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The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values ) may be used as either a key or a value.

Description

Map objects are collections of key-value pairs. A key in the Map may only occur once ; it is unique in the Map 's collection. A Map object is iterated by key-value pairs — a for...of loop returns a 2-member array of [key, value] for each iteration. Iteration happens in insertion order , which corresponds to the order in which each key-value pair was first inserted into the map by the set() method (that is, there wasn't a key with the same value already in the map when set() was called).

The specification requires maps to be implemented "that, on average, provide access times that are sublinear on the number of elements in the collection". Therefore, it could be represented internally as a hash table (with O(1) lookup), a search tree (with O(log(N)) lookup), or any other data structure, as long as the complexity is better than O(N).

Key equality

Value equality is based on the SameValueZero algorithm. (It used to use SameValue , which treated 0 and -0 as different. Check browser compatibility .) This means NaN is considered the same as NaN (even though NaN !== NaN ) and all other values are considered equal according to the semantics of the === operator.

Objects vs. Maps

Object is similar to Map —both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. For this reason (and because there were no built-in alternatives), Object has been used as Map historically.

However, there are important differences that make Map preferable in some cases:

Setting object properties

Setting Object properties works for Map objects as well, and can cause considerable confusion.

Therefore, this appears to work in a way:

But that way of setting a property does not interact with the Map data structure. It uses the feature of the generic object. The value of 'bla' is not stored in the Map for queries. Other operations on the data fail:

The correct usage for storing data in the Map is through the set(key, value) method.

Map-like browser APIs

Browser Map -like objects (or "maplike objects") are Web API interfaces that behave in many ways like a Map .

Just like Map , entries can be iterated in the same order that they were added to the object. Map -like objects and Map also have properties and methods that share the same name and behavior. However unlike Map they only allow specific predefined types for the keys and values of each entry.

The allowed types are set in the specification IDL definition. For example, RTCStatsReport is a Map -like object that must use strings for keys and objects for values. This is defined in the specification IDL below:

Map -like objects are either read-only or read-writable (see the readonly keyword in the IDL above).

  • Read-only Map -like objects have the property size , and the methods: entries() , forEach() , get() , has() , keys() , values() , and @@iterator .
  • Writeable Map -like objects additionally have the methods: clear() , delete() , and set() .

The methods and properties have the same behavior as the equivalent entities in Map , except for the restriction on the types of the keys and values.

The following are examples of read-only Map -like browser objects:

  • AudioParamMap
  • RTCStatsReport
  • EventCounts
  • KeyboardLayoutMap
  • MIDIInputMap
  • MIDIOutputMap

Constructor

Creates a new Map object.

Static properties

The constructor function that is used to create derived objects.

Static methods

Groups the elements of a given iterable using the values returned by a provided callback function. The final returned Map uses the unique values from the test function as keys, which can be used to get the array of elements in each group.

Instance properties

These properties are defined on Map.prototype and shared by all Map instances.

The constructor function that created the instance object. For Map instances, the initial value is the Map constructor.

Returns the number of key/value pairs in the Map object.

The initial value of the @@toStringTag property is the string "Map" . This property is used in Object.prototype.toString() .

Instance methods

Removes all key-value pairs from the Map object.

Returns true if an element in the Map object existed and has been removed, or false if the element does not exist. map.has(key) will return false afterwards.

Returns a new Iterator object that contains a two-member array of [key, value] for each element in the Map object in insertion order.

Calls callbackFn once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach , it will be used as the this value for each callback.

Returns the value associated to the passed key, or undefined if there is none.

Returns a boolean indicating whether a value has been associated with the passed key in the Map object or not.

Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.

Sets the value for the passed key in the Map object. Returns the Map object.

Returns a new Iterator object that contains the values for each element in the Map object in insertion order.

Using the Map object

Using nan as map keys.

NaN can also be used as a key. Even though every NaN is not equal to itself ( NaN !== NaN is true), the following example works because NaN s are indistinguishable from each other:

Iterating Map with for...of

Maps can be iterated using a for...of loop:

Iterating Map with forEach()

Maps can be iterated using the forEach() method:

Relation with Array objects

Cloning and merging maps.

Just like Array s, Map s can be cloned:

Note: Keep in mind that the data itself is not cloned.

Maps can be merged, maintaining key uniqueness:

Maps can be merged with Arrays, too:

Specifications

Browser compatibility.

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

  • Polyfill for Map in core-js

Home » JavaScript Tutorial » JavaScript Map Object

JavaScript Map Object

Summary : in this tutorial, you will learn about the JavaScript Map object that maps a key to a value.

Introduction to JavaScript Map object

Before ES6, we often used an object to emulate a map by mapping a key to a value of any type. But using an object as a map has some side effects:

  • An object always has a default key like the prototype .
  • A key of an object must be a string or a symbol , you cannot use an object as a key.
  • An object does not have a property that represents the size of the map.

ES6 provides a new collection type called Map that addresses these deficiencies.

By definition, a Map object holds key-value pairs. Keys are unique in a Map’s collection. In other words, a key in a Map object only appears once.

Keys and values of a Map can be any values.

When iterating a Map object, each iteration returns a 2-member array of [key, value] . The iteration order follows the insertion order which corresponds to the order in which each key-value pair was first inserted into the Map by the set() method.

To create a new Map , you use the following syntax:

The Map() accepts an optional iterable object whose elements are key-value pairs.

Useful JavaScript Map methods

  • clear() – removes all elements from the map object.
  •   delete(key) – removes an element specified by the key. It returns if the element is in the map, or false if it does not.
  •   entries() – returns a new Iterator object that contains an array of [key, value] for each element in the map object. The order of objects in the map is the same as the insertion order.
  •   forEach(callback[, thisArg]) – invokes a callback for each key-value pair in the map in the insertion order. The optional thisArg parameter sets the this value for each callback.
  •  get(key) – returns the value associated with the key. If the key does not exist, it returns undefined.
  •  has(key) – returns true if a value associated with the key exists or false otherwise.
  •   keys() – returns a new Iterator that contains the keys for elements in insertion order.
  •   set(key, value) – sets the value for the key in the map object. It returns the map object itself therefore you can chain this method with other methods.
  •   values() returns a new iterator object that contains values for each element in insertion order.

JavaScript Map examples

Let’s take some examples of using a Map object.

