length : Number

Reflects the number of elements in an array.

The value of the length property is an integer with a positive sign and a value less than 2 to the 32 power (232).

You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements does not increase; for example, if you set length to 3 when it is currently 2, the array still contains only 2 elements.

In the following example the array numbers is iterated through by looking at the length property to see how many elements it has. Each value is then doubled.

var numbers = [1,2,3,4,5];
for (var i = 0; i < numbers.length; i++) {
    numbers[i] *= 2;
}
// numbers is now [2,4,6,8,10];

The following example shortens the array statesUS to a length of 50 if the current length is greater than 50.

if (statesUS.length > 50) {
    statesUS.length=50
}

concat ( values ) : Array

Returns a new array comprised of this array joined with other array(s) and/or value(s).

concat creates a new array consisting of the elements in the this object on which it is called, followed in order by, for each argument, the elements of that argument (if the argument is an array) or the argument itself (if the argument is not an array).

concat does not alter this or any of the arrays provided as arguments but instead returns a "one level deep" copy that contains copies of the same elements combined from the original arrays. Elements of the original arrays are copied into the new array as follows: Object references (and not the actual object): concat copies object references into the new array. Both the original and new array refer to the same object. That is, if a referenced object is modified, the changes are visible to both the new and original arrays. Strings and numbers (not String and Number objects): concat copies the values of strings and numbers into the new array.

Any operation on the new array will have no effect on the original arrays, and vice versa.

Concatenating two arrays

The following code concatenates two arrays:

var alpha = ["a", "b", "c"];
var numeric = [1, 2, 3];

// creates array ["a", "b", "c", 1, 2, 3]; alpha and numeric are unchanged
var alphaNumeric = alpha.concat(numeric);

Concatenating three arrays

The following code concatenates three arrays:

var num1 = [1, 2, 3];
var num2 = [4, 5, 6];
var num3 = [7, 8, 9];

// creates array [1, 2, 3, 4, 5, 6, 7, 8, 9]; num1, num2, num3 are unchanged
var nums = num1.concat(num2, num3);

Concatenating values to an array

The following code concatenates three values to an array:

var alpha = ['a', 'b', 'c'];

// creates array ["a", "b", "c", 1, 2, 3], leaving alpha unchanged
var alphaNumeric = alpha.concat(1, [2, 3]);

Parameters

Returns

constructor ( [items] )

Creates new Array object.

Parameters

every ( callback, [thisObject] ) : Boolean

Tests whether all elements in the array pass the test implemented by the provided function.

every executes the provided callback function once for each element present in the array until it finds one where callback returns a false value. If such an element is found, the every method immediately returns false. Otherwise, if callback returned a true value for all elements, every will return true. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.

If a thisObject parameter is provided to every, it will be used as the this for each invocation of the callback. If it is not provided, or is null, the global object associated with callback is used instead.

every does not mutate the array on which it is called.

The range of elements processed by every is set before the first invocation of callback. Elements which are appended to the array after the call to every begins will not be visited by callback. If existing elements of the array are changed, their value as passed to callback will be the value at the time every visits them; elements that are deleted are not visited.

every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true. (It is vacuously true that all elements of the empty set satisfy any given condition.)

The following example tests whether all elements in the array are bigger than 10.

function isBigEnough(element, index, array) {
  return (element >= 10);
}
var passed = [12, 5, 8, 130, 44].every(isBigEnough);
// passed is false
passed = [12, 54, 18, 130, 44].every(isBigEnough);
// passed is true

NOTE: This method is part of the ECMAScript 5 standard.

Parameters

Returns

filter ( callback, [thisObject] ) : Array

Creates a new array with all elements that pass the test implemented by the provided function.

filter calls a provided callback function once for each element in an array, and constructs a new array of all the values for which callback returns a true value. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values. Array elements which do not pass the callback test are simply skipped, and are not included in the new array.

If a thisObject parameter is provided to filter, it will be used as the this for each invocation of the callback. If it is not provided, or is null, the global object associated with callback is used instead.

filter does not mutate the array on which it is called.

