noUninvokedArrayCallback (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.6
Diagnostic Category: lint/nursery/noUninvokedArrayCallback
This rule doesn’t have a fix.
The default severity of this rule is error.
Sources:
- Inspired from oxc/uninvoked-array-callback

How to configure

1
{
2
  "linter": {
3
    "rules": {
4
      "nursery": {
5
        "noUninvokedArrayCallback": "error"
6
      }
7
    }
8
  }
9
}

Description

Disallow array methods with callbacks on arrays that only have empty slots.

Calling new Array(5) or Array(5) with a single number doesn’t create an array of five undefined values. It creates an array with a length of 5 and five empty slots, which hold no value at all. Methods such as map(), filter(), and forEach() skip empty slots, so the callback passed to them never runs. For example, new Array(3).map(() => 0) gives an array with three empty slots, not [0, 0, 0].

This rule reports calls to every(), filter(), flatMap(), forEach(), map(), reduce(), reduceRight(), some(), sort(), and toSorted() with a callback, directly on an array created by calling Array with a single number.

To run the callback for every slot, fill the array first with fill(), or create it with Array.from({ length: 5 }, callback).

Examples

Invalid

1
const list = new Array(5).map(() => createElement());

code-block.js:1:27 lint/nursery/noUninvokedArrayCallback ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The callback passed to map() is never called.
  
  > 1 │ const list = new Array(5).map(() => createElement());
      │                           ^^^
    2 │ 
  
  ℹ This array only has empty slots.
  
  > 1 │ const list = new Array(5).map(() => createElement());
      │              ^^^^^^^^^^^^
    2 │ 
  
  ℹ Calling Array with a single number creates an array with that many empty slots instead of undefined values. map() doesn't call its callback for empty slots.
  
  ℹ Fill the array first, for example with new Array(length).fill(), or create it with Array.from({ length }, callback).
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

1
const list = Array(5).filter(function (item) {
2
    return item !== undefined;
3
});

code-block.js:1:23 lint/nursery/noUninvokedArrayCallback ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The callback passed to filter() is never called.
  
  > 1 │ const list = Array(5).filter(function (item) {
      │                       ^^^^^^
    2 │     return item !== undefined;
    3 │ });
  
  ℹ This array only has empty slots.
  
  > 1 │ const list = Array(5).filter(function (item) {
      │              ^^^^^^^^
    2 │     return item !== undefined;
    3 │ });
  
  ℹ Calling Array with a single number creates an array with that many empty slots instead of undefined values. filter() doesn't call its callback for empty slots.
  
  ℹ Fill the array first, for example with new Array(length).fill(), or create it with Array.from({ length }, callback).
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

1
new Array(5).forEach((_, index) => console.log(index));

code-block.js:1:14 lint/nursery/noUninvokedArrayCallback ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The callback passed to forEach() is never called.
  
  > 1 │ new Array(5).forEach((_, index) => console.log(index));
      │              ^^^^^^^
    2 │ 
  
  ℹ This array only has empty slots.
  
  > 1 │ new Array(5).forEach((_, index) => console.log(index));
      │ ^^^^^^^^^^^^
    2 │ 
  
  ℹ Calling Array with a single number creates an array with that many empty slots instead of undefined values. forEach() doesn't call its callback for empty slots.
  
  ℹ Fill the array first, for example with new Array(length).fill(), or create it with Array.from({ length }, callback).
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

Valid

1
const list = new Array(5).fill().map(() => createElement());
2
const items = Array.from({ length: 5 }, () => createElement());
3
const values = new Array(1, 2, 3).map((value) => value * 2);
4
const index = new Array(5).findIndex((item) => item === undefined);