Create a new Map object

Suppose you have a list of user objects as follows:

Assuming that you have to create a map of users and roles. In this case, you use the following code:

The userRoles is an instance of the Map object and its type is an object as illustrated in the following example:

Add elements to a Map

To assign a role to a user, you use the set() method:

The set() method maps user john with the admin role. Since the set() method is chainable, you can save some typing as shown in this example:

Initialize a map with an iterable object

As mentioned earlier, you can pass an iterable object to the Map() constructor:

Get an element from a map by key

If you want to see the roles of John , you use the get() method:

If you pass a key that does not exist, the get() method will return undefined .

Check the existence of an element by key

To check if a key exists in the map, you use the has() method.

Get the number of elements in the map

The size property returns the number of entries of the Map object.

Iterate over map keys

To get the keys of a Map object, you use the keys() method. The keys() returns a new iterator object that contains the keys of elements in the map.

The following example displays the username of the users in the userRoles map object.

Iterate over map values

Similarly, you can use the values() method to get an iterator object that contains values for all the elements in the map:

Iterate over map elements

Also, the entries() method returns an iterator object that contains an array of [key,value] of each element in the Map object:

To make the iteration more natural, you can use destructuring as follows:

In addition to for...of loop, you can use the forEach() method of the map object:

Convert map keys or values to an array

Sometimes, you want to work with an array instead of an iterable object, in this case, you can use the spread operator .

The following example converts keys for each element into an array of keys:

The following converts the values of elements to an array:

Delete an element by key

To delete an entry in the map, you use the delete() method.

Map and Set

Till now, we’ve learned about the following complex data structures:

  • Objects are used for storing keyed collections.
  • Arrays are used for storing ordered collections.

But that’s not enough for real life. That’s why Map and Set also exist.

Map is a collection of keyed data items, just like an Object . But the main difference is that Map allows keys of any type.

Methods and properties are:

  • new Map() – creates the map.
  • map.set(key, value) – stores the value by the key.
  • map.get(key) – returns the value by the key, undefined if key doesn’t exist in map.
  • map.has(key) – returns true if the key exists, false otherwise.
  • map.delete(key) – removes the element (the key/value pair) by the key.
  • map.clear() – removes everything from the map.
  • map.size – returns the current element count.

For instance:

As we can see, unlike objects, keys are not converted to strings. Any type of key is possible.

Although map[key] also works, e.g. we can set map[key] = 2 , this is treating map as a plain JavaScript object, so it implies all corresponding limitations (only string/symbol keys and so on).

So we should use map methods: set , get and so on.

Map can also use objects as keys.

Using objects as keys is one of the most notable and important Map features. The same does not count for Object . String as a key in Object is fine, but we can’t use another Object as a key in Object .

As visitsCountObj is an object, it converts all Object keys, such as john and ben above, to same string "[object Object]" . Definitely not what we want.

To test keys for equivalence, Map uses the algorithm SameValueZero . It is roughly the same as strict equality === , but the difference is that NaN is considered equal to NaN . So NaN can be used as the key as well.

This algorithm can’t be changed or customized.

Every map.set call returns the map itself, so we can “chain” the calls:

Iteration over Map

For looping over a map , there are 3 methods:

  • map.keys() – returns an iterable for keys,
  • map.values() – returns an iterable for values,
  • map.entries() – returns an iterable for entries [key, value] , it’s used by default in for..of .

The iteration goes in the same order as the values were inserted. Map preserves this order, unlike a regular Object .

Besides that, Map has a built-in forEach method, similar to Array :

Object.entries: Map from Object

When a Map is created, we can pass an array (or another iterable) with key/value pairs for initialization, like this:

If we have a plain object, and we’d like to create a Map from it, then we can use built-in method Object.entries(obj) that returns an array of key/value pairs for an object exactly in that format.

So we can create a map from an object like this:

Here, Object.entries returns the array of key/value pairs: [ ["name","John"], ["age", 30] ] . That’s what Map needs.

Object.fromEntries: Object from Map

We’ve just seen how to create Map from a plain object with Object.entries(obj) .

There’s Object.fromEntries method that does the reverse: given an array of [key, value] pairs, it creates an object from them:

We can use Object.fromEntries to get a plain object from Map .

E.g. we store the data in a Map , but we need to pass it to a 3rd-party code that expects a plain object.

Here we go:

A call to map.entries() returns an iterable of key/value pairs, exactly in the right format for Object.fromEntries .

We could also make line (*) shorter:

That’s the same, because Object.fromEntries expects an iterable object as the argument. Not necessarily an array. And the standard iteration for map returns same key/value pairs as map.entries() . So we get a plain object with same key/values as the map .

A Set is a special type collection – “set of values” (without keys), where each value may occur only once.

Its main methods are:

  • new Set([iterable]) – creates the set, and if an iterable object is provided (usually an array), copies values from it into the set.
  • set.add(value) – adds a value, returns the set itself.
  • set.delete(value) – removes the value, returns true if value existed at the moment of the call, otherwise false .
  • set.has(value) – returns true if the value exists in the set, otherwise false .
  • set.clear() – removes everything from the set.
  • set.size – is the elements count.

The main feature is that repeated calls of set.add(value) with the same value don’t do anything. That’s the reason why each value appears in a Set only once.

For example, we have visitors coming, and we’d like to remember everyone. But repeated visits should not lead to duplicates. A visitor must be “counted” only once.

Set is just the right thing for that:

The alternative to Set could be an array of users, and the code to check for duplicates on every insertion using arr.find . But the performance would be much worse, because this method walks through the whole array checking every element. Set is much better optimized internally for uniqueness checks.

Iteration over Set

We can loop over a set either with for..of or using forEach :

Note the funny thing. The callback function passed in forEach has 3 arguments: a value , then the same value valueAgain , and then the target object. Indeed, the same value appears in the arguments twice.

That’s for compatibility with Map where the callback passed forEach has three arguments. Looks a bit strange, for sure. But this may help to replace Map with Set in certain cases with ease, and vice versa.

The same methods Map has for iterators are also supported:

  • set.keys() – returns an iterable object for values,
  • set.values() – same as set.keys() , for compatibility with Map ,
  • set.entries() – returns an iterable object for entries [value, value] , exists for compatibility with Map .

