JavaScript Array Methods

JavaScript Arrays

JavaScript arrays are used to store multiple values in a single variable.

Displaying Arrays

In this tutorial we will use a script to display arrays inside a <p> element with id="demo":

Example
<p id="demo"></p>

The first line (in the script) creates an array named cars.

The second line "finds" the element with id="demo", and "displays" the array in the "innerHTML" of it.

Example
var cars = ["Saab", "Volvo", "BMW"];

Spaces and line breaks are not important. A declaration can span multiple lines:

Example
var cars = [
    "Saab",
    "Volvo",
    "BMW"
];

Never put a comma after the last element (like "BMW",). The effect is inconsistent across browsers.

What is an Array?

An array is a special variable, which can hold more than one value at a time.

If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

var car1 = "Saab";
var car2 = "Volvo";
var car3 = "BMW";

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?

The solution is an array!

An array can hold many values under a single name, and you can access the values by referring to an index number.

Creating an Array

Using an array literal is the easiest way to create a JavaScript Array.

Syntax:

var array-name = [item1, item2, ...];

Example:

var cars = ["Saab", "Volvo", "BMW"];

Using the JavaScript Keyword new

The following example also creates an Array, and assigns values to it:

Example

var cars = new Array("Saab", "Volvo", "BMW");

The two examples above do exactly the same. There is no need to use new Array().
For simplicity, readability and execution speed, use the first one (the array literal method).

Access the Elements of an Array

You refer to an array element by referring to the index number.

This statement accesses the value of the first element in cars:

var name = cars[0];

This statement modifies the first element in cars:

cars[0] = "Opel";

[0] is the first element in an array. [1] is the second. Array indexes start with 0.

You Can Have Different Objects in One Array

JavaScript variables can be objects. Arrays are special kinds of objects.

Because of this, you can have variables of different types in the same Array.

You can have objects in an Array. You can have functions in an Array. You can have arrays in an Array:

myArray[0] = Date.now;
myArray[1] = myFunction;
myArray[2] = myCars;

Arrays are Objects

Arrays are a special type of objects. The typeof operator in JavaScript returns "object" for arrays.

But, JavaScript arrays are best described as arrays.

Arrays use numbers to access its "elements". In this example, person[0] returns John:

Array:

var person = ["John", "Doe", 46];

Objects use names to access its "members". In this example, person.firstName returns John:

Object:

var person = {firstName:"John", lastName:"Doe", age:46};

Array Properties and Methods

The real strength of JavaScript arrays are the built-in array properties and methods:

Examples

var x = cars.length; // The length property returns the number of elements in cars
var y = cars.sort(); // The sort() method sort cars in alphabetical order

Array methods are covered in the next chapter.

The length Property

The length property of an array returns the length of an array (the number of array elements).

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.length; // the length of fruits is 4

The length property is always one more than the highest array index.

Adding Array Elements

The easiest way to add a new element to an array is using the push method:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.push("Lemon"); // adds a new element (Lemon) to fruits

New element can also be added to an array using the length property:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[fruits.length] = "Lemon"; // adds a new element (Lemon) to fruits

Adding elements with high indexes can create undefined "holes" in an array:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[10] = "Lemon"; // adds a new element (Lemon) to fruits

Looping Array Elements

The best way to loop through an array, is using a "for" loop:

Example

var index;
var fruits = ["Banana", "Orange", "Apple", "Mango"];
for (index = 0; index < fruits.length; index++) {
text += fruits[index];
}

Associative Arrays?

Many programming languages support arrays with named indexes.

Arrays with named indexes are called associative arrays (or hashes).

JavaScript does not support arrays with named indexes.

In JavaScript, arrays always use numbered indexes.

Example:

var person = [];
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
var x = person.length; // person.length will return 3
var y = person[0]; // person[0] will return "John"

WARNING !!
If you use a named index, JavaScript will redefine the array to a standard object.
After that, all array methods and properties will produce incorrect results.

Example:

var person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
var x = person.length; // person.length will return 0
var y = person[0]; // person[0] will return undefined

The Difference Between Arrays and Objects?

In JavaScript, arrays use numbered indexes.

In JavaScript, objects use named indexes.

Arrays are a special kind of objects, with numbered indexes.

When to Use Arrays? When to use Objects?

JavaScript does not support associative arrays.

You should use objects when you want the element names to be strings (text).

You should use arrays when you want the element names to be numbers.

Avoid new Array()

There is no need to use the JavaScript's built-in array constructor new Array().

Use [] instead.

These two different statements both create a new empty array named points:

var points = new Array(); // Bad
var points = []; // Good

These two different statements both create a new array containing 6 numbers:

var points = new Array(40, 100, 1, 5, 25, 10) // Bad
var points = [40, 100, 1, 5, 25, 10]; // Good

The new keyword complicates your code and produces nasty side effects:

var points = new Array(40, 100); // Creates an array with two elements (40 and 100)

What if I remove one of the elements?

var points = new Array(40); // Creates an array with 40 undefined elements !!!!!

How to Recognize an Array?

A common question is: How do I know if a variable is an array?

The problem is that the JavaScript operator typeof returns "object":

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

typeof fruits; // typeof returns object

The typeof operator returns object because a JavaScript array is an object.