The range of elements processed by filter is set before the first invocation of callback. Elements which are appended to the array after the call to filter 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 filter visits them; elements that are deleted are not visited.

The following example uses filter to create a filtered array that has all elements with values less than 10 removed.

function isBigEnough(element, index, array) {
  return (element >= 10);
}
var filtered = [12, 5, 8, 130, 44].filter(isBigEnough);
// filtered is [12, 130, 44]

NOTE: This method is part of the ECMAScript 5 standard.

Parameters

Returns

forEach ( callback, [thisArg] )

Executes a provided function once per array element.

forEach executes the provided function (callback) once for each element present in the array. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.

If a thisArg parameter is provided to forEach, it will be used as the this value for each callback invocation as if callback.call(thisArg, element, index, array) was called. If thisArg is undefined or null, the this value within the function depends on whether the function is in strict mode or not (passed value if in strict mode, global object if in non-strict mode).

The range of elements processed by forEach is set before the first invocation of callback. Elements which are appended to the array after the call to forEach 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 forEach visits them; elements that are deleted are not visited.

The following code logs a line for each element in an array:

function logArrayElements(element, index, array) {
    console.log("a[" + index + "] = " + element);
}
[2, 5, 9].forEach(logArrayElements);
// logs:
// a[0] = 2
// a[1] = 5
// a[2] = 9

NOTE: This method is part of the ECMAScript 5 standard.

Parameters

indexOf ( searchElement, [fromIndex] ) : Number

Returns the first index at which a given element can be found in the array, or -1 if it is not present.

indexOf compares searchElement to elements of the Array using strict equality (the same method used by the ===, or triple-equals, operator).

var array = [2, 5, 9];
var index = array.indexOf(2);
// index is 0
index = array.indexOf(7);
// index is -1

NOTE: This method is part of the ECMAScript 5 standard.

Parameters

Returns

join ( separator ) : String

Joins all elements of an array into a string.

The string conversions of all array elements are joined into one string.

The following example creates an array, a, with three elements, then joins the array three times: using the default separator, then a comma and a space, and then a plus.

var a = new Array("Wind","Rain","Fire");
var myVar1 = a.join();      // assigns "Wind,Rain,Fire" to myVar1
var myVar2 = a.join(", ");  // assigns "Wind, Rain, Fire" to myVar2
var myVar3 = a.join(" + "); // assigns "Wind + Rain + Fire" to myVar3

Parameters

Returns

lastIndexOf ( searchElement, [fromIndex] ) : Number

Returns the last index at which a given element can be found in the array, or -1 if it is not present. The array is searched backwards, starting at fromIndex.

lastIndexOf compares searchElement to elements of the Array using strict equality (the same method used by the ===, or triple-equals, operator).

var array = [2, 5, 9, 2];
var index = array.lastIndexOf(2);
// index is 3
index = array.lastIndexOf(7);
// index is -1
index = array.lastIndexOf(2, 3);
// index is 3
index = array.lastIndexOf(2, 2);
// index is 0
index = array.lastIndexOf(2, -2);
// index is 0
index = array.lastIndexOf(2, -1);
// index is 3

NOTE: This method is part of the ECMAScript 5 standard.

Parameters

Returns

map ( callback, [thisObject] ) : Array

Creates a new array with the results of calling a provided function on every element in this array.

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 which have been deleted or which have never been assigned values.

If a thisArg parameter is provided to map, it will be used as the this for each invocation of the callback. If it is not provided, or is null, the global object associated with callback is used instead.

map does not mutate the array on which it is called.

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.

The following code creates an array of "plural" forms of nouns from an array of their singular forms.

function fuzzyPlural(single) {
  var result = single.replace(/o/g, 'e');
  if( single === 'kangaroo'){
    result += 'se';
  }
  return result;
}

var words = ["foot", "goose", "moose", "kangaroo"];
console.log(words.map(fuzzyPlural));

// ["feet", "geese", "meese", "kangareese"]

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]

NOTE: This method is part of the ECMAScript 5 standard.

