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    Array.prototype.map()

    Summary

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

    Syntax

    arr.map(callback[, thisArg])

    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 executing callback.

    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; it is not invoked for indexes that are undefined, those which have been deleted or which have never been assigned values.

    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

    Example: 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]
    

    Example: 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]
    

    Example: 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]
    

    Example: 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 = [].map.call(elems, function(obj) {
      return obj.value;
    });
    

    Example: Using map to reverse a string

    var str = '12345';
    [].map.call(str, function(x) {
      return x;
    }).reverse().join(''); 
    
    // Output: '54321'
    // Bonus: use '===' to test if original string was a palindrome
    

    Example: 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 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 6 (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

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