

For features specific to array or object destructuring, refer to the individual examples below.

Binding and assignment

For both object and array destructuring, there are two kinds of destructuring patterns: binding pattern and assignment pattern , with slightly different syntaxes.

In binding patterns, the pattern starts with a declaration keyword ( var , let , or const ). Then, each individual property must either be bound to a variable or further destructured.

All variables share the same declaration, so if you want some variables to be re-assignable but others to be read-only, you may have to destructure twice — once with let , once with const .

In many other syntaxes where the language binds a variable for you, you can use a binding destructuring pattern. These include:

  • The looping variable of for...in for...of , and for await...of loops;
  • Function parameters;
  • The catch binding variable.

In assignment patterns, the pattern does not start with a keyword. Each destructured property is assigned to a target of assignment — which may either be declared beforehand with var or let , or is a property of another object — in general, anything that can appear on the left-hand side of an assignment expression.

Note: The parentheses ( ... ) around the assignment statement are required when using object literal destructuring assignment without a declaration.

{ a, b } = { a: 1, b: 2 } is not valid stand-alone syntax, as the { a, b } on the left-hand side is considered a block and not an object literal according to the rules of expression statements . However, ({ a, b } = { a: 1, b: 2 }) is valid, as is const { a, b } = { a: 1, b: 2 } .

If your coding style does not include trailing semicolons, the ( ... ) expression needs to be preceded by a semicolon, or it may be used to execute a function on the previous line.

Note that the equivalent binding pattern of the code above is not valid syntax:

You can only use assignment patterns as the left-hand side of the assignment operator. You cannot use them with compound assignment operators such as += or *= .

Default value

Each destructured property can have a default value . The default value is used when the property is not present, or has value undefined . It is not used if the property has value null .

The default value can be any expression. It will only be evaluated when necessary.

Rest property

You can end a destructuring pattern with a rest property ...rest . This pattern will store all remaining properties of the object or array into a new object or array.

The rest property must be the last in the pattern, and must not have a trailing comma.

Array destructuring

Basic variable assignment, destructuring with more elements than the source.

In an array destructuring from an array of length N specified on the right-hand side of the assignment, if the number of variables specified on the left-hand side of the assignment is greater than N , only the first N variables are assigned values. The values of the remaining variables will be undefined.

Swapping variables

Two variables values can be swapped in one destructuring expression.

Without destructuring assignment, swapping two values requires a temporary variable (or, in some low-level languages, the XOR-swap trick ).

Parsing an array returned from a function

It's always been possible to return an array from a function. Destructuring can make working with an array return value more concise.

In this example, f() returns the values [1, 2] as its output, which can be parsed in a single line with destructuring.

Ignoring some returned values

You can ignore return values that you're not interested in:

You can also ignore all returned values:

Using a binding pattern as the rest property

The rest property of array destructuring assignment can be another array or object binding pattern. The inner destructuring destructures from the array created after collecting the rest elements, so you cannot access any properties present on the original iterable in this way.

These binding patterns can even be nested, as long as each rest property is the last in the list.

On the other hand, object destructuring can only have an identifier as the rest property.

Unpacking values from a regular expression match

When the regular expression exec() method finds a match, it returns an array containing first the entire matched portion of the string and then the portions of the string that matched each parenthesized group in the regular expression. Destructuring assignment allows you to unpack the parts out of this array easily, ignoring the full match if it is not needed.

Using array destructuring on any iterable

Array destructuring calls the iterable protocol of the right-hand side. Therefore, any iterable, not necessarily arrays, can be destructured.

Non-iterables cannot be destructured as arrays.

Iterables are only iterated until all bindings are assigned.

The rest binding is eagerly evaluated and creates a new array, instead of using the old iterable.

Object destructuring

Basic assignment, assigning to new variable names.

A property can be unpacked from an object and assigned to a variable with a different name than the object property.