Map – is a collection of keyed values.

Methods and properties:

  • new Map([iterable]) – creates the map, with optional iterable (e.g. array) of [key,value] pairs for initialization.
  • map.set(key, value) – stores the value by the key, returns the map itself.
  • map.delete(key) – removes the element by the key, returns true if key existed at the moment of the call, otherwise false .

The differences from a regular Object :

  • Any keys, objects can be keys.
  • Additional convenient methods, the size property.

Set – is a collection of unique values.

  • new Set([iterable]) – creates the set, with optional iterable (e.g. array) of values for initialization.
  • set.add(value) – adds a value (does nothing if value exists), returns the set itself.

Iteration over Map and Set is always in the insertion order, so we can’t say that these collections are unordered, but we can’t reorder elements or directly get an element by its number.

Filter unique array members

Let arr be an array.

Create a function unique(arr) that should return an array with unique items of arr .

P.S. Here strings are used, but can be values of any type.

P.P.S. Use Set to store unique values.

Open a sandbox with tests.

Open the solution with tests in a sandbox.

Filter anagrams

Anagrams are words that have the same number of same letters, but in different order.

Write a function aclean(arr) that returns an array cleaned from anagrams.

From every anagram group should remain only one word, no matter which one.

To find all anagrams, let’s split every word to letters and sort them. When letter-sorted, all anagrams are same.

We’ll use the letter-sorted variants as map keys to store only one value per each key:

Letter-sorting is done by the chain of calls in the line (*) .

For convenience let’s split it into multiple lines:

Two different words 'PAN' and 'nap' receive the same letter-sorted form 'anp' .

The next line put the word into the map:

If we ever meet a word the same letter-sorted form again, then it would overwrite the previous value with the same key in the map. So we’ll always have at maximum one word per letter-form.

At the end Array.from(map.values()) takes an iterable over map values (we don’t need keys in the result) and returns an array of them.

Here we could also use a plain object instead of the Map , because keys are strings.

That’s how the solution can look:

Iterable keys

We’d like to get an array of map.keys() in a variable and then apply array-specific methods to it, e.g. .push .

But that doesn’t work:

Why? How can we fix the code to make keys.push work?

That’s because map.keys() returns an iterable, but not an array.

We can convert it into an array using Array.from :

  • 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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Related Articles

  • Solve Coding Problems
  • JavaScript Map() Constructor
  • JavaScript Map constructor Property
  • JavaScript Map size Property
  • JavaScript Map clear() Method
  • JavaScript Map delete() Method
  • JavaScript Map entries() Method
  • JavaScript Map forEach() Method
  • JavaScript Map get() Method
  • JavaScript Map has() Method

JavaScript Map keys() Method

  • JavaScript Map set() Method
  • JavaScript Map values() Method

The Map.keys() method is used to extract the keys from a given map object and return the iterator object of keys. The keys are returned in the order they were inserted.

Syntax: 

Parameters: This method does not accept any parameters.

Return Value: This returns the iterator object that contains keys in the map.

Example 1: Below is the basic example of the Map.keys() method.

Example 2: In this example, we will create an object and print its keys in the console.

Example 3: Updating the value of the key in the map and printing values using the iterator object.

Output: 

We have a complete list of Javascript Map Methods, to check those please go through Javascript Map Complete Reference article.

Supported Browsers: 

  • Chrome 38 and above
  • Opera 25 and above
  • Edge 12 and above
  • Firefox 20 and above
  • Safari 8 and above

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Js versions, js functions, js html dom, js browser bom, js web apis, js vs jquery, js graphics, js examples, js references, javascript maps.

A Map holds key-value pairs where the keys can be any datatype.

A Map remembers the original insertion order of the keys.

A Map has a property that represents the size of the map.

Map Methods

How to create a map.

You can create a JavaScript Map by:

  • Passing an Array to new Map()
  • Create a Map and use Map.set()

You can create a Map by passing an Array to the new Map() constructor:

You can add elements to a Map with the set() method:

The set() method can also be used to change existing Map values:

The get() method gets the value of a key in a Map:

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The size property returns the number of elements in a Map:

Map.delete()

The delete() method removes a Map element:

Map.clear()

The clear() method removes all the elements from a Map:

The has() method returns true if a key exists in a Map:

Maps are Objects

typeof returns object:

instanceof Map returns true:

JavaScript Objects vs Maps

Differences between javascript objects and maps:, map.foreach().

The forEach() method invokes a callback for each key/value pair in a Map:

Map.entries()

The entries() method returns an iterator object with the [key,values] in a Map:

The keys() method returns an iterator object with the keys in a Map:

Map.values()

The values() method returns an iterator object with the values in a Map:

You can use the values() method to sum the values in a Map:

Objects as Keys

Being able to use objects as keys is an important Map feature.

Remember: The key is an object (apples), not a string ("apples"):

Browser Support

JavaScript Maps are supported in all browsers, except Internet Explorer:

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Introduction to Maps in JavaScript – All You Need to Know

Introduction to Maps in JavaScript feature image

Table of Contents

Have you ever heard about maps in JavaScript? Maps are a new object type that was introduced in ES2015. In this tutorial, you will learn all you need to know about this lesser known object type. You will learn about what maps in JavaScript are, how they work and how to use them.

Quick introduction to maps

As a JavaScript developer you probably know JavaScript objects . Objects allow you to store data in the form of a key-value pair. Maps are very similar to JavaScript objects. When you want to store some data in maps, you store those data also in the form of a key-value pair.

Also just like with objects, you can add new keys and delete existing, and retrieve their values from maps. When you compare maps and objects, there are some differences you should know about. Let’s take a look at those differences before moving further.

Maps vs Objects

One of the most important differences is that, when it comes to maps in JavaScript, you can use any data type to create keys. You can use even objects or functions. Objects will allow you to use only strings or a symbols . Another important difference is the order of keys-value pairs.

In maps, keys are ordered based on the order you’ve added them to the map. If you iterate over a map, you will get its keys in the same order in which you’ve created them. In case of objects, this is true since ES2015 and only for JavaScript engines that support this specification. Before ES2015, keys in objects were not ordered.

Another difference is how easy it is to get the size of a map. Like set , every map has a size property that says how many key-value pairs it contains. With objects, you would have to use keys() or values() to get an array of keys or values. Then, use length to get the length of this array to finally get the size of an object.

