How to Use forEach() to Iterate Through a JavaScript Map

JavaScript's Map object has a handy function, forEach() , which operates similarly to arrays' forEach() function . JavaScript calls the forEach() callback with 3 parameters: the value, the key, and the map itself.

Map#entries()

JavaScript maps don't have chainable helpers like filter() or map() for arrays. If you want to use filter() with a map, you should use Map#entries() to first convert the map to an iterator, and then use the the spread operator or the Array.from() function to convert the iterator to an array.

Map#keys() and Map#values()

If you only need the keys or the values of the Map , you can use Map#keys() or Map#values() . Map#keys() returns an iterator over the map's keys, and Map#values() returns an iterator over the map's values. Make sure you convert the iterator to an array using the spread operator or Array.from() if you want to use filter() or map() !

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How to iterate through a Map object in JavaScript

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Last updated: Mar 3, 2024 Reading time · 4 min

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# Table of Contents

  • Iterate through a Map object in JavaScript
  • Iterate over a Map in Reverse Order

# Iterate through a Map object in JavaScript

Use the Map.forEach() method to iterate through a Map object.

The forEach method takes a function that gets invoked for each key-value pair in the Map in insertion order.

iterate through map object

The function we passed to the Map.forEach() method gets called for each key-value pair in the Map object.

The function gets passed 3 arguments on each iteration:

  • the value of the current iteration
  • the key of the current iteration
  • the Map object that is being iterated

The Map.forEach() method returns undefined .

# Iterate through a Map object using a for...of loop

Alternatively, you can use a for...of loop.

The for...of loop assigns the key and the value of the current iteration to variables.

iterate through map object using for of

The for...of statement is used to loop over iterable objects like arrays, strings, Map , Set and NodeList objects and generators .

We used destructuring assignment to assign the key and value variables.

The for...of loop might be your preferred approach if you have to use the break keyword to exit the loop prematurely.

Using the break keyword is not supported in the forEach() method.

# Iterating over the Keys or Values of a Map object

This is a two-step process:

  • Use the Map.keys() method to get an iterator object of the Map's keys.
  • Use the Map.values() method to get an iterator object of the Map's values.
  • Use a for...of loop to iterate over the keys or values.

iterating over keys or values of map object

If you need to iterate over the Map object's values, use the Map.values() method.

The Map.keys method returns an iterator object of the Map's keys.

The Map.values() method returns an iterator object of the Map's values.

The for...of loop is useful when you need to exit the loop prematurely if a certain condition is met.

If the condition in the if statement is met, the break statement exits the for...of loop.

Note that the return values of the Map.keys() and Map.values() methods are not arrays, they are iterator objects.

You can use the Array.from method if you want to convert the Map's values or keys to an array, e.g. to be able to use the forEach method.

You can also use the spread syntax (...) to unpack the values from the iterator object into an array.

The last 2 examples achieve the same result. They both convert the iterator objects to arrays.

# Iterate over a Map in Reverse Order

To iterate over a Map in reverse order:

  • Use the Array.from() method to convert the Map to an array.
  • Use the reverse() method to reverse the array.
  • Use the forEach() method to iterate over the reversed array.

iterate over map in reverse order

The first step is to convert the Map to an array.

We used the Array.reverse() method to reverse the array.

You can use the Array.forEach method to iterate over the reversed array.

The function we passed to the forEach method gets called with each element of the array.

You can also use the spread syntax (...) to convert the Map to an array .

This code sample achieves the same result, however, this time we used the spread syntax (...) to unpack the key-value pairs of the Map into an array.

In some very rare cases, the spread syntax (...) doesn't play nice when using TypeScript. This issue doesn't occur when using Array.from() .

# Additional Resources

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

  • How to sort a Map in JavaScript
  • Get the Sum of all Values in a Map using JavaScript
  • How to update a Value in a Map in JavaScript
  • How to merge Maps in JavaScript or TypeScript
  • How to Initialize a Map with Values in JavaScript
  • Increment a Value in an Object or a Map in JavaScript
  • How to Zip two or more Arrays in JavaScript - Complete Guide
  • How to iterate over a FileList in JavaScript [5 Ways]
  • Resource interpreted as stylesheet but transferred with MIME type text/html
  • Iterate over all keys stored in localStorage in JavaScript

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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.

