callbackFn

A function to execute for each element in the array. Its return value is added as a single element in the new array. The function is called with the following arguments:

element

The current element being processed in the array.

index

The index of the current element being processed in the array.

array

The arraymap() was called upon.

thisArgOptional

A value to use asthis when executingcallbackFn. Seeiterative methods.

A new array with each element being the result of the callback function.

Themap() method is aniterative method. It calls a providedcallbackFn function once for each element in an array and constructs a new array from the results. Read theiterative methods section for more information about how these methods work in general.

callbackFn is invoked only for array indexes which have assigned values. It is not invoked for empty slots insparse arrays.

Themap() method isgeneric. It only expects thethis value to have alength property and integer-keyed properties.

Sincemap builds a new array, calling it without using the returned array is an anti-pattern; useforEach orfor...of instead.

The following code takes an array of numbers and creates a new array containing the square roots of the numbers in the first array.

const numbers = [1, 4, 9];const roots = numbers.map((num) => Math.sqrt(num));// roots is now     [1, 2, 3]// numbers is still [1, 4, 9]

The following code takes an array of objects and creates a new array containing the newly reformatted objects.

const kvArray = [  { key: 1, value: 10 },  { key: 2, value: 20 },  { key: 3, value: 30 },];const reformattedArray = kvArray.map(({ key, value }) => ({ [key]: value }));console.log(reformattedArray); // [{ 1: 10 }, { 2: 20 }, { 3: 30 }]console.log(kvArray);// [//   { key: 1, value: 10 },//   { key: 2, value: 20 },//   { key: 3, value: 30 }// ]

It is common to use the callback with one argument (the element being traversed). Certain functions are also commonly used with one argument, even though they take additional optional arguments. These habits may lead to confusing behaviors. Consider:

["1", "2", "3"].map(parseInt);

While one might expect[1, 2, 3], the actual result is[1, NaN, NaN].

parseInt is often used with one argument, but takes two. The first is an expression and the second is the radix to the callback function,Array.prototype.map passes 3 arguments: the element, the index, and the array. The third argument is ignored byparseInt — butnot the second one! This is the source of possible confusion.

Here is a concise example of the iteration steps:

/* first iteration  (index is 0): */ parseInt("1", 0); // 1/* second iteration (index is 1): */ parseInt("2", 1); // NaN/* third iteration  (index is 2): */ parseInt("3", 2); // NaN

To solve this, define another function that only takes one argument:

["1", "2", "3"].map((str) => parseInt(str, 10)); // [1, 2, 3]

You can also use theNumber function, which only takes one argument:

["1", "2", "3"].map(Number); // [1, 2, 3]// But unlike parseInt(), Number() will also return a float or (resolved) exponential notation:["1.1", "2.2e2", "3e300"].map(Number); // [1.1, 220, 3e+300]// For comparison, if we use parseInt() on the array above:["1.1", "2.2e2", "3e300"].map((str) => parseInt(str, 10)); // [1, 2, 3]

SeeA JavaScript optional argument hazard by Allen Wirfs-Brock for more discussions.

Whenundefined or nothing is returned, the resulting array containsundefined. If you want to delete the element instead, chain afilter() method, or use theflatMap() method and return an empty array to signify deletion.

const numbers = [1, 2, 3, 4];const filteredNumbers = numbers.map((num, index) => {  if (index < 3) {    return num;  }});// index goes from 0, so the filterNumbers are 1,2,3 and undefined.// filteredNumbers is [1, 2, 3, undefined]// numbers is still [1, 2, 3, 4]

The callback can have side effects.

const cart = [5, 15, 25];let total = 0;const withTax = cart.map((cost) => {  total += cost;  return cost * 1.2;});console.log(withTax); // [6, 18, 30]console.log(total); // 45

This is not recommended, because copying methods are best used with pure functions. In this case, we can choose to iterate the array twice.

const cart = [5, 15, 25];const total = cart.reduce((acc, cost) => acc + cost, 0);const withTax = cart.map((cost) => cost * 1.2);

Sometimes this pattern goes to its extreme and theonly useful thing thatmap() does is causing side effects.

const products = [  { name: "sports car" },  { name: "laptop" },  { name: "phone" },];products.map((product) => {  product.price = 100;});

As mentioned previously, this is an anti-pattern. If you don't use the return value ofmap(), useforEach() or afor...of loop instead.

products.forEach((product) => {  product.price = 100;});

Or, if you want to create a new array instead:

const productsWithPrice = products.map((product) => ({  ...product,  price: 100,}));

Thearray argument is useful if you want to access another element in the array, especially when you don't have an existing variable that refers to the array. The following example first usesfilter() to extract the positive values and then usesmap() to create a new array where each element is the average of its neighbors and itself.

const numbers = [3, -1, 1, 4, 1, 5, 9, 2, 6];const averaged = numbers  .filter((num) => num > 0)  .map((num, idx, arr) => {    // Without the arr argument, there's no way to easily access the    // intermediate array without saving it to a variable.    const prev = arr[idx - 1];    const next = arr[idx + 1];    let count = 1;    let total = num;    if (prev !== undefined) {      count++;      total += prev;    }    if (next !== undefined) {      count++;      total += next;    }    const average = total / count;    // Keep two decimal places    return Math.round(average * 100) / 100;  });console.log(averaged); // [2, 2.67, 2, 3.33, 5, 5.33, 5.67, 4]

Thearray argument isnot the array that is being built — there is no way to access the array being built from the callback function.

A sparse array remains sparse aftermap(). The indices of empty slots are still empty in the returned array, and the callback function won't be called on them.

console.log(  [1, , 3].map((x, index) => {    console.log(`Visit ${index}`);    return x * 2;  }),);// Visit 0// Visit 2// [2, empty, 6]

Themap() method reads thelength property ofthis and then accesses each property whose key is a nonnegative integer less thanlength.

const arrayLike = {  length: 3,  0: 2,  1: 3,  2: 4,  3: 5, // ignored by map() since length is 3};console.log(Array.prototype.map.call(arrayLike, (x) => x ** 2));// [ 4, 9, 16 ]

This example shows how to iterate through a collection of objects collected byquerySelectorAll. This is becausequerySelectorAll returns aNodeList (which is a collection of objects). In this case, we return all the selectedoptions' values on the screen:

const elems = document.querySelectorAll("select option:checked");const values = Array.prototype.map.call(elems, ({ value }) => value);

You can also useArray.from() to transformelems to an array, and then access themap() method.

• Polyfill ofArray.prototype.map incore-js
• es-shims polyfill ofArray.prototype.map
• Indexed collections guide
• Array
• Array.prototype.forEach()
• Array.from()
• TypedArray.prototype.map()
• Map