Array.prototype.map() - JavaScript 编辑

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

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

let newArray = arr.map(callback(currentValue[, index[, array]]) {
  // return element for newArray, after executing something
}[, thisArg]);

Parameters

callback

Function that is called for every element of arr. Each time callback executes, the returned value is added to newArray.

The callback function accepts the following arguments:

currentValue

The current element being processed in the array.

indexOptional

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

arrayOptional

The array map was called upon.

thisArgOptional

Value to use as this when executing callback.

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;
• indexes which have been deleted.

When not to use map()

Since map builds a new array, using it when you aren't using the returned array is an anti-pattern; use forEach or for...of instead.

You shouldn't be using map if:

• you're not using the array it returns; and/or
• you're not returning a value from the callback.

Parameters in Detail

callback is invoked with three arguments: the value of the element, the index of the element, and the array object being mapped.

If a thisArg parameter is provided, it will be used as callback's this value. Otherwise, the value undefined will be used 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 after the call to map, their value will be the value at the time callback visits them. Elements that are deleted after the call to map begins and before being visited are not visited.

Due to the algorithm defined in the specification, if the array which map was called upon is sparse, resulting array will also be sparse keeping same indices blank.

Polyfill

map was added to the ECMA-262 standard in the 5th edition. Therefore, 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: https://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: https://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 = arguments[1];
    }

    // 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;
  };
}

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.

let numbers = [1, 4, 9]
let roots = numbers.map(function(num) {
    return Math.sqrt(num)
})
// 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.

let kvArray = [{key: 1, value: 10},
               {key: 2, value: 20},
               {key: 3, value: 30}]

let reformattedArray = kvArray.map(obj => {
   let 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 from each element of the array as map loops through the original array.

let numbers = [1, 4, 9]
let 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:

let map = Array.prototype.map
let 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. This is because querySelectorAll returns a NodeList (which is a collection of objects).

In this case, we return all the selected options' values on the screen:

let elems = document.querySelectorAll('select option:checked')
let values = Array.prototype.map.call(elems, function(obj) {
  return obj.value
})

An easier way would be the Array.from() method.

Tricky use case

(inspired by this blog post)

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
• the array

The third argument is ignored by parseInt—but not the second one! This is the source of possible confusion.

Here is a concise example of the iteration steps:

// parseInt(string, radix) -> map(parseInt(value, index))
/*  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

Then let's talk about solutions.

function returnInt(element) {
  return parseInt(element, 10)
}

['1', '2', '3'].map(returnInt); // [1, 2, 3]
// Actual result is an array of numbers (as expected)

// Same as above, but using the concise arrow function syntax
['1', '2', '3'].map( str => parseInt(str) )

// A simpler way to achieve the above, while avoiding the "gotcha":
['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) ) // [1, 2, 3]

One alternative output of the map method being called with parseInt as a parameter runs as follows:

let xs = ['10', '10', '10']

xs = xs.map(parseInt)

console.log(xs)
// Actual result of 10,NaN,2 may be unexpected based on the above description.

Mapped array contains undefined

When undefined or nothing is returned:

let numbers = [1, 2, 3, 4]
let filteredNumbers = numbers.map(function(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]

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'Array.prototype.map' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• Array.prototype.forEach()
• Map object
• Array.from()