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:

在这个例子中,item 数组的 length 为 2,实际上,items[0]items[1]并没有值。数组中只有逗号是有效的,再加上 length 被设置,没有实际的值被设置,这些情况让很多开发者对稀疏数组感到困惑。考虑下面的情况:

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?

在这个例子中,colors 数值的 length 是 3。但是否是开发者想让数组中间出现一个空元素?或者只是一个书写错误?

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 disallows sparse array literals which have “holes” where commas are not preceded by elements. It does not apply to a trailing comma following the last element.


Examples of incorrect code for this rule:

错误 代码示例:

/*eslint no-sparse-arrays: "error"*/

var items = [,];
var colors = [ "red",, "blue" ];

Examples of correct code for this rule:

正确 代码示例:

/*eslint no-sparse-arrays: "error"*/

var items = [];
var items = new Array(23);

// trailing comma (after the last element) is not a problem
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


This rule was introduced in ESLint 0.4.0.

该规则在 ESLint 0.4.0 中被引入。