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    Array

    Summary

    The JavaScript Array global object is a constructor for arrays, which are high-level, list-like objects.

    Syntax

    [element0, element1, ..., elementN]
    new Array(element0, element1, ..., elementN)
    new Array(arrayLength)
    
    element0, element1, ..., elementN
    A JavaScript array is initialized with the given elements, except in the case where a single argument is passed to the Array constructor and that argument is a number. (See below.) Note that this special case only applies to JavaScript arrays created with the Array constructor, not array literals created with the bracket syntax.
    arrayLength
    If the only argument passed to the Array constructor is an integer between 0 and 232-1 (inclusive), this returns a new JavaScript array with length set to that number. If the argument is any other number, a RangeError exception is thrown.

    Description

    Arrays are list-like objects whose prototype has methods to perform traversal and mutation operations. Neither the length of a JavaScript array nor the types of its elements are fixed. Since an array's size length grow or shrink at any time, JavaScript arrays are not guaranteed to be dense. In general, these are convenient characteristics; but if these features are not desirable for your particular use, you might consider using typed arrays.

    Some people think that you shouldn't use an array as an associative array. In any case, you can use plain objects instead, although doing so comes with its own caveats. See the post Lightweight JavaScript dictionaries with arbitrary keys as an example.

    Accessing array elements

    JavaScript arrays are zero-indexed: the first element of an array is at index 0, and the last element is at the index equal to the value of the array's length property minus 1.

    var arr = ["this is the first element", "this is the second element"];
    console.log(arr[0]);              // prints "this is the first element"
    console.log(arr[1]);              // prints "this is the second element"
    console.log(arr[arr.length - 1]); // prints "this is the second element"
    

    Array elements are object properties in the same way that toString is a property, but trying to access an element of an array as follows throws a syntax error, because the property name is not valid:

    console.log(arr.0);
    

    There is nothing special about JavaScript arrays and their properties that causes this. JavaScript properties that begin with a digit cannot be referenced with dot notation; and must be accessed using bracket notation. For example, if you had an object with a property named "3d", it can only be referenced using bracket notation. E.g.:

    var years = [1950, 1960, 1970, 1980, 1990, 2000, 2010];
    console.log(years.0); // a syntax error
    console.log(years[0]); // works properly
    
    renderer.3d.setTexture(model, "character.png"); // a syntax error
    renderer["3d"].setTexture(model, "character.png"); // works properly
    

    Note that in the 3d example, "3d" had to be quoted. It's possible to quote the JavaScript array indexes as well (e.g., years["2"] instead of years[2]), although it's not necessary. The 2 in years[2] is coerced into a string by the JavaScript engine through an implicit toString conversion. It is for this reason that "2" and "02" would refer to two different slots on the years object and the following example could be true:

    console.log(years["2"] != years["02"]);
    

    Similarly, object properties which happen to be reserved words(!) can only be accessed as string literals in bracket notation:

    var promise = {'var'  : 'text',
                   'array': [1, 2, 3, 4] }
    
    console.log(promise['array'])

    Relationship between length and numerical properties

    A JavaScript array's length property and numerical properties are connected. Several of the built-in array methods (e.g., join, slice, indexOf, etc.) take into account the value of an array's length property when they're called. Other methods (e.g., push, splice, etc.) also result in updates to an array's length property.

    var fruits = [];
    fruits.push("banana", "apple", "peach");
    
    console.log(fruits.length); // 3

    When setting a property on a JavaScript array when the property is a valid array index and that index is outside the current bounds of the array, the engine will update the array's length property accordingly:

    fruits[5] = "mango";
    console.log(fruits[5]); // "mango"
    console.log(Object.keys(fruits));  // ["0", "1", "2", "5"]
    console.log(fruits.length); // 6

    Increasing the length property does not create additional elements.

    fruits.length = 10;
    console.log(Object.keys(fruits)); // ["0", "1", "2", "5"]
    console.log(fruits.length); // 10
    

    Decreasing the length property does, however, delete elements.

    fruits.length = 2;
    console.log(Object.keys(fruits)); // ["0", "1"]
    console.log(fruits.length); // 2
    
    This is explained further on the Array.length page.