Here, for example, const { p: foo } = o takes from the object o the property named p and assigns it to a local variable named foo .

Assigning to new variable names and providing default values

A property can be both

  • Unpacked from an object and assigned to a variable with a different name.
  • Assigned a default value in case the unpacked value is undefined .

Unpacking properties from objects passed as a function parameter

Objects passed into function parameters can also be unpacked into variables, which may then be accessed within the function body. As for object assignment, the destructuring syntax allows for the new variable to have the same name or a different name than the original property, and to assign default values for the case when the original object does not define the property.

Consider this object, which contains information about a user.

Here we show how to unpack a property of the passed object into a variable with the same name. The parameter value { id } indicates that the id property of the object passed to the function should be unpacked into a variable with the same name, which can then be used within the function.

You can define the name of the unpacked variable. Here we unpack the property named displayName , and rename it to dname for use within the function body.

Nested objects can also be unpacked. The example below shows the property fullname.firstName being unpacked into a variable called name .

Setting a function parameter's default value

Default values can be specified using = , and will be used as variable values if a specified property does not exist in the passed object.

Below we show a function where the default size is 'big' , default co-ordinates are x: 0, y: 0 and default radius is 25.

In the function signature for drawChart above, the destructured left-hand side has a default value of an empty object = {} .

You could have also written the function without that default. However, if you leave out that default value, the function will look for at least one argument to be supplied when invoked, whereas in its current form, you can call drawChart() without supplying any parameters. Otherwise, you need to at least supply an empty object literal.

For more information, see Default parameters > Destructured parameter with default value assignment .

Nested object and array destructuring

For of iteration and destructuring, computed object property names and destructuring.

Computed property names, like on object literals , can be used with destructuring.

Invalid JavaScript identifier as a property name

Destructuring can be used with property names that are not valid JavaScript identifiers by providing an alternative identifier that is valid.

Destructuring primitive values

Object destructuring is almost equivalent to property accessing . This means if you try to destruct a primitive value, the value will get wrapped into the corresponding wrapper object and the property is accessed on the wrapper object.

Same as accessing properties, destructuring null or undefined throws a TypeError .

This happens even when the pattern is empty.

Combined array and object destructuring

Array and object destructuring can be combined. Say you want the third element in the array props below, and then you want the name property in the object, you can do the following:

The prototype chain is looked up when the object is deconstructed

When deconstructing an object, if a property is not accessed in itself, it will continue to look up along the prototype chain.

Specifications

Browser compatibility.

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

  • Assignment operators
  • ES6 in Depth: Destructuring on hacks.mozilla.org (2015)
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Work with the JavaScript Editor

Use the JavaScript editor to add custom JavaScript functions that meet your business requirements, for example, a JavaScript function to validate whether required fields in a form have values.

Any JavaScript code that you add will have a defined scope based on the editor where you write the code. If your code will only be called from within a specific page (for example, to load some data when the page loads), you can write your code in the page-level JavaScript editor. If you want a JavaScript function to be used on multiple pages (for example, to load libraries for customizing navigation elements or custom web components), then you'll need to use the JavaScript editor for the flow or the application. You can also define JavaScript functions at the layout and fragment level.

The JavaScript editor displays a particular artifact's JS file. So if you open the main flow's JavaScript editor (for example), you're seeing the contents of the main-flow.js file. Each artifact has its own JS file: an application artifact uses the app-flow.js file, a flow uses flow-name -flow.js , and a page uses page-name -page.js . A layout uses layout.js and a fragment uses fragment-name -fragment.js .

Description of js-code-complete-example.png follows

Selecting a structure will let you easily switch the variables in the structure.

  • https://www.w3schools.com/js/js_es6.asp
  • https://www.javascripttutorial.net/es6/

Add a Custom JavaScript Function

To add a custom JavaScript function, you define the function within the class provided in the JavaScript editor for your page, flow, or application . You can also add JavaScript functions to layouts and fragments.

