JavaScript Array : Object

constructor

Arrays are containers that allow access to its items through numerical indices.

Array() : Array

Creates an empty Array (length = 0). Can also be constructed as [].

Example:

// The following are equivalent
var x = [];
var y = Array();
var z = new Array();

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Array(length : Number) : Array

Creates an Array of the specified length where each item starts as undefined.

Example:

var squares = Array(5);
for (var i = 0; i < squares.length; i++) {
  squares[i] = i * i;
}
console.log('squares = ' + squares);

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Array(item0 : Object, [item1 : Object, [...]]) : Array

Creates an Array with the specified parameters as the 0th, 1st, ... items of the Array. Can also be constructed as [item0, item1, ...].

Example:

// The following are equivalent
var x = ['a', 'b', 'c'];
var y = Array('a', 'b', 'c');
var z = new Array('a', 'b', 'c');

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Constructors

new Array() : Array

Same as Array().

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new Array(length : Number) : Array

Same as Array(length).

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new Array(item0 : Object, [item1 : Object, [...]]) : Array

Same as Array(item0, item1, ...).

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Instance Properties

length : Number

The number of items in this. It is 1 greater than the index of the last item.

Example:

console.log([].length);
console.log(['a', 'b', 'c'].length);
console.log(Array(100).length);
var x = [];
x[50] = 'foo';
console.log(x.length);

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Instance Methods

concat(item0 : Object, [item1 : Object, [...]]) : Array

Returns a new Array composed of the items of this followed by item0, item1, .... To concat with another Array, you can use Function.call to pass the second Array's items to concat.

Example:

var x = [1, 2, 3];
console.log(x.concat(4, 5));
console.log(x);
console.log(Array.prototype.concat.call(x, [4, 5]));

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every(callback : Function, [thisArg : Object]) : Boolean

callback(item : Object, index : Number, array : Array) : Boolean

Returns true if callback returns true for every item in this. Otherwise returns false. The Array passed to callback is the this of the call to every.

Example:

var isEven = function(x) { return (x % 2) === 0; }
console.log([2, 22, 36].every(isEven));
console.log([6, 19, 18].every(isEven));
console.log([5, 19, 17].every(isEven));

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filter(callback : Function, [thisArg : Object]) : Array

callback(item : Object, index : Number, array : Array) : Boolean

Returns a new Array containing only the items in this that callback returned true for. The Array passed to callback is the this of the call to filter.

Example:

var isEven = function(x) { return (x % 2) === 0; }
var numbers = [1, 2, 4, 5, 7, 9, 12];
console.log(numbers.filter(isEven));

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forEach(callback : Function, [thisArg : Object]) : undefined

callback(item : Object, index : Number, array : Array) : undefined

Calls callback for each item in this. The Array passed to callback is the this of the call to forEach.

Example:

var x = ['a', 'b', 'c'];
x.forEach(function(item, index) {
  console.log('item at ' + index + ' = ' + item);
});

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indexOf(item : Object, [startingIndex = 0 : Number]) : Number

Returns the first location of item in this starting the search from startingIndex. If startingIndex is negative, this.length is added to it before starting the search. Returns -1 if item is not found.

Example:

var x = ['a', 'b', 'c', 'd', 'b', 'c', 'a'];
console.log(x.indexOf('b'));
console.log(x.indexOf('b', 2));
console.log(x.indexOf('b', -4));
console.log(x.indexOf('e', 2));

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join([separator = ',' : String]) : String

Returns a String created by joining the toString() of each item of this separated by separator.

Example:

var x = ['a', 'b', 'c'];
console.log(x.join());
console.log(x.join(' -- '));

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lastIndexOf(item : Object, [startingIndex : Number]) : Number

Returns the location of item in this starting the search from startingIndex by searching backwards through the array. If startingIndex is not specified, the search starts from the end of the array. If startingIndex is negative, this.length is added to it before starting the search. Returns -1 if item is not found.

Example:

var x = ['a', 'b', 'c', 'd', 'b', 'c', 'a'];
console.log(x.lastIndexOf('b'));
console.log(x.lastIndexOf('b', 3));
console.log(x.lastIndexOf('b', -4));
console.log(x.lastIndexOf('e', 2));

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map(callback : Function, [thisArg : Object]) : Array

callback(item : Object, index : Number, array : Array) : Object

Returns a new Array with where each item is the result of calling callback on each item in this. The Array passed to callback is the this of the call to map.