Parameters

Returns

pop Object

The pop method removes the last element from an array and returns that value to the caller.

pop is intentionally generic; this method can be called or applied to objects resembling arrays. Objects which do not contain a length property reflecting the last in a series of consecutive, zero-based numerical properties may not behave in any meaningful manner.

var myFish = ["angel", "clown", "mandarin", "surgeon"];
var popped = myFish.pop();
alert(popped); // Alerts 'surgeon'

Returns

push ( elements ) : Number

Adds one or more elements to the end of an array and returns the new length of the array.

push is intentionally generic. This method can be called or applied to objects resembling arrays. The push method relies on a length property to determine where to start inserting the given values. If the length property cannot be converted into a number, the index used is 0. This includes the possibility of length being nonexistent, in which case length will also be created.

The only native, array-like objects are strings, although they are not suitable in applications of this method, as strings are immutable.

Adding elements to an array

The following code creates the sports array containing two elements, then appends two elements to it. After the code executes, sports contains 4 elements: "soccer", "baseball", "football" and "swimming".

var sports = ["soccer", "baseball"];
sports.push("football", "swimming");

Parameters

Returns

reduce ( callback, [initialValue] ) : Mixed

Applies a function against an accumulator and each value of the array (from left-to-right) as to reduce it to a single value.

reduce executes the callback function once for each element present in the array, excluding holes in the array.

The first time the callback is called, previousValue and currentValue can be one of two values. If initialValue is provided in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was provided, then previousValue will be equal to the first value in the array and currentValue will be equal to the second.

Suppose the following use of reduce occurred:

[0,1,2,3,4].reduce(function(previousValue, currentValue, index, array){
  return previousValue + currentValue;
});

The callback would be invoked four times, with the arguments and return values in each call being as follows:

The value returned by reduce would be that of the last callback invocation (10).

If you were to provide an initial value as the second argument to reduce, the result would look like this:

[0,1,2,3,4].reduce(function(previousValue, currentValue, index, array){
  return previousValue + currentValue;
}, 10);

The value returned by reduce this time would be, of course, 20.

Example: Sum up all values within an array:

var total = [0, 1, 2, 3].reduce(function(a, b) {
    return a + b;
});
// total == 6

Example: Flatten an array of arrays:

var flattened = [[0, 1], [2, 3], [4, 5]].reduce(function(a, b) {
    return a.concat(b);
});
// flattened is [0, 1, 2, 3, 4, 5]

NOTE: This method is part of the ECMAScript 5 standard.

Parameters

Returns

reduceRight ( callback, [initialValue] ) : Mixed

Applies a function simultaneously against two values of the array (from right-to-left) as to reduce it to a single value.

reduceRight executes the callback function once for each element present in the array, excluding holes in the array.

The first time the callback is called, previousValue and currentValue can be one of two values. If initialValue is provided in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was provided, then previousValue will be equal to the last value in the array and currentValue will be equal to the second-to-last value.

Some example run-throughs of the function would look like this:

[0, 1, 2, 3, 4].reduceRight(function(previousValue, currentValue, index, array) {
    return previousValue + currentValue;
});

// First call
previousValue = 4, currentValue = 3, index = 3

// Second call
previousValue = 7, currentValue = 2, index = 2

// Third call
previousValue = 9, currentValue = 1, index = 1

// Fourth call
previousValue = 10, currentValue = 0, index = 0

// array is always the object [0,1,2,3,4] upon which reduceRight was called

// Return Value: 10

And if you were to provide an initialValue, the result would look like this:

[0, 1, 2, 3, 4].reduceRight(function(previousValue, currentValue, index, array) {
    return previousValue + currentValue;
}, 10);

// First call
previousValue = 10, currentValue = 4, index = 4

// Second call
previousValue = 14, currentValue = 3, index = 3

// Third call
previousValue = 17, currentValue = 2, index = 2

// Fourth call
previousValue = 19, currentValue = 1, index = 1

// Fifth call
previousValue = 20, currentValue = 0, index = 0

// array is always the object [0,1,2,3,4] upon which reduceRight was called

// Return Value: 20

NOTE: This method is part of the ECMAScript 5 standard.

Parameters

Returns

reverse Array

Reverses the order of the elements of an array -- the first becomes the last, and the last becomes the first.