Another nice thing is that maps, like array, are iterable. You don’t have to get the keys or values first in order to iterate over them. You can do it right away. For example, you can use forEach() method , just like with array. You can also use for…of loop , just like with objects.

The last difference, that is good to know, is that maps are optimized for adding and removing of key-value pairs. Objects are not. This may not matter if you don’t need to manipulate with data often. If you do using maps may help you improve performance of your JavaScript code.

Creating maps in JavaScript

Maps are similar to objects. One thing that is different between them, among the things we just discussed, is how you create them. When you want to create a new object, there are multiple options to do that. For example, you can use new Object() , Object.create() , object literal or object constructor.

When you want to create a new map, there are two ways to do it. Well, theoretically. The first option to create new maps is by creating new empty Map object using new Map() and assigning it values later.

From array to map

The second option is also about using new Map() to create new Map object. However, you can also pass in an array. To make this work, this array has to be structured in a specific way. It has to contain nested array for each key-value pair. Each array (key-value pair) has to contain two items, the key and the value.

From object to map

You can use the second option also with objects. You can take existing object and get all its entries with entries() method. The entries() method returns all entries in the same format as the array you saw in the previous example. So, you can pass the result of calling entries() method to the Map() object.

Adding values to maps

When you want to add values, key-value pairs, to an object there are two ways to do that. Well, three, if you count adding values during object initialization. The first is using do notation. The second is using square brackets. The second way, square brackets, works also with maps.

That said, adding values to maps using square brackets is not a good practice. The problem is that when you do this you will lose performance optimizations maps have. The right way to add values to maps is by using set() method. This method accepts two parameters. First is for the key and second for the value .

When you want to add multiple key-value pairs to a map you can do it only one at the time. One interesting thing is that maps support chaining. So, yes, you have to use set() method for every key-value pair you want to add. However, you can chain these methods so you don’t have to use the name of the map over and over again.

Removing values from maps

When you want to remove values from maps the process is simple. There is a method you have to use called delete() . This method accepts one parameter, the key of the key-value pair you want to remove. If the deletion is successful, the delete() method will return true . If the key doesn’t exist, it will return false .

One thing to remember about delete() method is that it works only with one key at the time. You can’t pass in multiple keys. If you try it the delete() method will remove only the first key. It will ignore the rest.

Removing all values from maps

Removing values with delete() method is handy when you want to remove just one or few values. If you want to remove all values in a map at once, there is a better and faster way to do it. Aside from overwriting the map, you can do this also with clear() method. This method doesn’t accept any parameters.

Retrieving values from maps

Adding and removing values from maps is straightforward. You can say the same about retrieving them. When you want to retrieve a specific value from a map you can use get() method. This method accepts one parameter, a key that is associated with the value you want to retrieve.

If the key its value you want to retrieve exists, it will return the value. If it doesn’t exist it will return undefined .

Checking if value exists in map

In some sense, the get() method can also help you check if some key exists in a map. However, there is a method dedicated for this. This method is called has() . Similarly to get() , the has() method also accepts one parameter, the key you are looking for. If the key exists has() returns true . If not, it returns false .

Getting the size of the map

In the “Maps vs Objects” we discussed that one of the benefits of maps is that it is easy to find out their size. This is true. Every Map object has its own size property. This property is analogous to the length property that exists on arrays. Using this size property will quickly tell you how many key-value pairs are there in a specific map.

Iterating over maps

You know how to add values to maps and how to remove them. You also know how to retrieve values, one by one. Question is, what if you want to retrieve all values from a map? There are four options you can choose from. These options are keys() , values() , entries() and forEach() methods.

Map.keys(), Map.values() and Map.entries()

The first three methods all return Iterator object hat contain specific data. The first method, keys() , returns an Iterator with keys, one key for each pair in the Map. The second method, values() Iterator with values, also one value for each pair in the Map. The third method entries() returns an iterable for all entries.

These entries are returned in the form of a [key, value] . When you use these three methods you can then iterate over the returned Iterator object with next() method and its value property. Every use of next() method, along with value , will return the next value in the iterator, next value, key or entry in the map.

Maps, iterators and for…of loop

Using the next() and value will not be the best tool if you want to get all data from Iterator object at once. For this, a better option will be for...of loop. This loop allows you to iterate over an Iterator and get all data inside, without the need to use next() multiple times.

Map.forEach()

The forEach() method is a bit different. It doesn’t return Iterator object like keys() , values() and entries() and lets you iterate over those values manually. Instead, forEach() iterates over the map directly It also automatically iterates over all key-value pairs.

When it iterates over the pairs it executes a callback function for each of them. This callback function accepts three parameters. All these parameters are optional. These parameters are value , key and map . The value allows you to access current value in each iteration.

The key allows you to access current key in the iteration. The last one, the map , allows you to access the whole map you are iterating over.

From maps to objects

You know that you can create maps from objects. You can also do the opposite. You can take existing map and use it to create new object. This can be done with fromEntries() method. In the beginning, you used entries() method to create an array from entries that exist inside an object.

The fromEntries() method does the opposite thing. It takes an array, in the form of [key, value] , and transforms it into an object. The entries() method that exists on map will help you transform any map into an array you need. You can then use that array with fromEntries() method to create new object.

Conclusion: Introduction to maps in JavaScript

Maps are one of the lesser known, and less frequently used, object types in JavaScript. I hope this tutorial helped you learn what maps in JavaScript are, how they work and how to use them. I also hope it helped you distinguish when they can be a better choice than objects.

Do you have any questions, recommendations, thoughts, advice or tip you would like to share with other readers of this blog, and me? Please share it in a comment. You can also send me a mail . I would love to hear from you.

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Understanding Map and Set in JavaScript

This article was originally written for DigitalOcean .

In JavaScript, developers often spend a lot of time deciding the correct data structure to use. This is because choosing the correct data structure can make it easier to manipulate that data later on, saving time and making code easier to comprehend. The two predominant data structures for storing collections of data are Objects and Arrays (a type of object). Developers use Objects to store key/value pairs and Arrays to store indexed lists. However, to give developers more flexibility, the ECMAScript 2015 specification introduced two new types of iterable objects: Maps, which are ordered collections of key/value pairs, and Sets, which are collections of unique values.