Delete all elements in the map

To delete all entries in the Map object, you use the clear() method:

Hence, the size of the map now is zero.

A WeakMap is similar to a Map except for the keys of a WeakMap must be objects. It means that when a reference to a key (an object) is out of scope, the corresponding value is automatically released from the memory.

A WeakMap only has subset methods of a Map object:

  •   get(key)
  •   set(key, value)
  •   has(key)
  •   delete(key)

Here are the main differences between a Map and a WeekMap :

  • Elements of a WeakMap cannot be iterated.
  • Cannot clear all elements at once.
  • Cannot check the size of a WeakMap.

In this tutorial, you have learned how to work with the JavaScript Map object and its useful methods to manipulate entries in the map.

JavaScript Map forEach() method explained

The code below shows how the forEach() method works:

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  • Map.prototype.forEach()

Return value

Printing the contents of a map object.

The forEach() method executes a provided function once per each key/value pair in the Map object, in insertion order.

undefined .

Description

The forEach method executes the provided callback  once for each key of the map which actually exist. It is not invoked for keys which have been deleted. However, it is executed for values which are present but have the value undefined .

callback is invoked with three arguments :

  • the element value
  • the element key
  • the Map object being traversed

If a thisArg parameter is provided to forEach , it will be passed to callback when invoked, for use as its this value.  Otherwise, the value undefined will be passed for use as its this value.  The this value ultimately observable by callback is determined according to the usual rules for determining the this seen by a function .

Each value is visited once, except in the case when it was deleted and re-added before forEach has finished. callback is not invoked for values deleted before being visited. New values added before forEach has finished will be visited.

forEach executes the callback function once for each element in the Map object; it does not return a value.

The following code logs a line for each element in an Map object:

Specifications

Specification Status Comment
Standard Initial definition.
Living Standard  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support 38 (Yes) (25.0) 11 25 7.1
Feature Android Chrome for Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support 38 (Yes) 25.0 (25.0) No support No support 8
  • Array.prototype.forEach()
  • Set.prototype.forEach()

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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How to iterate a Map in Javascript - Map Part 2

js . Srijan

Table of Contents

Introduction, iterating a map, iterating map with for..of, iterating map with foreach().

If you missed reading the first article in which we discussed how to do basic operations over a Map, you can read it here .

In this article, we will learn how to iterate over a Map using different methods available. So, let’s dive in and learn how to loop over a Map.

For iterating over a Map, we can use the following javascript constructs:

Let's create a Map first to loop over from the knowledge gained in our previous article .

We can use for..of to iterate a Map in Javascript.

Map also provides these three methods, which comes in handy while iterating over a Map.

  • map.keys() - Returns an iterable for keys
  • map.values() - Returns an iterable for values
  • map.entries() - Returns an iterable of [key,value]

We can also iterate through a Map using forEach().

Map preserves the order in which values are inserted. So, while iterating over a Map, elements will be in the same order in which they are inserted.
  • To iterate over a Map, we can use for..of and forEach() loop constructs.
  • Map provides three methods that return iterable: map.keys(), map.values() and map.entries() .
  • Iteration over Maps is always in insertion order .

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JS Reference

Html events, html objects, other references, javascript array map().

Return a new array with the square root of all element values:

Multiply all the values in an array with 10:

More examples below.

Description

map() creates a new array from calling a function for every array element.

map() does not execute the function for empty elements.

map() does not change the original array.

Array Iteration Methods:

The Array entries() Method

The Array every() Method

The Array filter() Method

The Array forEach() Method

The Array keys() Method

The Array map() Method

Parameter Description
Required.
A function to be run for each array element.
Required.
The value of the current element.
Optional.
The index of the current element.
Optional.
The array of the current element.
Optional.
Default value .
A value passed to the function to be used as its value.