To solve this problem you can create your own isArray() function:

function isArray(myArray) {
return myArray.constructor.toString().indexOf("Array") > -1;
}

The function above always return true if the argument is an array.

Or more precisely: it returns true if the object prototype of the argument is "[object array]".

JavaScript Array Methods

The strength of JavaScript arrays lies in the array methods.

Converting Arrays to Strings

In JavaScript, all objects have the valueOf() and toString() methods.

The valueOf() method is the default behavior for an array. It returns an array as a string:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.valueOf();

For JavaScript arrays, valueOf() and toString() are equal.

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.toString();

The join() method also joins all array elements into a string.

It behaves just like toString(), but you can specify the separator:

Example

<p id="demo"></p>

<script>
var fruits = ["Banana", "Orange","Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.join(" * ");
</script>

Popping and Pushing

When you work with arrays, it is easy to remove elements and add new elements.

This is what popping and pushing is: Popping items out of an array, or pushing items into an array.

The pop() method removes the last element from an array:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.pop(); // Removes the last element ("Mango") from fruits

The push() method adds a new element to an array (at the end):

Remember: [0] is the first element in an array. [1] is the second. Array indexes start with 0.

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.push("Kiwi"); // Adds a new element ("Kiwi") to fruits

The pop() method returns the string that was "popped out".

The push() method returns the new array length.

Shifting Elements

Shifting is equivalent to popping, working on the first element instead of the last.

The shift() method removes the first element of an array, and "shifts" all other elements one place down.

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.shift(); // Removes the first element "Banana" from fruits

The unshift() method adds a new element to an array (at the beginning), and "unshifts" older elements:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.unshift("Lemon"); // Adds a new element "Lemon" to fruits

The shift() method returns the string that was "shifted out".

The unshift() method returns the new array length.

Changing Elements

Array elements are accessed using their index number:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[0] = "Kiwi"; // Changes the first element of fruits to "Kiwi"

The length property provides an easy way to append a new element to an array:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[fruits.length] = "Kiwi"; // Appends "Kiwi" to fruit

Deleting Elements

Since JavaScript arrays are objects, elements can be deleted by using the JavaScript operator delete:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
delete fruits[0]; // Changes the first element in fruits to undefined

	Using delete on array elements leaves undefined holes in the array. Use pop() or splice() instead.

Splicing an Array

The splice() method can be used to add new items to an array:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(2, 0, "Lemon", "Kiwi");

The first parameter (2) defines the position where new elements should be added (spliced in).

The second parameter (0) defines how many elements should be removed.

The rest of the parameters ("Lemon" , "Kiwi") define the new elements to be added.

Using splice() to Remove Elements

With clever parameter setting, you can use splice() to remove elements without leaving "holes" in the array:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(0, 1); // Removes the first element of fruits

The first parameter (0) defines the position where new elements should be added (spliced in).

The second parameter (1) defines how many elements should be removed.

The rest of the parameters are omitted. No new elements will be added.

Sorting an Array

The sort() method sorts an array alphabetically:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.sort(); // Sorts the elements of fruits

Reversing an Array

The reverse() method reverses the elements in an array.

You can use it to sort an array in descending order:

Example

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.sort(); // Sorts the elements of fruits
fruits.reverse(); // Reverses the order of the elements

Numeric Sort

By default, the sort() function sorts values as strings.

This works well for strings ("Apple" comes before "Banana").

However, if numbers are sorted as strings, "25" is bigger than "100", because "2" is bigger than "1".

Because of this, the sort() method will produce incorrect result when sorting numbers.

You can fix this by providing a compare function:

Example

var points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return a-b});

Use the same trick to sort an array descending:

Example

var points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return b-a});

The Compare Function

The purpose of the compare function is to define an alternative sort order.

The compare function should return a negative, zero, or positive value, depending on the arguments:

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

When the sort() function compares two values, it sends the values to the compare function, and sorts the values according to the returned (negative, zero, positive) value.

Example:

When comparing 40 and 100, the sort() method calls the compare function(40,100).

The function calculates 40-100, and returns -60 (a negative value).

The sort function will sort 40 as a value lower than 100.

Find the Highest (or Lowest) Value

How to find the highest value in an array?

Example

var points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return b-a});
// now points[0] contains the highest value

And the lowest:

Example

var points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return a-b});
// now points[0] contains the lowest value

Joining Arrays

The concat() method creates a new array by concatenating two arrays:

Example

var myGirls = ["Cecilie", "Lone"];
var myBoys = ["Emil", "Tobias","Linus"];
var myChildren = myGirls.concat(myBoys); // Concatenates (joins) myGirls and myBoys

The concat() method can take any number of array arguments:

Example

var arr1 = ["Cecilie", "Lone"];
var arr2 = ["Emil", "Tobias","Linus"];
var arr3 = ["Robin", "Morgan"];
var myChildren = arr1.concat(arr2, arr3); // Concatenates arr1 with arr2 and arr3

Slicing an Array

The slice() method slices out a piece of an array into a new array:

Example

var fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = fruits.slice(1, 3);

The slice() method selects elements starting at the start argument, and ends at, but does not include, the end argument.

If the end argument is omitted, the slice() method slices out the rest of the array:

Example

var fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = fruits.slice(1);

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