In this article, you will go over the Map and Set objects, what makes them similar or different to Objects and Arrays, the properties and methods available to them, and examples of some practical uses.

A Map is a collection of key/value pairs that can use any data type as a key and can maintain the order of its entries. Maps have elements of both Objects (a unique key/value pair collection) and Arrays (an ordered collection), but are more similar to Objects conceptually. This is because, although the size and order of entries is preserved like an Array, the entries themselves are key/value pairs like Objects.

Maps can be initialized with the new Map() syntax:

This gives us an empty Map:

Adding Values to a Map

You can add values to a map with the set() method. The first argument will be the key, and the second argument will be the value.

The following adds three key/value pairs to map :

Here we begin to see how Maps have elements of both Objects and Arrays. Like an Array, we have a zero-indexed collection, and we can also see how many items are in the Map by default. Maps use the => syntax to signify key/value pairs as key => value :

This example looks similar to a regular object with string-based keys, but we can use any data type as a key with Map.

In addition to manually setting values on a Map, we can also initialize a Map with values already. We do this using an Array of Arrays containing two elements that are each key/value pairs, which looks like this:

Using the following syntax, we can recreate the same Map:

Incidentally, this syntax is the same as the result of calling Object.entries() on an Object. This provides a ready-made way to convert an Object to a Map, as shown in the following code block:

Alternatively, you can turn a Map back into an Object or an Array with a single line of code.

The following converts a Map to an Object:

This will result in the following value of obj :

Now, let's convert a Map to an Array:

This will result in the following Array for arr :

Maps accept any data type as a key, and do not allow duplicate key values. We can demonstrate this by creating a map and using non-string values as keys, as well as setting two values to the same key.

First, let's initialize a map with non-string keys:

This example will override the first key of 1 with the subsequent one, and it will treat '1' the string and 1 the number as unique keys:

Although it is a common belief that a regular JavaScript Object can already handle Numbers, booleans, and other primitive data types as keys, this is actually not the case, because Objects change all keys to strings.

As an example, initialize an object with a numerical key and compare the value for a numerical 1 key and a stringified "1" key:

This is why if you attempt to use an Object as a key, it will print out the string object Object instead.

As an example, create an Object and then use it as the key of another Object:

This will yield the following:

This is not the case with Map. Try creating an Object and setting it as the key of a Map:

The key of the Map element is now the object we created.

There is one important thing to note about using an Object or Array as a key: the Map is using the reference to the Object to compare equality, not the literal value of the Object. In JavaScript {} === {} returns false , because the two Objects are not the same two Objects, despite having the same (empty) value.

That means that adding two unique Objects with the same value will create a Map with two entries:

But using the same Object reference twice will create a Map with one entry.

Which will result in the following:

The second set() is updating the same exact key as the first, so we end up with a Map that only has one value.

Getting and Deleting Items from a Map

One of the disadvantages of working with Objects is that it can be difficult to enumerate them, or work with all the keys or values. The Map structure, by contrast, has a lot of built-in properties that make working with their elements more direct.

We can initialize a new Map to demonstrate the following methods and properties: delete() , has() , get() , and size .

Use the has() method to check for the existence of an item in a map. has() will return a Boolean.

Use the get() method to retrieve a value by key.

One particular benefit Maps have over Objects is that you can find the size of the Map at any time, like you can with an Array. You can get the count of items in a Map with the size property. This involves fewer steps than converting an Object to an Array to find the length.

Use the delete() method to remove an item from a Map by key. The method will return a Boolean— true if an item existed and was deleted, and false if it did not match any item.

This will result in the following Map:

Finally, a Map can be cleared of all values with map.clear() .

This will yield:

Keys, Values, and Entries for Maps

Objects can retrieve keys, values, and entries by using the properties of the Object constructor. Maps, on the other hand, have prototype methods that allow us to get the keys, values, and entries of the Map instance directly.

The keys() , values() , and entries() methods all return a MapIterator , which is similar to an Array in that you can use for...of to loop through the values.

Here is another example of a Map, which we can use to demonstrate these methods.

The keys() method returns the keys:

The values() method returns the values:

The entries() method returns an array of key/value pairs:

Iteration with Map

Map has a built-in forEach method, similar to an Array, for built-in iteration. However, there is a bit of a difference in what they iterate over. The callback of a Map's forEach iterates through the value , key , and map itself, while the Array version iterates through the item , index , and array itself.

This is a big advantage for Maps over Objects, as Objects need to be converted with keys() , values() , or entries() , and there is not an simple way to retrieve the properties of an Object without converting it.

To demonstrate this, let's iterate through our Map and log the key/value pairs to the console:

This will give:

Since a for...of loop iterates over iterables like Map and Array, we can get the exact same result by destructuring the array of Map items:

Map Properties and Methods

The following table shows a list of Map properties and methods for quick reference:

When to Use Map

Summing up, Maps are similar to Objects in that they hold key/value pairs, but Maps have several advantages over objects:

  • Size - Maps have a size property, whereas Objects do not have a built-in way to retrieve their size.
  • Iteration - Maps are directly iterable, whereas Objects are not.
  • Flexibility - Maps can have any data type (primitive or Object) as the key to a value, while Objects can only have strings.
  • Ordered - Maps retain their insertion order, whereas objects do not have a guaranteed order.

Due to these factors, Maps are a powerful data structure to consider. However, Objects haves some important advantages as well:

  • JSON - Objects work flawlessly with JSON.parse() and JSON.stringify() , two essential functions for working with JSON , a common data format that many REST APIs deal with.
  • Working with a single element - Working with a known value in an Object, you can access it directly with the key without the need to use a method, such as Map's get() .

This list will help you decide if a Map or Object is the right data structure for your use case.

A Set is a collection of unique values. Unlike a Map, a Set is conceptually more similar to an Array than an Object, since it is a list of values and not key/value pairs. However, Set is not a replacement for Arrays, but rather a supplement for providing additional support for working with duplicated data.

You can initialize Sets with the new Set() syntax.

This gives us an empty Set:

Items can be added to a Set with the add() method. (This is not to be confused with the set() method available to Map, although they are similar.)

Since Sets can only contain unique values, any attempt to add a value that already exists will be ignored.

Note : The same equality comparison that applies to Map keys applies to Set items. Two objects that have the same value but do not share the same reference will not be considered equal.

