Array

Summary

The JavaScript Array object is a global object that is used in the construction of arrays; which are high-level, list-like objects.

Create an Array

var fruits = ["Apple", "Banana"];

console.log(fruits.length);
// 2

Access (index into) an Array item

var first = fruits[0];
// Apple

var last = fruits[fruits.length - 1];
// Banana

Loop over an Array

fruits.forEach(function (item, index, array) {
  console.log(item, index);
});
// Apple 0
// Banana 1

Add to the end of an Array

var newLength = fruits.push("Orange");
// ["Apple", "Banana", "Orange"]

Remove from the end of an Array

var last = fruits.pop(); // remove Orange (from the end)
// ["Apple", "Banana"];

Remove from the front of an Array

var first = fruits.shift(); // remove Apple from the front
// ["Banana"];

Add to the front of an Array

var newLength = fruits.unshift("Strawberry") // add to the front
// ["Strawberry", "Banana"];

Find the index of an item in the Array

fruits.push("Mango");
// ["Strawberry", "Banana", "Mango"]

var pos = fruits.indexOf("Banana");
// 1

Remove an item by Index Position

var removedItem = fruits.splice(pos, 1); // this is how to remove an item
// ["Strawberry", "Mango"]

Copy an Array

var shallowCopy = fruits.slice(); // this is how to make a copy
// ["Strawberry", "Mango"]

Syntax

[element0, element1, ..., elementN]
new Array(element0, element1[, ...[, elementN]])
new Array(arrayLength)

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]);              // logs 'this is the first element'
console.log(arr[1]);              // logs 'this is the second element'
console.log(arr[arr.length - 1]); // logs '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); // a syntax error

There is nothing special about JavaScript arrays and the properties that cause 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(but it can be accessed by dot notation in firefox 40.0a2 at least):

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.

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

Array.length: The Array constructor's length property whose value is 1.
Array.prototype: Allows the addition of properties to all array objects.

Methods

Array.isArray(): Returns true if a variable is an array, if not false.

`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

Methods

Mutator methods

Accessor methods

Iteration methods

`Array` generic methods

Array generics are non-standard, deprecated and will get removed near future. Note that you can not rely on them cross-browser. However, there is a shim available on GitHub.

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

Generics are also available on String.

These are 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', 'find', 'findIndex', 'entries', 'keys',
      'values', 'copyWithin', 'includes'
    ],
    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

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.');
}

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

Created by Mozilla Contributors and licensed under CC-BY-SA 2.5

DroidScript

Array

Summary

Syntax

Description

Accessing array elements

Relationship between length and numerical properties

Creating an array using the result of a match

Properties

Methods

Array instances

Properties

Methods

Mutator methods

Accessor methods

Iteration methods

Array generic methods

Examples

Creating an array

Creating a two-dimensional array

Relationship between `length` and numerical properties

`Array` instances

`Array` generic methods