Description of page-functions-editor.png follows

An application uses the app-flow.js file, a flow uses flow-name -flow.js , and a page uses page-name -page.js . A layout uses layout.js and a fragment fragment-name -fragment.js .

Just as flow-level functions use FlowModule , application functions use AppModule and page-level functions use PageModule .

Description of javascript-function-example.png follows

  • In an action chain, use the Call Function action. See Add a Call Function Action .

Description of js_functions_variablepicker.png follows

To write efficient expressions that handle situations where a referenced field might not be available or the field's value could be null, see How Do I Write Expressions If a Referenced Field Might Not Be Available Or Its Value Could Be Null?

Use RequireJS to Reference External JavaScript Files

If you want to use RequireJS to refer to external JavaScript libraries in your application , you can add a requirejs statement to your application 's definition, then import the library.

  • In the Web Apps pane, select your application node, then click the JSON tab, or
  • In the Source view, locate the file for your application under webapps .

Either way, make sure the requirejs entry is a sibling of the id or description entries. If a requirejs section already exists, simply add your entry under paths .

  • To load and use your library in a module, use the define statement to make your library a dependency for your module. In your JS file, enter, for example: define(['myLib'], (MyLib) => { 'use strict'; ...

Use Variables with a JavaScript Module

You can't directly get or set variables from within your JavaScript modules. However, you can use the Call Module Function action to access your JS module. This action takes an array of parameters which can include variables and can return a result that you can assign to a variable.

This approach ensures that the variable has a consistent state from the beginning to the end of your action chain's execution.

To "get" a value, pass the variable in as a parameter to the module function that you are calling using a callModuleFunction action in the action chain.

To "set" a variable based on the return value from that callModuleFunction , use an Assign Variables action to copy the result of the function into the desired variable in whatever scope.

How to Use LocalStorage in JavaScript

In modern web development, having a way to persist data helps developers improve performance and create a better user experience. And using local storage is an effective way of persisting data in an application.

In this article, you will learn what local storage is and how to use it in modern web applications. You will also learn the advantages of using local storage, as well as some of its limitations.

Table of Contents

What is local storage, differences between local storage and session storage, how to use local storage, a practical example, how to view local storage in devtools, benefits of using local storage.

  • Limitations of Local Storage

Local storage is a feature in web browsers that allows developers to save data in the user’s browser. It’s part of the web storage API, together with session storage.

Local storage works by accepting data in key-value pairs. It retains the data even when the user refreshes the page or closes the tab or browser.

As I mentioned earlier, the web storage API in modern browsers provides two main features for data storage. These are local storage and session storage.

The key differences between the two are the lifespan of the stored data and their scope.

Data in local storage remains available even when the tab/browser is closed. But closing the tab/browser clears any data stored in session storage.

Also, data in local storage is accessible across multiple browser tabs and windows. On the other hand, data in session storage is only accessible within specific browser tabs and is not shared.

The local storage object provides different methods you can use to interact with it. With these methods, you can add, read, and delete data from local storage.

How to Store Data in Local Storage

To store data in local storage, you use the setItem() method. This method takes in two arguments, a key and a value.

If the key does not exist in local storage, the setItem() method will create a new key and assign the given value to it. But if a key with the same name exists in local storage, it will update the value of the key with the provided value.

How to Read Data From Local Storage

To retrieve and use data from local storage, you use the getItem() method. This method takes in a key as an argument.

If the given key exists in local storage, the method returns the value of that key. If it doesn’t, the method returns null .

How to Store and Read Complex Data Values in Local Storage

Local storage can only store strings. This means if you need to store values like objects or arrays, you first need to get a string representation of the value. You do this using the JSON.stringify() method.

The JSON.stringify() method converts the userObj object into a string representation before sending it to local storage.

Now, when you want to retrieve the data back from local storage, you also need to change it from its string representation back to the original form. And you do that using the JSON.parse() method.

