Disallow Sparse Arrays (no-sparse-arrays)
Sparse arrays contain empty slots, most frequently due to multiple commas being used in an array literal, such as:
var items = [,,];
While the items
array in this example has a length
of 2, there are actually no values in items[0]
or items[1]
. The fact that the array literal is valid with only commas inside, coupled with the length
being set and actual item values not being set, make sparse arrays confusing for many developers. Consider the following:
var colors = [ "red",, "blue" ];
In this example, the colors
array has a length
of 3. But did the developer intend for there to be an empty spot in the middle of the array? Or is it a typo?
The confusion around sparse arrays defined in this manner is enough that it’s recommended to avoid using them unless you are certain that they are useful in your code.
Rule Details
This rule aims to eliminate sparse arrays that are defined by extra commas.
The following patterns are considered problems:
/*eslint no-sparse-arrays: 2*/
var items = [,]; /*error Unexpected comma in middle of array.*/
var colors = [ "red",, "blue" ]; /*error Unexpected comma in middle of array.*/
The following patterns are not considered problems:
/*eslint no-sparse-arrays: 2*/
var items = [];
var items = new Array(23);
// trailing comma is okay
var colors = [ "red", "blue", ];
When Not To Use It
If you want to use sparse arrays, then it is safe to disable this rule.
Further Reading
Version
This rule was introduced in ESLint 0.4.0.