The reverse method transposes the elements of the calling array object in place, mutating the array, and returning a reference to the array.

The following example creates an array myArray, containing three elements, then reverses the array.

var myArray = ["one", "two", "three"];
myArray.reverse();

This code changes myArray so that:

• myArray[0] is "three"
• myArray[1] is "two"
• myArray[2] is "one"

Returns

shift Object

Removes the first element from an array and returns that element.

The shift method removes the element at the zeroeth index and shifts the values at consecutive indexes down, then returns the removed value.

shift is intentionally generic; this method can be called or applied to objects resembling arrays. Objects which do not contain a length property reflecting the last in a series of consecutive, zero-based numerical properties may not behave in any meaningful manner.

The following code displays the myFish array before and after removing its first element. It also displays the removed element:

// assumes a println function is defined
var myFish = ["angel", "clown", "mandarin", "surgeon"];
println("myFish before: " + myFish);
var shifted = myFish.shift();
println("myFish after: " + myFish);
println("Removed this element: " + shifted);

This example displays the following:

myFish before: angel,clown,mandarin,surgeon
myFish after: clown,mandarin,surgeon
Removed this element: angel

Returns

slice ( begin, end ) : Array

Extracts a section of an array and returns a new array.

slice does not alter the original array, but returns a new "one level deep" copy that contains copies of the elements sliced from the original array. Elements of the original array are copied into the new array as follows:

• For object references (and not the actual object), slice copies object references into the new array. Both the original and new array refer to the same object. If a referenced object changes, the changes are visible to both the new and original arrays.
• For strings and numbers (not String and Number objects), slice copies strings and numbers into the new array. Changes to the string or number in one array does not affect the other array.

If a new element is added to either array, the other array is not affected.

Using slice

In the following example, slice creates a new array, newCar, from myCar. Both include a reference to the object myHonda. When the color of myHonda is changed to purple, both arrays reflect the change.

// Using slice, create newCar from myCar.
var myHonda = { color: "red", wheels: 4, engine: { cylinders: 4, size: 2.2 } };
var myCar = [myHonda, 2, "cherry condition", "purchased 1997"];
var newCar = myCar.slice(0, 2);

// Print the values of myCar, newCar, and the color of myHonda
//  referenced from both arrays.
print("myCar = " + myCar.toSource());
print("newCar = " + newCar.toSource());
print("myCar[0].color = " + myCar[0].color);
print("newCar[0].color = " + newCar[0].color);

// Change the color of myHonda.
myHonda.color = "purple";
print("The new color of my Honda is " + myHonda.color);

// Print the color of myHonda referenced from both arrays.
print("myCar[0].color = " + myCar[0].color);
print("newCar[0].color = " + newCar[0].color);

This script writes:

myCar = [{color:"red", wheels:4, engine:{cylinders:4, size:2.2}}, 2, "cherry condition",
"purchased 1997"]
newCar = [{color:"red", wheels:4, engine:{cylinders:4, size:2.2}}, 2]
myCar[0].color = red
newCar[0].color = red
The new color of my Honda is purple
myCar[0].color = purple
newCar[0].color = purple

Parameters

Returns

some ( callback, [thisObject] ) : Boolean

Tests whether some element in the array passes the test implemented by the provided function.

some executes the callback function once for each element present in the array until it finds one where callback returns a true value. If such an element is found, some immediately returns true. Otherwise, some returns false. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.

If a thisObject parameter is provided to some, it will be used as the this for each invocation of the callback. If it is not provided, or is null, the global object associated with callback is used instead.

some does not mutate the array on which it is called.

The range of elements processed by some is set before the first invocation of callback. Elements that are appended to the array after the call to some begins will not be visited by callback. If an existing, unvisited element of the array is changed by callback, its value passed to the visiting callback will be the value at the time that some visits that element's index; elements that are deleted are not visited.

The following example tests whether some element in the array is bigger than 10.

function isBigEnough(element, index, array) {
  return (element >= 10);
}
var passed = [2, 5, 8, 1, 4].some(isBigEnough);
// passed is false
passed = [12, 5, 8, 1, 4].some(isBigEnough);
// passed is true

NOTE: This method is part of the ECMAScript 5 standard.