In the example above, we first check if there is data for ‘user’ in local storage before using the JSON.parse() method. This is important because if it does not exist in local storage, JSON.parse() will be applied to a null value (which will result in an error).

How to Delete Data from Local Storage

There are two methods available for deleting data from local storage. One is the removeItem() method and the other is the clear() method.

You use the removeItem() method when you want to delete a single item from local storage. The method takes in a key as an argument and deletes the corresponding key-value pair from local storage.

But what if, instead of deleting a single key-value pair, you want to clear all data from the local storage? Well, local storage has a method for that - the clear() method.

The clear() method deletes all key-value pairs in the local storage for the current domain.

How to Get the Name of a Key in Local Storage

If you want to get the name of a key at a particular index in local storage, you can use the key() method. It takes in a number as an argument and returns the name of the key at that specified index.

The example above will return the name of the key at index 0. If there is no key at the specified index, the method will return null.

The following shows a practical demo of the difference between local storage and session storage.

In this example, we'll save the user's name in local storage and save the age in session storage.

The markup includes two header elements. One for userName and the other for userAge . It also includes two input elements for name and age. Each input has an associated button we'll use for saving the data.

Now, let's use the querySelector method to select the various elements.

Code Example for Local Storage

First, we get the value of the name input, set it as the textContent of userNameText . And then use the setItem() of local storage to save the userName value in local storage.

Next, let's see how we can get the name value from local storage when we need it.

The displayUserName function gets nameFromLocalStorage using the getItem() method. If the value exists in local storage, we set it as the textContent of the userNameText element. If it's null or doesn't exist, then we set textContent to the string "No name data in local storage" .

Code Example for Session Storage

Now, let's do the same thing for the age value. The only difference here will be using session storage instead of local storage.

The setItem and getItem methods also works for session storage.

As you can see from the demo above, when you close and reopen the page, the name data from local storage persists. But the age data from session storage is cleared once the page closes.

You can follow the steps below to inspect the contents of local storage in your browser's developer tools.

First, open DevTools. You can do that by right clicking on the web page and selecting "Inspect".

Then, select the "Application" tab on the DevTools panel. Depending on your browser, this panel may have a different name. For example, it's called "Storage" in Safari and Firefox.

Locate the "Storage" section on the sidebar showing a list of the various web storage options.

Click on "Local Storage" to expand and view its contents.

You can click on individual items to view the corresponding key-value pair.

The following are some of the benefits local storage has over other storage mechanisms in modern web development.

  • Persistent data: When you use local storage, the stored data remains even when the user closes the tab or the browser. This is useful for saving user preferences, settings, and other relevant data. It can help create a seamless user experience.
  • Offline access: You can use local storage as a means to cache data which can be accessed even with limited or no internet. This makes it a useful feature for apps that rely on caching data for offline use like news readers, productivity apps, and so on.
  • More storage capacity: Compared to other storage means, local storage has a relatively high capacity. For example, cookies are limited to 4 kilobytes per domain. But local storage can store up to 5 megabytes of data per domain.

Limitations of Using Local Storage

  • Stores only strings: As you learned earlier, local storage can only store string values. You can use the JSON stringify and parse methods to work around it. But some web developers may not prefer it as it can lead to writing complex code that’s difficult to debug.
  • Security concerns: Data in the local storage can be prone to attacks like cross-site scripting (XSS). As such, you should be cautious when working with sensitive information. It’s advisable to assess security implications and consider other alternatives where necessary.
  • Not accessible to web workers: Local storage is part of the Window object. As such, it’s tied to the main execution thread of the web page. This means it's not accessible to web workers. So if you run any background processes, you cannot use local storage within the web worker scripts.

Local storage is a feature in modern web browsers that makes it easy for web developers to store and persist data between browser sessions.

Compared to traditional cookies, it provides larger storage capacities. Also, unlike cookies, it does not rely on server-side processes. This reduces the need for frequent server requests and helps improve performance.