Return Value

Type Description
An arrayThe results of a function for each array element.

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Browser Support

map() is an ECMAScript5 (ES5) feature.

ES5 (JavaScript 2009) is fully supported in all modern browsers since July 2013:

Chrome
23
IE/Edge
11
Firefox
21
Safari
6
Opera
15
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Difference Between for…in and Object.keys() in JavaScript

The for…in and Object.keys() in JavaScript are used to iterate over the properties of an object. While they might seem similar at first glance they have distinct usage, behavior, and characteristics. This article will explore these differences in detail.

These are the following topics that we are going to discuss:

Table of Content

What is for…in?

What is object.keys(), difference between for…in and object.keys().

The for…in statement is a looping construct that iterates over the enumerable properties of an object. It goes through each property name (key) in the object and allows to perform the actions with these properties.

Characteristics:

  • Iterates over all enumerable properties including the inherited properties.
  • Can iterate over properties added to the object via the prototype chain.
  • Useful for general property enumeration when dealing with both own and inherited properties.

Applications:

  • Iterating over all properties of an object.
  • Useful when we want to include the inherited properties in the iteration.

Example: In this example, the for…in loop iterates over the properties name and age of the person object as well as the gender property inherited from the Object.prototype.

The Object.keys() method returns an array of the given object’s own enumerable property names in the same order as provided by the for…in loop but without including the properties from the prototype chain.

  • Returns an array of the object’s own enumerable properties.
  • Does not include the inherited properties.
  • Useful for obtaining a list of the property names for an object’s own properties.
  • Extracting a list of the object’s own property names.
  • Avoiding the enumeration of the properties from the prototype chain.

Example: In this example Object.keys(person) returns an array containing only the name and age properties excluding the inherited gender property.

Characteristics

for…in

Object.keys()

Iterates over

All enumerable properties

Own enumerable properties only

Returns

The Property names as strings

The Array of property names

Includes inherited properties

Yes

No

Use case

When you need to iterate over all properties

When you need a list of the own property names

Understanding the differences between the for…in and Object.keys() is crucial for effectively working with the objects in JavaScript. While for…in is useful for the iterating over all enumerable properties Object.keys() provides a cleaner approach for retrieving only an object’s own properties. Choosing the right method depends on the specific use case and the properties we need to the work with.

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For..In loops in JavaScript - key value pairs

I was wondering if there's a way to do something like a PHP foreach loop in JavaScript. The functionality I'm looking for is something like this PHP Snippet:

I was looking at the JS for..in loop, but there seems to be no way to specify the as . If I do this with a 'normal' for loop ( for(var i = 0; i < data.length; i++ ), is there a way to grab the key => value pairs?

TTT's user avatar

19 Answers 19

hasOwnProperty is used to check if your target really has that property, rather than having inherited it from its prototype. A bit simpler would be:

It just checks that k is not a method (as if target is array you'll get a lot of methods alerted, e.g. indexOf , push , pop ,etc.)

Michael Levy's user avatar

  • 92 Another way to iterate only over "own" properties is Object.keys . Object.keys(target).forEach(function (key) { target[key]; }); . –  katspaugh Commented Aug 30, 2011 at 10:50
  • 5 not going to work if target is created using Object.create(null) , code should be changed target.hasOwnProperty(k) -> Object.prototype.hasOwnProperty.call(target,k) –  Azder Commented Sep 30, 2015 at 9:49
  • 1 why not to use variables given in question example? What here is k , target and property ? For me, non-javascripter this area of undefined :) –  Gediminas Šukys Commented Mar 30, 2017 at 13:24
  • 2 Object.keys(target).forEach((key) => { target[key]; }); for Angular –  askilondz Commented Aug 18, 2017 at 17:30
  • I would say that almost every time you see obj.hasOwnProperty(k) it should be rewritten as Object.prototype.hasOwnProperty.call(obj, k) . If you don't know whether or not an object has an own property k , then you probably also don't know for sure whether it has an own property named "hasOwnProperty" ; and if it does, you don't want that one, you want the one from Object.prototype . So IMO making hasOwnProperty a method at all was a design flaw in the language; nobody ever wants its behaviour to be overridden in practice. –  kaya3 Commented Jan 17, 2022 at 1:56

If you can use ES6 natively or with Babel (js compiler) then you could do the following:

const test = {a: 1, b: 2, c: 3}; for (const [key, value] of Object.entries(test)) { console.log(key, value); }

Which will print out this output:

The Object.entries() method returns an array of a given object's own enumerable property [key, value] pairs, in the same order as that provided by a for...in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well) .