You can also initialize Sets with an Array of values. If there are duplicate values in the array, they will be removed from the Set.

Conversely, a Set can be converted into an Array with one line of code:

Set has many of the same methods and properties as Map, including delete() , has() , clear() , and size .

Note that Set does not have a way to access a value by a key or index, like Map.get(key) or arr[index] .

Keys, Values, and Entries for Sets

Map and Set both have keys() , values() , and entries() methods that return an Iterator. However, while each one of these methods have a distinct purpose in Map, Sets do not have keys, and therefore keys are an alias for values. This means that keys() and values() will both return the same Iterator, and entries() will return the value twice. It makes the most sense to only use values() with Set, as the other two methods exist for consistency and cross-compatibility with Map.

Iteration with Set

Like Map, Set has a built-in forEach() method. Since Sets don't have keys, the first and second parameter of the forEach() callback return the same value, so there is no use case for it outside of compatibility with Map. The parameters of forEach() are (value, key, set) .

Both forEach() and for...of can be used on Set. First, let's look at forEach() iteration:

Then we can write the for...of version:

Both of these strategies will yield the following:

Set Properties and Methods

The following table shows a list of Set properties and methods for quick reference:

When to Use Set

Set is a useful addition to your JavaScript toolkit, particularly for working with duplicate values in data.

In a single line, we can create a new Array without duplicate values from an Array that has duplicate values.

Set can be used for finding the union, intersection, and difference between two sets of data. However, Arrays have a significant advantage over Sets for additional manipulation of the data due to the sort() , map() , filter() , and reduce() methods, as well as direct compatibility with JSON methods.

In this article, you learned that a Map is a collection of ordered key/value pairs, and that a Set is a collection of unique values. Both of these data structures add additional capabilities to JavaScript and simplify common tasks such as finding the length of a key/value pair collection and removing duplicate items from a data set, respectively. On the other hand, Objects and Arrays have been traditionally used for data storage and manipulation in JavaScript, and have direct compatibility with JSON, which continues to make them the most essential data structures, especially for working with REST APIs. Maps and Sets are primarily useful as supporting data structures for Objects and Arrays.

If you would like to learn more about JavaScript, check out the homepage for our How To Code in JavaScript series , or browse our How to Code in Node.js series for articles on back-end development.

How to add a Key/Value pair to a Map in JavaScript

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Last updated: Jan 6, 2023 Reading time · 3 min

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# Add a Key/Value pair to a Map object in JavaScript

Use the set() method to add a key-value pair to a Map , e.g. map.set('myKey', 'myValue') .

The set() method adds or updates the element with the provided key and value and returns the Map object.

add key value pair to map object

We used the Map.set() method to add key-value pairs to a Map object.

The method takes 2 parameters:

  • the key of the element
  • the value of the element

If the key already exists in the Map object, its value is updated.

We called the set() method with an existing key ( name ), so the value of the key got updated.

You can also pass an array of key-value pairs to the Map constructor to initialize the Map with values.

The Map() constructor can be called with an array of key-value pairs.

If you need to add the key-value pairs of an object to a Map, use the Object.entries() method.

The Object.entries() method returns an array of key-value pairs.

# Chaining calls to the Map.set() method

The Map.set() method returns the Map object. This allows us to chain multiple calls to the set() method.

chaining calls to map set

On each call to the set() method, the Map object is returned, so we can chain multiple calls to the method.

# Conditionally adding key-value pairs to a Map object

You can add key-value pairs to a Map object conditionally by using the Map.has() method.

conditionally adding key value pairs to map object

We used the Map.has() method to check if the name key is contained in the Map object before setting a value for the key.

The Map.has() method returns true if the specified key is contained in the Map and false otherwise.

# The keys and values of a Map can be of any type

Note that the keys and values of a Map can be of any type, e.g. an object.

the keys and values of map can be of any type

The code sample uses an object as a key in the Map .

I've also written an article on how to add a key-value pair to an object .

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

  • How to Add a Key/Value pair to an Object in JavaScript
  • Add a Key/Value pair to all Objects in Array in JavaScript
  • Check if all Object Properties are Null in JavaScript

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Implement a Map with multiple keys in JavaScript

This post will discuss how to implement a Map with multiple keys in JavaScript.

There are several ways to implement a Map with multiple keys in JavaScript, which is a data structure that allows us to map multiple keys to a single value. Here are some possible functions:

1. Using a nested map

One option to implement a map with multiple keys is to use a nested map object, where each level of the map represents a key and the last level of the map contains the value. This creates a map of maps, where each key is mapped to another map that contains the next key and the value. This method works well for a fixed number of keys, but it requires a lot of nesting and accessing. The following code illustrates this:

Download    Run Code

2. Using an array as a key

Another option is to create an array of keys and uses it as a single key for the map. This method works well for any number of keys, but it requires a custom hash function to use arrays as keys in a Map in JavaScript . The following code illustrates this:

3. Using a third-party library

A better option is to use an existing library that provides a multikeymap implementation for JavaScript. For example, we can use the multikeymap-js library , which allows us to create and manipulate multikeymaps with various functions and options. Here, an entry is keyed by an array of values. This library does not place restrictions on the order of keys to set and get. The following code example demonstrates its usage:

Download Code

4. Extending Map class

This function creates a custom class that inherits from the built-in Map class and overrides some of its functions to support multiple keys. The custom class can use an internal Map object to store the key-value pairs, and use a hashing function to generate a unique hash for each combination of keys. The hashing function can be any function that returns a string or a number that is unique for each set of keys, regardless of their order. The custom class can then use the hashing function to create and retrieve the key-value pairs from the internal Map object. For example, one possible implementation of the custom class could be:

That’s all about implementing a Map with multiple keys in JavaScript.

Implement a multikey map in JavaScript
Extract keys and values from a Map in JavaScript
Conditionally remove keys from a map in JavaScript

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Create a form in Word that users can complete or print

In Word, you can create a form that others can fill out and save or print.  To do this, you will start with baseline content in a document, potentially via a form template.  Then you can add content controls for elements such as check boxes, text boxes, date pickers, and drop-down lists. Optionally, these content controls can be linked to database information.  Following are the recommended action steps in sequence.  