In this article, you learn about how to use local storage. We covered saving, retrieving, and deleting data from local storage. You also learned about some of the benefits of using local storage in your project, and some of its limitations too.

Thanks for reading. And happy coding! For more in-depth tutorials, feel free to subscribe to my YouTube channel .

Software Developer | Technical Writer

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COMMENTS

  1. JavaScript Arrays

    Using an array literal is the easiest way to create a JavaScript Array. Syntax: const array_name = [ item1, item2, ... ]; It is a common practice to declare arrays with the const keyword. Learn more about const with arrays in the chapter: JS Array Const. Example const cars = ["Saab", "Volvo", "BMW"]; Try it Yourself »

  2. Add a property to a JavaScript array

    Add a property to a JavaScript array Ask Question Asked 11 years, 11 months ago Modified 1 year, 4 months ago Viewed 70k times 66 Arrays have a "length" property by default. Can I add custom properties to them? Without having to make them objects. javascript arrays Share Improve this question Follow edited Apr 22, 2021 at 12:03 Peter Mortensen

  3. How to Manipulate Arrays in JavaScript

    map() creates a new array by manipulating the values in an array. reduce() calculates a single value based on an array. forEach() iterates through an array, it applies a function on all items in an array

  4. JavaScript Arrays: Create, Access, Add & Remove Elements

    JavaScript array is a special type of variable, which can store multiple values using a special syntax. The following declares an array with five numeric values. let numArr = [10, 20, 30, 40, 50]; In the above array, numArr is the name of an array variable.

  5. Array

    Array The Array object, as with arrays in other programming languages, enables storing a collection of multiple items under a single variable name, and has members for performing common array operations. Description In JavaScript, arrays aren't primitives but are instead Array objects with the following core characteristics:

  6. Arrays

    Create an array styles with items "Jazz" and "Blues". Append "Rock-n-Roll" to the end. Replace the value in the middle with "Classics". Your code for finding the middle value should work for any arrays with odd length. Strip off the first value of the array and show it. Prepend Rap and Reggae to the array. The array in the process:

  7. The JavaScript Array Handbook

    In JavaScript, the array index starts with 0, and it increases by one with each element. So, for example, in the above array, the element 100 is at index 0, true is at index 1, 'freeCodeCamp' is at index 2, and so on. The number of elements in the array determines its length. For example, the length of the above array is four.

  8. Understanding Arrays in JavaScript

    An array in JavaScript is a type of global object that is used to store data. Arrays consist of an ordered collection or list containing zero or more data types, ... We can overwrite any value in an array by assigning a new value using the assignment operator, just like we would with a regular variable. ...

  9. JavaScript Arrays

    JavaScript arrays are zero-indexed, meaning the first element is at index 0. Array Length: ... Elements in an array can be modified by assigning a new value to their corresponding index. Javascript // Creating an Array and Initializing with Values. let courses = ["HTML", ...

  10. Indexed collections

    An array is an ordered list of values that you refer to with a name and an index. For example, consider an array called emp, which contains employees' names indexed by their numerical employee number. So emp [0] would be employee number zero, emp [1] employee number one, and so on. JavaScript does not have an explicit array data type.

  11. JavaScript Array Tutorial

    The array length will be the value of the index of the last element inside the array + 1, since the indexing starts at zero. Multidimensional Arrays . JavaScript arrays can hold any allowed values, arrays included. An array inside another array is called a nested array. This situation creates the possibility of many array objects nested at ...

