Javascript Arrays

An array is a basic data structure in JavaScript.

Creating an array is easy:

// Create an array using the bracket syntax
demoArray = ['apple', 'orange', 'banana', 'watermelon', 'strawberry'];

We can now access our array via the demoArray variable.

The first thing we’ll want to do is access the data within the array.

Arrays are numerically indexed, so let’s access the first element via the numerical index:

// Access the first element of the array.
// This should display "apple"
demoArray[0];

Wait, wha? Why demoArray[0] and not demoArray[1]?

Arrays are zero-indexed. The first element is available at 0. This is a common gotcha. Access the second element with demoArray[1], the third with demoArray[2] and so forth.

What if we don’t know how many elements we added to the array? You can get its length by accessing the "length" property of the array:

// Show length of the array.
// This should display 5
demoArray.length;

So we should be able to access the last element of an array like this…​right?

// Access the last element of the array, right??
demoArray[demoArray.length];
// Nope... array.length != last element

To access the last element of an array we need to do this:

// *Actually* access the last element of the array.
demoArray[demoArray.length - 1];

So what else can we do with arrays? Arrays are a basic building block for any application.

You might want to add an element to existing array—​if so, push() is your friend:

// Add an element to the end of the array
// returns the new length of the array
demoArray.push('mango');

You can also use push() to append many elements to the end of the array:

demoArray.push('grape', 'rasberry', 'blackberry');

unshift() lets you add elements to the beginning of an array:

demoArray.unshift('lemon');

Let’s take a look at our array again:

demoArray;

Let’s make it a little more orderly with sort():

// Sort array
// Our elements should now appear in alphabetical order.
demoArray.sort();

That’s better. The array is now in alphabetical order.

We can also remove elements from the array with shift(), pop(), and splice().

// shift() removes and returns the first element of an array
demoArray.length;
demoArray.shift();
// You'll see that the demo array is one element shorter now:
'The demoArray has ' + demoArray.length + ' elements now.';
// pop() removes and returns last element of an array:
demoArray.pop();
// Again, You'll see that the demo array is one element shorter:
'The demoArray has ' + demoArray.length + ' elements now.';
// splice() removes and returns elements from anywhere in the array.
// remove third element in the array. Will return an array of removed elements.
demoArray.splice(3, 1);

Let’s take a look at our array again:

demoArray;

This is just beginning of what you can do with arrays.

Learn more about them at the Mozilla Developer Network.

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