Show the Developer tab

In Word, be sure you have the Developer tab displayed in the ribbon.  (See how here:  Show the developer tab .)

Open a template or a blank document on which to base the form

You can start with a template or just start from scratch with a blank document.

Start with a form template

Go to File > New .

In the  Search for online templates  field, type  Forms or the kind of form you want. Then press Enter .

In the displayed results, right-click any item, then select  Create. 

Start with a blank document 

Select Blank document .

Add content to the form

Go to the  Developer  tab Controls section where you can choose controls to add to your document or form. Hover over any icon therein to see what control type it represents. The various control types are described below. You can set properties on a control once it has been inserted.

To delete a content control, right-click it, then select Remove content control  in the pop-up menu. 

Note:  You can print a form that was created via content controls. However, the boxes around the content controls will not print.

Insert a text control

The rich text content control enables users to format text (e.g., bold, italic) and type multiple paragraphs. To limit these capabilities, use the plain text content control . 

Click or tap where you want to insert the control.

Rich text control button

To learn about setting specific properties on these controls, see Set or change properties for content controls .

Insert a picture control

A picture control is most often used for templates, but you can also add a picture control to a form.

Picture control button

Insert a building block control

Use a building block control  when you want users to choose a specific block of text. These are helpful when you need to add different boilerplate text depending on the document's specific purpose. You can create rich text content controls for each version of the boilerplate text, and then use a building block control as the container for the rich text content controls.

building block gallery control

Select Developer and content controls for the building block.

Developer tab showing content controls

Insert a combo box or a drop-down list

In a combo box, users can select from a list of choices that you provide or they can type in their own information. In a drop-down list, users can only select from the list of choices.

combo box button

Select the content control, and then select Properties .

To create a list of choices, select Add under Drop-Down List Properties .

Type a choice in Display Name , such as Yes , No , or Maybe .

Repeat this step until all of the choices are in the drop-down list.

Fill in any other properties that you want.

Note:  If you select the Contents cannot be edited check box, users won’t be able to click a choice.

Insert a date picker

Click or tap where you want to insert the date picker control.

Date picker button

Insert a check box

Click or tap where you want to insert the check box control.

Check box button

Use the legacy form controls

Legacy form controls are for compatibility with older versions of Word and consist of legacy form and Active X controls.

Click or tap where you want to insert a legacy control.

Legacy control button

Select the Legacy Form control or Active X Control that you want to include.

Set or change properties for content controls

Each content control has properties that you can set or change. For example, the Date Picker control offers options for the format you want to use to display the date.

Select the content control that you want to change.

Go to Developer > Properties .

Controls Properties  button

Change the properties that you want.

Add protection to a form

If you want to limit how much others can edit or format a form, use the Restrict Editing command:

Open the form that you want to lock or protect.

Select Developer > Restrict Editing .

Restrict editing button

After selecting restrictions, select Yes, Start Enforcing Protection .

Restrict editing panel

Advanced Tip:

If you want to protect only parts of the document, separate the document into sections and only protect the sections you want.

To do this, choose Select Sections in the Restrict Editing panel. For more info on sections, see Insert a section break .

Sections selector on Resrict sections panel

If the developer tab isn't displayed in the ribbon, see Show the Developer tab .

Open a template or use a blank document

To create a form in Word that others can fill out, start with a template or document and add content controls. Content controls include things like check boxes, text boxes, and drop-down lists. If you’re familiar with databases, these content controls can even be linked to data.

Go to File > New from Template .

New from template option

In Search, type form .

Double-click the template you want to use.

Select File > Save As , and pick a location to save the form.

In Save As , type a file name and then select Save .

Start with a blank document

Go to File > New Document .

New document option

Go to File > Save As .

Go to Developer , and then choose the controls that you want to add to the document or form. To remove a content control, select the control and press Delete. You can set Options on controls once inserted. From Options, you can add entry and exit macros to run when users interact with the controls, as well as list items for combo boxes, .

Adding content controls to your form

In the document, click or tap where you want to add a content control.

On Developer , select Text Box , Check Box , or Combo Box .

Developer tab with content controls

To set specific properties for the control, select Options , and set .

Repeat steps 1 through 3 for each control that you want to add.

Set options

Options let you set common settings, as well as control specific settings. Select a control and then select Options to set up or make changes.

Set common properties.

Select Macro to Run on lets you choose a recorded or custom macro to run on Entry or Exit from the field.

Bookmark Set a unique name or bookmark for each control.

Calculate on exit This forces Word to run or refresh any calculations, such as total price when the user exits the field.

Add Help Text Give hints or instructions for each field.

OK Saves settings and exits the panel.

Cancel Forgets changes and exits the panel.

Set specific properties for a Text box

Type Select form Regular text, Number, Date, Current Date, Current Time, or Calculation.

Default text sets optional instructional text that's displayed in the text box before the user types in the field. Set Text box enabled to allow the user to enter text into the field.

Maximum length sets the length of text that a user can enter. The default is Unlimited .

Text format can set whether text automatically formats to Uppercase , Lowercase , First capital, or Title case .

Text box enabled Lets the user enter text into a field. If there is default text, user text replaces it.

Set specific properties for a Check box .

Default Value Choose between Not checked or checked as default.

Checkbox size Set a size Exactly or Auto to change size as needed.

Check box enabled Lets the user check or clear the text box.

Set specific properties for a Combo box

Drop-down item Type in strings for the list box items. Press + or Enter to add an item to the list.

Items in drop-down list Shows your current list. Select an item and use the up or down arrows to change the order, Press - to remove a selected item.

Drop-down enabled Lets the user open the combo box and make selections.

Protect the form

Go to Developer > Protect Form .

Protect form button on the Developer tab

Note:  To unprotect the form and continue editing, select Protect Form again.

Save and close the form.

Test the form (optional)

If you want, you can test the form before you distribute it.

Protect the form.

Reopen the form, fill it out as the user would, and then save a copy.

Creating fillable forms isn’t available in Word for the web.

You can create the form with the desktop version of Word with the instructions in Create a fillable form .

When you save the document and reopen it in Word for the web, you’ll see the changes you made.