    Creating an array using the result of a match

    The result of a match between a regular expression and a string can create a JavaScript array. This array has properties and elements which provide information about the match. Such an array is returned by RegExp.exec, String.match, and String.replace. To help explain these properties and elements, look at the following example and then refer to the table below:

    // Match one d followed by one or more b's followed by one d
    // Remember matched b's and the following d
    // Ignore case
    
    var myRe = /d(b+)(d)/i;
    var myArray = myRe.exec("cdbBdbsbz");
    

    The properties and elements returned from this match are as follows:

    Property/Element Description Example
    input A read-only property that reflects the original string against which the regular expression was matched. cdbBdbsbz
    index A read-only property that is the zero-based index of the match in the string. 1
    [0] A read-only element that specifies the last matched characters. dbBd
    [1], ...[n] Read-only elements that specify the parenthesized substring matches, if included in the regular expression. The number of possible parenthesized substrings is unlimited. [1]: bB
    [2]: d

    Properties

    For properties available on Array instances, see Properties of Array instances.
    Array.length
    The Array constructor's length property whose value is 1.
    Array.prototype
    Allows the addition of properties to all array objects.
    Properties inherited from Function:

    Methods

    For methods available on Array instances, see Methods of Array instances.
    Array.isArray()
    Returns true if a variable is an array, if not false.
    Array.of()
    Creates a new Array instance with a variable number of arguments, regardless of number or type of the arguments.

    Array instances

    All Array instances inherit from Array.prototype. The prototype object of the Array constructor can be modified to affect all Array instances.

    Properties

    Array.prototype.constructor
    Specifies the function that creates an object's prototype.
    Array.prototype.length
    Reflects the number of elements in an array.

    Methods

    Mutator methods

    These methods modify the array:

    Array.prototype.fill()
    Fills all the elements of an array from a start index to an end index with a static value.
    Array.prototype.pop()
    Removes the last element from an array and returns that element.
    Array.prototype.push()
    Adds one or more elements to the end of an array and returns the new length of the array.
    Array.prototype.reverse()
    Reverses the order of the elements of an array -- the first becomes the last, and the last becomes the first.
    Array.prototype.shift()
    Removes the first element from an array and returns that element.
    Array.prototype.sort()
    Sorts the elements of an array in place and returns the array.
    Array.prototype.splice()
    Adds and/or removes elements from an array.
    Array.prototype.unshift()
    Adds one or more elements to the front of an array and returns the new length of the array.

    Accessor methods

    These methods do not modify the array and return some representation of the array.

    Array.prototype.concat()
    Returns a new array comprised of this array joined with other array(s) and/or value(s).
    Array.prototype.join()
    Joins all elements of an array into a string.
    Array.prototype.slice()
    Extracts a section of an array and returns a new array.
    Array.prototype.toSource()
    Returns an array literal representing the specified array; you can use this value to create a new array. Overrides the Object.prototype.toSource() method.
    Array.prototype.toString()
    Returns a string representing the array and its elements. Overrides the Object.prototype.toString() method.
    Array.prototype.toLocaleString()
    Returns a localized string representing the array and its elements. Overrides the Object.prototype.toLocaleString() method.
    Array.prototype.indexOf()
    Returns the first (least) index of an element within the array equal to the specified value, or -1 if none is found.
    Array.prototype.lastIndexOf()
    Returns the last (greatest) index of an element within the array equal to the specified value, or -1 if none is found.

    Iteration methods

    Several methods take as arguments functions to be called back while processing the array. When these methods are called, the length of the array is sampled, and any element added beyond this length from within the callback is not visited. Other changes to the array (setting the value of or deleting an element) may affect the results of the operation if the method visits the changed element afterwards. While the specific behavior of these methods in such cases is well-defined, you should not rely upon it so as not to confuse others who might read your code. If you must mutate the array, copy into a new array instead.

    Array.prototype.forEach()
    Calls a function for each element in the array.
    Array.prototype.entries()
    Returns a new Array Iterator object that contains the key/value pairs for each index in the array.
    Array.prototype.every()
    Returns true if every element in this array satisfies the provided testing function.
    Array.prototype.some()
    Returns true if at least one element in this array satisfies the provided testing function.
    Array.prototype.filter()
    Creates a new array with all of the elements of this array for which the provided filtering function returns true.
    Array.prototype.find()
    Returns the found value in the array, if an element in the array satisfies the provided testing function or undefined if not found.
    Array.prototype.findIndex()
    Returns the found index in the array, if an element in the array satisfies the provided testing function or -1 if not found.
    Array.prototype.keys()
    Returns a new Array Iterator that contains the keys for each index in the array.
    Array.prototype.map()
    Creates a new array with the results of calling a provided function on every element in this array.
    Array.prototype.reduce()
    Apply a function against an accumulator and each value of the array (from left-to-right) as to reduce it to a single value.
    Array.prototype.reduceRight()
    Apply a function against an accumulator and each value of the array (from right-to-left) as to reduce it to a single value.