  • Object.entries documentation
  • for...of documentation
  • Destructuring assignment documentation
  • Enumerability and ownership of properties documentation

Muhammad Dyas Yaskur's user avatar

  • 1 This works perfect, just wondering - compared to "for key in object and then get value by object[key]", which one gives better performance? –  lsheng Commented Jul 9, 2017 at 5:44
  • 2 In this specific case I would assume it's slower because of the Object.entries call. I didn't run any tests though. –  Francesco Casula Commented Jul 9, 2017 at 6:44
  • 7 this is the best answer to the question at hand, which asked about grabbing both key and value in the for loop. –  cipak Commented Jan 13, 2018 at 18:12
  • 4 The accepted answer should be updated since this actually answers the question, although it wasn't available at time of the question. –  Marc Qualie Commented May 13, 2018 at 20:44
  • 2 You might want to check this question: stackoverflow.com/questions/47213651/… which seems to indicate that a syntax of this type would be recommended: Object.keys(myObject).forEach(key => {... –  Will59 Commented Dec 21, 2019 at 0:09

No one has mentioned Object.keys so I'll mention it.

Afshin Moazami's user avatar

  • 3 Note: Not supported by IE8 and below. –  Edwin Stoteler Commented Dec 8, 2014 at 7:57
  • 2 At this point you should be using an ES5 shim. If you're living in the nice ES6 future use for of tc39wiki.calculist.org/es6/for-of –  goatslacker Commented Dec 11, 2014 at 9:15
  • 24 It is worth noting, that "There is no way to stop or break a forEach() loop other than by throwing an exception" –  rluks Commented Jan 3, 2016 at 16:43
  • Object.keys(obj).forEach((key) => { }); for Angular –  askilondz Commented Aug 18, 2017 at 17:29
  • It doesn't work on ES6 or I don't understand it. Felix has a better and more readable answer below: data.forEach(function(value, index) { console.log(index); // 0 , 1, 2... }); –  gtamborero Commented Jan 30, 2019 at 22:15

for...in will work for you.

In modern JavaScript you can also do this:

Paul's user avatar

The php syntax is just sugar.

JD.'s user avatar

  • This will also iterate on all inherited properties. To avoid this, use .hasOwnProperty(). –  LSerni Commented Sep 5, 2019 at 8:40

I assume you know that i is the key and that you can get the value via data[i] (and just want a shortcut for this).

ECMAScript5 introduced forEach [MDN] for arrays (it seems you have an array):

The MDN documentation provides a shim for browsers not supporting it.

Of course this does not work for objects, but you can create a similar function for them:

Since you tagged the question with jquery , jQuery provides $.each [docs] which loops over both, array and object structures.

Felix Kling's user avatar

  • That's Array forEach , not object forEach . –  user193476 Commented Aug 30, 2011 at 10:38
  • 2 So? Apparently the OP is looping over an array. –  Felix Kling Commented Aug 30, 2011 at 10:41
  • Also Mozilla (Firefox, SpiderMonkey-C, Rhino &c) has an non-standard extension that allows for each syntax. for each (let val in myObj) console.log(val); . –  katspaugh Commented Aug 30, 2011 at 10:43
  • 2 @katspaugh: Right, but as it is Mozilla only, it does not seem to be very useful. –  Felix Kling Commented Aug 30, 2011 at 10:45
  • Thanks a lot for your answer. I'll read over the information you provided. Your assumption at the start of the answer was right, I knew that, except I got so much on my head with this project that I can't focus and forgot about it.. Thank you. –  Joris Ooms Commented Aug 30, 2011 at 10:47

There are three options to deal with keys and values of an object:

Select values:

Select keys:

Select keys and values:

Tonatio's user avatar

In javascript, every object has a bunch of built-in key-value pairs that have meta-information. When you loop through all the key-value pairs for an object you're looping through them too. The use of hasOwnProperty() filters these out.