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IMAGES

  1. JavaScript Map

    map keys and values javascript

  2. JavaScript Map keys() Method

    map keys and values javascript

  3. Javascript Map object iterating over keys & values

    map keys and values javascript

  4. How to Initialize a Map with Values in JavaScript

    map keys and values javascript

  5. 34 Javascript Loop Map Key Value

    map keys and values javascript

  6. JavaScript Map Data Structure with Examples

    map keys and values javascript

COMMENTS

  1. Map

    Description Map objects are collections of key-value pairs. A key in the Map may only occur once; it is unique in the Map 's collection. A Map object is iterated by key-value pairs — a for...of loop returns a 2-member array of [key, value] for each iteration.

  2. Object.keys, values, entries

    Map Set Array Plain objects also support similar methods, but the syntax is a bit different. Object.keys, values, entries For plain objects, the following methods are available: Object.keys (obj) - returns an array of keys. Object.values (obj) - returns an array of values. Object.entries (obj) - returns an array of [key, value] pairs.

  3. JavaScript Maps

    A Map holds key-value pairs where the keys can be any datatype. A Map remembers the original insertion order of the keys. Essential Map Methods How to Create a Map You can create a JavaScript Map by: Passing an Array to new Map () Create a Map and use Map.set () The new Map () Method

  4. The Essential Guide to JavaScript Map: How To Use Maps Effectively

    Keys and values of a Map can be any values. When iterating a Map object, each iteration returns a 2-member array of [key, value]. The iteration order follows the insertion order which corresponds to the order in which each key-value pair was first inserted into the Map by the set () method. To create a new Map, you use the following syntax:

  5. Map and Set

    Data types November 14, 2022 Map and Set Till now, we've learned about the following complex data structures: Objects are used for storing keyed collections. Arrays are used for storing ordered collections. But that's not enough for real life. That's why Map and Set also exist. Map Map is a collection of keyed data items, just like an Object.

  6. Understanding Map and Set Objects in JavaScript

    Map(0) {} Keys, Values, and Entries for Maps. Objects can retrieve keys, values, and entries by using the properties of the Object constructor. Maps, on the other hand, have prototype methods that allow us to get the keys, values, and entries of the Map instance directly.

  7. How to create map with key and a list of values in JavaScript

    1 In Java we can create private map<String, List<String>> map = HashMap<String, ArrayList<String> (); How to create a map with keys and lists of values in JavaScript? I want to put key as country name and list of values are country states. How to do in JavaScript? javascript dictionary Share Improve this question Follow edited Sep 18, 2014 at 6:26

  8. JavaScript Map keys() Method

    Javascript let mp = new Map () mp.set ("a", 1); mp.set ("b", 2); mp.set ("c", 22); mp.set ("d", 12); console.log ("Type of mp.keys () is: ", typeof (mp.keys ())); console.log ("Keys in map mp are: ", mp.keys ()); Output: Type of mp.keys () is: object Keys in map mp are: MapIterator {'a', 'b', 'c', 'd'}

  9. Construct a map from keys and values in JavaScript

    To construct a map from arrays containing keys and values in JavaScript, we can use the Map () constructor that takes an iterable object of key-value pairs as an argument and creates a new map object with the elements of the iterable. For example, if we have an array of keys and an array of values, we can use the zip () function from the Lodash ...

  10. JavaScript Maps

    Sets the value for a key in a Map: get() Gets the value for a key in a Map: clear() Removes all the elements from a Map: delete() Removes a Map element specified by a key: has() Returns true if a key exists in a Map: forEach() Invokes a callback for each key/value pair in a Map: entries() Returns an iterator object with the [key, value] pairs ...

  11. Introduction to Maps in JavaScript

    Maps are very similar to JavaScript objects. When you want to store some data in maps, you store those data also in the form of a key-value pair. Also just like with objects, you can add new keys and delete existing, and retrieve their values from maps. When you compare maps and objects, there are some differences you should know about.

  12. Understanding Map and Set in JavaScript

    When to Use Map. Summing up, Maps are similar to Objects in that they hold key/value pairs, but Maps have several advantages over objects: Size - Maps have a size property, whereas Objects do not have a built-in way to retrieve their size.; Iteration - Maps are directly iterable, whereas Objects are not.; Flexibility - Maps can have any data type (primitive or Object) as the key to a value ...

  13. How to add a Key/Value pair to a Map in JavaScript

    We used the Map.set () method to add key-value pairs to a Map object. If the key already exists in the Map object, its value is updated. We called the set () method with an existing key ( name ), so the value of the key got updated. You can also pass an array of key-value pairs to the Map constructor to initialize the Map with values.

  14. Extract keys and values from a Map in JavaScript

    There are several methods to get an array of map keys and values in JavaScript. A map is an object that holds key-value pairs and remembers the original insertion order of the keys. Here are some of the methods that we can use to extract keys and values from a Map: 1. Using Array.from () function

  15. Implement a Map with multiple keys in JavaScript

    Here are some possible functions: 1. Using a nested map. One option to implement a map with multiple keys is to use a nested map object, where each level of the map represents a key and the last level of the map contains the value. This creates a map of maps, where each key is mapped to another map that contains the next key and the value.

  16. Converting a List of Maps to a Map Grouped by Key in Kotlin

    This code representation shows that the variable listOfMaps is a list that contains maps with a key-value pair. However, we want to combine all these maps into a single map that groups entries by keys to end up with expectedMap. Therefore, this new Map uses all the keys from the original listOfMaps and associates each key to a List of values.

  17. Create a form in Word that users can complete or print

    Show the Developer tab. If the developer tab isn't displayed in the ribbon, see Show the Developer tab.. Open a template or use a blank document. To create a form in Word that others can fill out, start with a template or document and add content controls.

  18. How to use .map () over Map keys in Javascript

    6 Answers Sorted by: 6 Map.keys () returns an iterator you can spread the iterator using spread syntax const map = new Map (); map.set (0, 'Zero'); map.set (1, 'One'); map.set (2, 'Two'); [...map.keys ()].forEach (key => { console.log (key); }) Share Follow edited Aug 25, 2019 at 9:59 answered Aug 25, 2019 at 3:18 Code Maniac 37.5k 5 39 61 2

  19. Russian landing ship Caesar Kunikov sunk off Crimea, says Ukraine

    A big Russian amphibious ship, the Caesar Kunikov, has been sunk off the coast of Russian-occupied Crimea, according to Ukraine's armed forces. Powerful explosions were heard early on Wednesday ...