What’s an array in JavaScript?

In the simplest of terms, it’s a list:

var list = ['item1', 'item2', 'item3'];
// array created using the literal notation

An array can hold any type of value:

var array = [
    'string',
    Number(1),
    Boolean(true),
    { object: '' },
    [ 'array' ],
    Function,
    null,
    undefined
];

Arrays are also zero-indexed, which means the first element starts at 0:

var array = ['first', 'second'];
array[0]; // 'first'
array[1]; // 'second'

Then, what’s the index of the last element?

var array = ['first', 'last'];
array.length; // 2
var lastIndex = array.length - 1;
array[lastIndex]; // 'last'

Although an array is an instance of Array, it’s actually a type Object:

var arr = [];
arr.constructor;      // [Function: Array]
arr instanceof Array; // true
typeof arr;           // 'object'

Therefore, you can think of arrays as a hash:

var array = ['apple', 'banana', 'carrot'];
var associativeArray  = {
    '0': 'apple',
    '1': 'banana',
    '2': 'carrot'
};

You can specifically set an array element to a value:

var array = [];
array[0] = 'initial';
array; // [ 'initial' ]
array[0] = 'override';
array; // [ 'override' ]

If it’s set to an index beyond the array’s length, the unset elements will be undefined:

var array = ['one'];
var array[2] = 'three';
array; // [ 'one', undefined, 'three' ]

And array elements can also be deleted:

var array = ['one', 'two', 'three'];
delete array[2];
array; // [ 'one', 'two', undefined ]

What to learn more about array methods? Check out the post for more information.