Speaking JavaScript

Chapter 18. Arrays

An array is a map from indices (natural numbers, starting at zero) to arbitrary values. The values (the range of the map) are called the array’s elements. The most convenient way of creating an array is via an array literal. Such a literal enumerates the array elements; an element’s position implicitly specifies its index.

In this chapter, I will first cover basic array mechanisms, such as indexed access and the length property, and then go over array methods.

Overview

This section provides a quick overview of arrays. Details are explained later.

As a first example, we create an array arr via an array literal (see Creating Arrays) and access elements (see Array Indices):

> var arr = [ 'a', 'b', 'c' ]; // array literal
> arr[0]  // get element 0
'a'
> arr[0] = 'x';  // set element 0
> arr
[ 'x', 'b', 'c' ]

We can use the array property length (see length) to remove and append elements:

> var arr = [ 'a', 'b', 'c' ];
> arr.length
3
> arr.length = 2;  // remove an element
> arr
[ 'a', 'b' ]
> arr[arr.length] = 'd';  // append an element
> arr
[ 'a', 'b', 'd' ]

The array method push() provides another way of appending an element:

> var arr = [ 'a', 'b' ];
> arr.push('d')
3
> arr
[ 'a', 'b', 'd' ]

Arrays Are Maps, Not Tuples

The ECMAScript standard specifies arrays as maps (dictionaries) from indices to values. In other words, arrays may not be contiguous and can have holes in them. For example:

> var arr = [];
> arr[0] = 'a';
'a'
> arr[2] = 'b';
'b'
> arr
[ 'a', , 'b' ]

The preceding ...