  12. Object.assign()

    Object.assign () The Object.assign () static method copies all enumerable own properties from one or more source objects to a target object. It returns the modified target object. Try it Syntax js Object.assign(target) Object.assign(target, source1) Object.assign(target, source1, source2)

  13. Javascript

    5,130 21 61 93 Add a comment 9 Answers Sorted by: 58 There's no built-in way, you'll have to loop over all of them: function setAll (a, v) { var i, n = a.length; for (i = 0; i < n; ++i) { a [i] = v; } } http://jsfiddle.net/alnitak/xG88A/ If you really want, do this:

  14. Reassign Objects & Arrays in Javascript

    const numbers = [ 1, 2, 3 ]; numbers.push ( 4 ); console. log (numbers) // Outpusts [1,2,3,4]; With methods, we can modify our array by adding another value to the end of the array using the push method. Another way you could modify the array is to remove an item from the end by using the pop method.

  15. JavaScript Arrays

    The most common way to create an array in JavaScript would be to assign that array to a variable like this: const books = ["The Great Gatsby", "War and Peace", "Hamlet", "Moby Dick"]; If we console.log the array, then it will show us all 4 elements listed in the array.

  16. Array() constructor

    If the only argument passed to the Array constructor is an integer between 0 and 2 32 - 1 (inclusive), this returns a new JavaScript array with its length property set to that number (Note: this implies an array of arrayLength empty slots, not slots with actual undefined values — see sparse arrays).

  17. Javascript assign array elements by reference or by value

    1 Is there a way to explicity set array elements by reference instead of by value? For example, this method sets the array elements by value: METHOD 1 var pushes = [2,1] for(var i=0; i<pushes.length; i++) { vm.allEmployeesOnJob[i] = vm.allEmployees[pushes[i]]; } And this method sets the array elements by reference: METHOD 2

  18. Functional programming with JavaScript arrays

    JavaScript's arrays can hold heterogeneous types, change size on the fly, and readily insert or remove elements. Traditional methods like slice, splice, and push/pop do this by operating on the ...

  19. Populating another array from array

    bar[i]=ar[i]; } alert(ar[1]); And, here is the fiddle: http://jsfiddle.net/vGycZ/ (The above is a simplification. The actual array is multidimensional.) javascript Share Improve this question Follow edited Aug 9, 2010 at 7:30 asked Aug 9, 2010 at 7:18 ina 19.3k 39 124 202 check this : hardcode.nl/subcategory_1/… - Pranay Rana Aug 9, 2010 at 7:25

  20. JavaScript 2D Array

    Note: All JavaScript array methods will operate on a 2D array because it is an array of arrays. You only have to be mindful when adjusting individual elements in the elements array. ... where you can update an array's value by using the index to assign another value. let array = [1, 2, 3]; array[1] = 5; console.log(array); // returns [1, 5, 3]

  21. Destructuring assignment

    Unpacking values from a regular expression match. When the regular expression exec() method finds a match, it returns an array containing first the entire matched portion of the string and then the portions of the string that matched each parenthesized group in the regular expression. Destructuring assignment allows you to unpack the parts out of this array easily, ignoring the full match if ...

  22. Work with the JavaScript Editor

    This approach ensures that the variable has a consistent state from the beginning to the end of your action chain's execution. To "get" a value, pass the variable in as a parameter to the module function that you are calling using a callModuleFunction action in the action chain.. To "set" a variable based on the return value from that callModuleFunction, use an Assign Variables action to copy ...

  23. Assigning values from an array into variables in javascript

    Assigning values from an array into variables in javascript Ask Question Asked 2 years, 4 months ago Modified 2 years, 4 months ago Viewed 2k times -4 I have a array with 18 elements (may vary, but only is multiples of 6), I want to assign these 18 elements to 6 variables lets say the array is

  24. How to Use LocalStorage in JavaScript

    This method takes in two arguments, a key and a value. localStorage.setItem(key, value) If the key does not exist in local storage, the setItem() method will create a new key and assign the given value to it. But if a key with the same name exists in local storage, it will update the value of the key with the provided value.

  25. javascript

    In my project I am using a textbox where user can paste a text with a set of questions and answers. I need to get the input split each line in an array and add an object to the setState array with index and the content of the line. The array of object in setState is. const [question, setQuestion] = React.useState([{quin:0,content:'',examid:''}])