    Array generic methods

    Sometimes you would like to apply array methods to strings or other array-like objects (such as function arguments). By doing this, you treat a string as an array of characters (or otherwise treat a non-array as an array). For example, in order to check that every character in the variable str is a letter, you would write:

    function isLetter(character) {
      return character >= "a" && character <= "z";
    }
    
    if (Array.prototype.every.call(str, isLetter)) {
        console.log("The string '" + str + "' contains only letters!");
    }

    This notation is rather wasteful and JavaScript 1.6 introduced a generic shorthand:

    if (Array.every(isLetter, str)) {
        console.log("The string '" + str + "' contains only letters!");
    }

    Generics are also available on String.

    These are currently not part of ECMAScript standards (though the ES6 Array.from() can be used to achieve this). The following is a shim to allow its use in all browsers:

    // Assumes Array extras already present (one may use polyfills for these as well)
    (function () {
        'use strict';
    
        var i,
            // We could also build the array of methods with the following, but the
            //   getOwnPropertyNames() method is non-shimable:
            // Object.getOwnPropertyNames(Array).filter(function (methodName) {
            //     return typeof Array[methodName] === 'function'});
            methods = [
                'join', 'reverse', 'sort', 'push', 'pop', 'shift', 'unshift',
                'splice', 'concat', 'slice', 'indexOf', 'lastIndexOf',
                'forEach', 'map', 'reduce', 'reduceRight', 'filter',
                'some', 'every'
            ],
            methodCount = methods.length,
            assignArrayGeneric = function (methodName) {
                if (!Array[methodName]) {
                    var method = Array.prototype[methodName];
                    if (typeof method === 'function') {
                        Array[methodName] = function () {
                            return method.call.apply(method, arguments);
                        };
                    }
                }
            };
    
        for (i = 0; i < methodCount; i++) {
            assignArrayGeneric(methods[i]);
        }
    }());

    Examples

    Example: Creating an array

    The following example creates an array, msgArray, with a length of 0, then assigns values to msgArray[0] and msgArray[99], changing the length of the array to 100.

    var msgArray = [];
    msgArray[0] = "Hello";
    msgArray[99] = "world";
    
    if (msgArray.length === 100) {
        console.log("The length is 100.");
    }

    Example: Creating a two-dimensional array

    The following creates a chess board as a two dimensional array of strings. The first move is made by copying the 'p' in (6,4) to (4,4). The old position 6,4 is made blank.

    var board = [ 
        ['R','N','B','Q','K','B','N','R'],
        ['P','P','P','P','P','P','P','P'],
        [' ',' ',' ',' ',' ',' ',' ',' '],
        [' ',' ',' ',' ',' ',' ',' ',' '],
        [' ',' ',' ',' ',' ',' ',' ',' '],
        [' ',' ',' ',' ',' ',' ',' ',' '],
        ['p','p','p','p','p','p','p','p'],
        ['r','n','b','q','k','b','n','r'] ];
    
    console.log(board.join('\n') + '\n\n');
    
    // Move King's Pawn forward 2
    board[4][4] = board[6][4];
    board[6][4] = ' ';
    console.log(board.join('\n'));
    

    Here is the output:

    R,N,B,Q,K,B,N,R
    P,P,P,P,P,P,P,P
     , , , , , , , 
     , , , , , , , 
     , , , , , , , 
     , , , , , , , 
    p,p,p,p,p,p,p,p
    r,n,b,q,k,b,n,r
    
    R,N,B,Q,K,B,N,R
    P,P,P,P,P,P,P,P
     , , , , , , , 
     , , , , , , , 
     , , , ,p, , , 
     , , , , , , , 
    p,p,p,p, ,p,p,p
    r,n,b,q,k,b,n,r
    

    Specifications

    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition.
    ECMAScript Language Specification 5.1th Edition (ECMA-262) Standard New methods added: Array.isArray, indexOf, lastIndexOf, every, some, forEach, map, filter, reduce, reduceRight  
    ECMAScript Language Specification 6th Edition (ECMA-262) Draft New methods added: Array.from, Array.of, find, findIndex, fill, copyWithin

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

    See also

    Document Tags and Contributors

    Contributors to this page: Sevenspade
    Last updated by: Sevenspade,
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