Example:

var numbers = [1, 2, 3, 4];
var squares = numbers.map(function(x) { return x * x; });
console.log(squares);
console.log(numbers);

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pop() : Object

Removes the last item from this and returns the removed item. Returns undefined if this is empty. Use with push to treat an Array as a stack.

Example:

var x = ['a', 'b', 'c']; 
console.log(x.pop());
console.log(x);

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push(item0 : Object, [item1 : Object, [...]]) : Number

Appends the specified items to the end of this and returns the new value of this.length. Use with pop to treat an Array as a stack.

Example:

var x = ['a', 'b', 'c']; 
console.log(x.push('d', 'e'));
console.log(x);

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reduce(callback : Function, [initialValue : Object]) : Object

callback(previous : Object, current : Object, index : Number, array : Array) : Object

Calls callback for each item in this and passes the return value of callback as the previous parameter to the next callback invocation. Returns the result from the last call to callback. If initialValue is not specified, callback will first be called on this[1] with previous set to this[0]. The Array passed to callback is the this of the call to reduce.

Example:

var join = function(prev, cur) { return prev + cur; } 
console.log(['b', 'c'].reduce(join, 'a'));

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reduceRight(callback : Function, [initialValue : Object]) : Object

callback(previous : Object, current : Object, index : Number, array : Array) : Object

Calls callback for each item in this in reverse order and passes the return value of callback as the previous parameter to the next callback invocation. Returns the result from the last call to callback. If initialValue is not specified, callback will first be called on this[this.length - 2] with previous set to this[this.length - 1]. The Array passed to callback is the this of the call to reduceRight.

Example:

var join = function(prev, cur) { return prev + cur; } 
console.log(['b', 'c'].reduceRight(join, 'a'));

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reverse() : Array

Reverses the order of the items in this and returns this.

Example:

var x = ['a', 'b', 'c']; 
console.log(x.reverse());
console.log(x);

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shift() : Object

Removes the first item of this and shifts the remaining items down by 1. Returns the removed item.

Example:

var x = ['a', 'b', 'c']; 
console.log(x.shift());
console.log(x);

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slice(start : Number, [end : Number]) : Array

Returns a new Array which is composed of the items this[start], this[start + 1], ..., this[end - 1]. Note that item[end] is not included. If start or end is negative, the value is added to this.length before performing the slice. If end is not specified, this.length is used.

Example:

var x = ['a', 'b', 'c', 'd', 'e']; 
console.log(x.slice(2));
console.log(x.slice(2, 3));
console.log(x.slice(1, -2));
console.log(x.slice(-4, -1));
console.log(x);

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some(callback : Function, [thisArg : Object]) : Boolean

callback(item : Object, index : Number, array : Array) : Boolean

Returns true if callback returns true for at least one item in this. Otherwise returns false. The Array passed to callback is the this of the call to some.

Example:

var isEven = function(x) { return (x % 2) === 0; }
console.log([2, 22, 36].some(isEven));
console.log([6, 19, 18].some(isEven));
console.log([5, 19, 17].some(isEven));

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sort([comparisonFunction : Function]) : Array

comparisonFunction(x : Object, y : Object) : Number

Sort the items of this using comparisonFunction to determine the sort order and returns this. The Number returned by comparisonFunction should be 0 if x and y are equal, negative if x is less than y, or positive if x is greater than y. If comparisonFunction is not specified, the toString() of the items will be sorted in alphanumeric order.

Example:

var x = [52, 7, 14, 22, 9]; 

// Sort alphanumerically
console.log(x.sort());
console.log(x);

// Sort numerically 
var numericComparison = function(x, y) { 
  return x - y;
};
console.log(x.sort(numericComparison));

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splice(start : Number, count : Number, [item0 : Object, [item1 : Object, [...]]]) : Array

Removes count items from this starting at index start. If the optional items are specified, they are inserted into this at start. Returns a new Array containing the removed items.

Example:

var x = ['a', 'b', 'c', 'd', 'e']; 
console.log(x.splice(2, 1));
console.log(x);
console.log(x.splice(1, 2, 'X', 'Y', 'Z'));
console.log(x);

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unshift(item0 : Object, [item1 : Object, [...]]) : Number

Inserts the specified items at the start of this. Returns the new value of this.length.

Example:

var x = ['c', 'd', 'e'];
console.log(x.unshift('a', 'b'));
console.log(x);

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Array Methods

isArray(value : Object) : Boolean

Returns true if value is an Array.

Example:

console.log(Array.isArray([]));
console.log(Array.isArray({}));
console.log(Array.isArray(53));

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