Siddhu's user avatar

You can use the for..in for that.

Christoph Winkler's user avatar

  • 6 You may add some explanation with the code you posted instead of posting a plain code which might be not understandable. –  AaoIi Commented Aug 17, 2018 at 12:30
  • 1 Object.entries pulls out an array of arrays based on the key/value pairs of the original object: [['a', 1],['b',2],['c',3]] . The forEach deconstructs each of the key/value arrays and sets the two variables to key and value , to be used as you want the in function - here output in console.log . –  Mark Swardstrom Commented Nov 18, 2019 at 22:16

In the last few year since this question was made, Javascript has added a few new features. One of them is the Object.Entries method.

Copied directly from MDN is the follow code snippet

David's user avatar

ES6 will provide Map.prototype.forEach(callback) which can be used like this

Stephen Murby's user avatar

  • 2 what's the parameter myMap for? –  phil294 Commented Mar 4, 2017 at 22:09
  • A map is not an object. They are completely separate things. –  Dakusan Commented Apr 15, 2017 at 6:02
  • 2 The forEach function does not contain the 'key' of the array but more the index of the element that you are currently iterating. –  Francis Malloch Commented Jul 25, 2017 at 3:26

You can use a 'for in' loop for this:

Richard Dalton's user avatar

Below is an example that gets as close as you get.

If you're using jQuery see: http://api.jquery.com/jQuery.each/

TjeerdJan's user avatar

If you are using Lodash , you can use _.forEach

Gil Epshtain's user avatar

why not simply this

gros chat's user avatar

  • 3 Why not include an educational explanation? The OP is wanting to grab the key and value from a 1-dimensional structure -- your answer is ignoring this requirement. –  mickmackusa Commented Jun 10, 2021 at 6:32

Please try the below code:

Nazmul Haque's user avatar

  • Why is array there if you aren't using it? How is your answer valuable versus the same advice from 5 years earlier? –  mickmackusa Commented Jun 10, 2021 at 6:35

yes, you can have associative arrays also in javascript:

Alex Pacurar's user avatar

  • @PaulPRO ... everything in javascript is a key-value pair (thus, an object is in fact an associative array of key-value pairs...) –  Alex Pacurar Commented Aug 30, 2011 at 10:59
  • 1 @AlexPacurar and assocative array has an order. An object is unordered. thats a big difference –  Raynos Commented Aug 30, 2011 at 13:40
  • @Raynos you may be right... it will be a great help to explain exactly how an object is unordered... given the above example, one would expect that the 'i' in the for loop to be [name, otherProperty, and finally date]... so in which situation the order of properties of an object will be mixed ? –  Alex Pacurar Commented Aug 30, 2011 at 14:14
  • @AlexPacurar the particular order in which it will loop over the object is browser specific. Some do it alphabetically, some do it order of definition, etc –  Raynos Commented Aug 30, 2011 at 14:58
  • 3 @Raynos: Are associative arrays necessarily ordered? I've often seen the term used more generally. For example, on the Associative array Wikipedia article . –  Jeremy Commented Aug 30, 2011 at 15:20

user278064's user avatar

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map keys foreach javascript

IMAGES

  1. JavaScript Map forEach() Method

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  2. 34 How To Use Foreach In Javascript

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  3. JavaScript Map forEach method example

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  4. JavaScript Map

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  5. How we can use forEach with Map and Set In JavaScript

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  6. La diferencia de usar forEach() y map() en javascript #es6