Parameters

Returns

sort ( compareFunction ) : Array

Sorts the elements of an array.

If compareFunction is not supplied, elements are sorted by converting them to strings and comparing strings in lexicographic ("dictionary" or "telephone book," not numerical) order. For example, "80" comes before "9" in lexicographic order, but in a numeric sort 9 comes before 80.

If compareFunction is supplied, the array elements are sorted according to the return value of the compare function. If a and b are two elements being compared, then: If compareFunction(a, b) is less than 0, sort a to a lower index than b. If compareFunction(a, b) returns 0, leave a and b unchanged with respect to each other, but sorted with respect to all different elements. Note: the ECMAscript standard does not guarantee this behaviour, and thus not all browsers respect this. If compareFunction(a, b) is greater than 0, sort b to a lower index than a. compareFunction(a, b) must always returns the same value when given a specific pair of elements a and b as its two arguments. If inconsistent results are returned then the sort order is undefined

So, the compare function has the following form:

function compare(a, b)
{
    if (a is less than b by some ordering criterion)
        return -1;
    if (a is greater than b by the ordering criterion)
       return 1;
    // a must be equal to b
    return 0;
}

To compare numbers instead of strings, the compare function can simply subtract b from a:

function compareNumbers(a, b)
{
return a - b;
}

The sort() method can be conveniently used with closures:

var numbers = [4, 2, 5, 1, 3];
numbers.sort(function(a, b) {
    return a - b;
});
print(numbers);

Parameters

Returns

splice ( index, howMany, elements ) : Array

Adds and/or removes elements from an array.

If you specify a different number of elements to insert than the number you're removing, the array will have a different length at the end of the call.

// assumes a print function is defined
var myFish = ["angel", "clown", "mandarin", "surgeon"];
print("myFish: " + myFish);

var removed = myFish.splice(2, 0, "drum");
print("After adding 1: " + myFish);
print("removed is: " + removed);

removed = myFish.splice(3, 1);
print("After removing 1: " + myFish);
print("removed is: " + removed);

removed = myFish.splice(2, 1, "trumpet");
print("After replacing 1: " + myFish);
print("removed is: " + removed);

removed = myFish.splice(0, 2, "parrot", "anemone", "blue");
print("After replacing 2: " + myFish);
print("removed is: " + removed);

This script displays:

myFish: angel,clown,mandarin,surgeon
After adding 1: angel,clown,drum,mandarin,surgeon
removed is:
After removing 1: angel,clown,drum,surgeon
removed is: mandarin
After replacing 1: angel,clown,trumpet,surgeon
removed is: drum
After replacing 2: parrot,anemone,blue,trumpet,surgeon
removed is: angel,clown

Parameters

Returns

toString String

Returns a string representing the array and its elements. Overrides the Object.prototype.toString method.

The Array object overrides the toString method of Object. For Array objects, the toString method joins the array and returns one string containing each array element separated by commas. For example, the following code creates an array and uses toString to convert the array to a string.

var monthNames = new Array("Jan","Feb","Mar","Apr");
myVar = monthNames.toString(); // assigns "Jan,Feb,Mar,Apr" to myVar

JavaScript calls the toString method automatically when an array is to be represented as a text value or when an array is referred to in a string concatenation.

Returns

unshift ( elements ) : Number

Adds one or more elements to the front of an array and returns the new length of the array.

The unshift method inserts the given values to the beginning of an array-like object.

unshift is intentionally generic; this method can be called or applied to objects resembling arrays. Objects which do not contain a length property reflecting the last in a series of consecutive, zero-based numerical properties may not behave in any meaningful manner.

The following code displays the myFish array before and after adding elements to it.

// assumes a println function exists
myFish = ["angel", "clown"];
println("myFish before: " + myFish);
unshifted = myFish.unshift("drum", "lion");
println("myFish after: " + myFish);
println("New length: " + unshifted);

This example displays the following:

myFish before: ["angel", "clown"]
myFish after: ["drum", "lion", "angel", "clown"]
New length: 4

Parameters

Returns