Arrays
In JavaScript, arrays are a special type of object. Unlike regular objects, array contents
have a natural order (element 0 will always come before element 1), and keys are
numeric and sequential. Arrays support a number of useful methods that make this
data type an extremely powerful way to express information, which we will cover in
.
If you’re coming from other languages, you’ll find that arrays in JavaScript are some‐
thing of a hybrid of the efficient, indexed arrays of C and more powerful dynamic
arrays and linked lists. Arrays in JavaScript have the following properties:
• Array size is not fixed; you can add or remove elements at any time.
• Arrays are not homogeneous; each individual element can be of any type.
• Arrays are zero-based. That is, the first element in the array is element 0.
Because arrays are special types of objects with some extra func‐
tionality, you can assign non-numeric (or fractional or negative)
keys to an array. While this is possible, it contradicts the intended
purpose of arrays, can lead to confusing behavior and difficult-to-
diagnose bugs, and is best avoided.
To create an array literal in JavaScript, use square brackets, with the elements of the
array separated by commas:
const
a1
=
[
1
,
2
,
3
,
4
];
// array containing numbers
const
a2
=
[
1
,
'two'
,
3
,
null
];
// array containing mixed types
const
a3
=
[
// array on multiple lines
"What the hammer? What the chain?"
,
"In what furnace was thy brain?"
,
"What the anvil? What dread grasp"
,
"Dare its deadly terrors clasp?"
,
];
const
a4
=
[
// array containing objects
{
name
:
"Ruby"
,
hardness
:
9
},
{
name
:
"Diamond"
,
hardness
:
10
},
{
name
:
"Topaz"
,
hardness
:
8
},
];
const
a5
=
[
// array containing arrays
[
1
,
3
,
5
],
[
2
,
4
,
6
],
];
Arrays have a property
length
, which returns the number of elements in the array:
const
arr
=
[
'a'
,
'b'
,
'c'
];
arr
.
length
;
// 3
Arrays | 49