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VIDEO

  1. Promises in ForEach vs For-Of Loops vs Map 💡 #JavaScript #javascriptdeveloper #javascriptinterview

  2. JavaScript forEach vs .map #azbytes #coding #javascript

  3. Array Iteration Methods

  4. JAVASCRIPT Tutorial:Foreach , Map , Reduce , Filter| Web Development Tutorials (JAVASCRIPT ) PART-11

  5. JavaScript: Difference between map & forEach Methods

  6. JavaScript Tip: Use Map for Key-Value Pairs

COMMENTS

  1. javascript

    JSBench with the methods as per the following. These are the results from fastest to slowest: Map.prototype.forEach() for of loop through Map.prototype.keys() for of loop through Map.prototype[Symbol.iterator]() Create array from Map.prototype.keys() and then Array.prototype.forEach() edited Jun 25, 2023 at 6:25. Chester Fung.

  2. Map.prototype.forEach ()

    The forEach() method of Map instances executes a provided function once per each key/value pair in this map, in insertion order.

  3. JavaScript Map forEach () Method

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

  4. javascript

    I have a Map() object that I need to iterate, so I can get the day of the week and a selected hour. The code below doesn't work, because Object.keys(newFieldReservationPrice).forEach is trying to l...

  5. How to Use forEach() to Iterate Through a JavaScript Map

    JavaScript maps have a `forEach()` function, as well as several helpers that let you iterate over a map's keys and values using `forEach()`. Here's what you need to know.

  6. How to iterate through a Map object in JavaScript

    The forEach method takes a function that gets invoked for each key-value pair in the Map in insertion order.

  7. Map

    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.

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

    This tutorial introduces you to the JavaScript Map object and shows you how to manipulate maps effectively.

  9. JavaScript Map forEach() method explained

    The forEach() method iterates over each key/value pair in your Map object in the same order the pair was inserted. The full syntax of the method is as shown below: forEach(function (value, key, map) { /* function body */ }, thisArg); The method will pass the 3 arguments to the callback function: value - The value of the current pair in the ...

  10. JavaScript Map forEach () Method

    JavaScript Map.forEach method is used to loop over the map with the given function and executes the given function over each key-value pair.

  11. How to iterate over Map elements in JavaScript

    Map () is very useful in JavaScript it organises elements into key-value pairs. This method iterates over each element in an array and applies a callback function to it, allowing for modifications before returning the updated array.". The Map () keyword is used to generate the object. The map () method loops through each object in the given ...

  12. Map.prototype.forEach ()

    The forEach () method executes a provided function once per each key/value pair in the Map object, in insertion order.

  13. Array.prototype.map ()

    The map() method of Array instances creates a new array populated with the results of calling a provided function on every element in the calling array.

  14. Map and Set

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

  15. How to get the Value by a Key in JavaScript Map?

    In this approach, iterate we will use forEach method to iterates over each key-value pair in the Map. It takes a callback function as an argument, which is called with each key and value in the Map. In this case, the callback function checks if the key matches the desired key and assigns the corresponding value to the value variable.

  16. How to map key/value pairs of a "map" in JavaScript?

    You can use Object.keys which will give an array of the keys present in that object , then use array map method on that array and access the value of each key and create a string

  17. How to iterate a Map in Javascript

    In this article, we will continue learning Map. We will learn how we can iterate a Map using different loop constructs in Javascript.

  18. Map.prototype.keys ()

    The keys() method of Map instances returns a new map iterator object that contains the keys for each element in this map in insertion order.

  19. JavaScript Array map () Method

    Learn how to use the map() method to create a new array with the results of calling a function on every element of an original array.

  20. Difference Between for…in and Object.keys() in JavaScript

    An object consists of key-value pairs where each key is a unique identifier associated with a corresponding value. Several methods can be used to count the number of keys/properties in an object, which are listed below: Table of Content Counting the Number of Keys using Object.keysCounting the Number of Keys using for-in loopCounting the Number of

  21. javascript

    Map.keys() returns a MapIterator object which can be converted to Array using Array.from:

  22. foreach

    In javascript, every object has a bunch of built-in key-value pairs that have meta-information. When you loop through all the key-value pairs for an object you're looping through them too.