Array.prototype.map()
The map()
method creates a new array with the results of calling a provided function on every element in this array.
Syntax
<var>arr</var>.map(<var>callback</var>[, <var>thisArg</var>])
Parameters
callback
- Function that produces an element of the new Array, taking three arguments:
currentValue
- The current element being processed in the array.
index
- The index of the current element being processed in the array.
array
- The array
map
was called upon.
thisArg
- Optional. Value to use as
this
when executingcallback
.
Return value
A new array with each element being the result of the callback function.
Description
map
calls a provided callback
function once for each element in an array, in order, and constructs a new array from the results. callback
is invoked only for indexes of the array which have assigned values, including undefined. It is not called for missing elements of the array (that is, indexes that have never been set, which have been deleted or which have never been assigned a value).
callback
is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.
If a thisArg
parameter is provided to map
, 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.
map
does not mutate the array on which it is called (although callback
, if invoked, may do so).
The range of elements processed by map
is set before the first invocation of callback
. Elements which are appended to the array after the call to map
begins will not be visited by callback
. If existing elements of the array are changed, or deleted, their value as passed to callback
will be the value at the time map
visits them; elements that are deleted are not visited.
Examples
Mapping an array of numbers to an array of square roots
The following code takes an array of numbers and creates a new array containing the square roots of the numbers in the first array.
var numbers = [1, 4, 9]; var roots = numbers.map(Math.sqrt); // roots is now [1, 2, 3], numbers is still [1, 4, 9]
Using map to reformat objects in an array
The following code takes an array of objects and creates a new array containing the newly reformatted objects.
var kvArray = [{key:1, value:10}, {key:2, value:20}, {key:3, value: 30}]; var reformattedArray = kvArray.map(function(obj){ var rObj = {}; rObj[obj.key] = obj.value; return rObj; }); // reformattedArray is now [{1:10}, {2:20}, {3:30}], // kvArray is still [{key:1, value:10}, {key:2, value:20}, {key:3, value: 30}]
Mapping an array of numbers using a function containing an argument
The following code shows how map works when a function requiring one argument is used with it. The argument will automatically be assigned to each element of the array as map loops through the original array.
var numbers = [1, 4, 9]; var doubles = numbers.map(function(num) { return num * 2; }); // doubles is now [2, 8, 18]. numbers is still [1, 4, 9]
Using map
generically
This example shows how to use map on a String
to get an array of bytes in the ASCII encoding representing the character values:
var map = Array.prototype.map; var a = map.call('Hello World', function(x) { return x.charCodeAt(0); }); // a now equals [72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100]
Using map
generically querySelectorAll
This example shows how to iterate through a collection of objects collected by querySelectorAll
. In this case we get all selected options on the screen and printed on the console:
var elems = document.querySelectorAll('select option:checked'); var values = Array.prototype.map.call(elems, function(obj) { return obj.value; });
Using map
to reverse a string
var str = '12345'; Array.prototype.map.call(str, function(x) { return x; }).reverse().join(''); // Output: '54321' // Bonus: use '===' to test if original string was a palindrome
Tricky use case
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 could 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, the array // The third argument is ignored by parseInt, but not the second one, // hence the possible confusion. See the blog post for more details function returnInt(element) { return parseInt(element, 10); } ['1', '2', '3'].map(returnInt); // [1, 2, 3] // Actual result is an array of numbers (as expected) // A simpler way to achieve the above, while avoiding the "gotcha": ['1', '2', '3'].map(Number); // [1, 2, 3]
Polyfill
map
was added to the ECMA-262 standard in the 5th edition; as such it may not be present in all implementations of the standard. You can work around this by inserting the following code at the beginning of your scripts, allowing use of map
in implementations which do not natively support it. This algorithm is exactly the one specified in ECMA-262, 5th edition, assuming Object
, TypeError
, and Array
have their original values and that callback.call
evaluates to the original value of
.Function.prototype.call
// Production steps of ECMA-262, Edition 5, 15.4.4.19 // Reference: http://es5.github.io/#x15.4.4.19 if (!Array.prototype.map) { Array.prototype.map = function(callback, thisArg) { var T, A, k; if (this == null) { throw new TypeError(' this is null or not defined'); } // 1. Let O be the result of calling ToObject passing the |this| // value as the argument. var O = Object(this); // 2. Let lenValue be the result of calling the Get internal // method of O with the argument "length". // 3. Let len be ToUint32(lenValue). var len = O.length >>> 0; // 4. If IsCallable(callback) is false, throw a TypeError exception. // See: http://es5.github.com/#x9.11 if (typeof callback !== 'function') { throw new TypeError(callback + ' is not a function'); } // 5. If thisArg was supplied, let T be thisArg; else let T be undefined. if (arguments.length > 1) { T = thisArg; } // 6. Let A be a new array created as if by the expression new Array(len) // where Array is the standard built-in constructor with that name and // len is the value of len. A = new Array(len); // 7. Let k be 0 k = 0; // 8. Repeat, while k < len while (k < len) { var kValue, mappedValue; // a. Let Pk be ToString(k). // This is implicit for LHS operands of the in operator // b. Let kPresent be the result of calling the HasProperty internal // method of O with argument Pk. // This step can be combined with c // c. If kPresent is true, then if (k in O) { // i. Let kValue be the result of calling the Get internal // method of O with argument Pk. kValue = O[k]; // ii. Let mappedValue be the result of calling the Call internal // method of callback with T as the this value and argument // list containing kValue, k, and O. mappedValue = callback.call(T, kValue, k, O); // iii. Call the DefineOwnProperty internal method of A with arguments // Pk, Property Descriptor // { Value: mappedValue, // Writable: true, // Enumerable: true, // Configurable: true }, // and false. // In browsers that support Object.defineProperty, use the following: // Object.defineProperty(A, k, { // value: mappedValue, // writable: true, // enumerable: true, // configurable: true // }); // For best browser support, use the following: A[k] = mappedValue; } // d. Increase k by 1. k++; } // 9. return A return A; }; }
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 5.1 (ECMA-262) The definition of 'Array.prototype.map' in that specification. |
Standard | Initial definition. Implemented in JavaScript 1.6. |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Array.prototype.map' in that specification. |
Standard | |
ECMAScript 2017 Draft (ECMA-262) The definition of 'Array.prototype.map' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | 1.5 (1.8) | 9 | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | 1.0 (1.8) | (Yes) | (Yes) | (Yes) |
See also
License
© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/javascript/reference/global_objects/array/map