JavaScript: The Definitive Guide, 6th Edition

Sparse Arrays

A sparse array is one in which the elements do not have contiguous indexes starting at 0. Normally, the length property of an array specifies the number of elements in the array. If the array is sparse, the value of the length property is greater than the number of elements. Sparse arrays can be created with the Array() constructor or simply by assigning to an array index larger than the current array length.

a = new Array(5);   // No elements, but a.length is 5.
a = [];             // Create an array with no elements and length = 0.
a[1000] = 0;        // Assignment adds one element but sets length to 1001.

We’ll see later that you can also make an array sparse with the delete operator.

Arrays that are sufficiently sparse are typically implemented in a slower, more memory-efficient way than dense arrays are, and looking up elements in such an array will take about as much time as regular object property lookup.

Note that when you omit a value in an array literal (using repeated commas as in [1,,3]) the resulting array is sparse and the omitted elements simply do not exist:

var a1 = [,];           // This array has no elements and length 1
var a2 = [undefined];   // This array has one undefined element
0 in a1                 // => false: a1 has no element with index 0
0 in a2                 // => true: a2 has the undefined value at index 0

Some older implementations (such as Firefox 3) incorrectly insert the undefined values in array literals with omitted values. In these implementations [1,,3] is the same as [1,undefined,3].

Understanding ...