noForIn

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JavaScript (and super languages)

Summary

Diagnostic Category: lint/nursery/noForIn
This rule doesn’t have a fix.
The default severity of this rule is information.
Sources:
- Inspired from @typescript-eslint/no-for-in-array

How to configure

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{
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  "linter": {
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    "rules": {
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      "nursery": {
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        "noForIn": "error"
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      }
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    }
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  }
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}

Description

Disallow iterating using a for-in loop.

A for-in loop (for (const i in o)) iterates over the properties of an Object. While it is legal to use for-in loops with array values, it is not common. There are several potential bugs with this:

It iterates over all enumerable properties, including non-index ones and the entire prototype chain. For example, RegExp.prototype.exec returns an array with additional properties, and for-in will iterate over them. Some libraries or even your own code may add additional methods to Array.prototype (either as polyfill or as custom methods), and if not done properly, they may be iterated over as well.
It skips holes in the array. While sparse arrays are rare and advised against, they are still possible and your code should be able to handle them.
The “index” is returned as a string, not a number. This can be caught by TypeScript, but can still lead to subtle bugs.

You may have confused for-in with for-of, which iterates over the elements of the array. If you actually need the index, use a regular for loop or the forEach method.

Examples

Invalid

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for (const i in array) {
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  console.log(i, array[i]);
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}

code-block.js:1:1 lint/nursery/noForIn ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Unexpected for-in loop.
  
  > 1 │ for (const i in array) {
      │ ^^^^^^^^^^^^^^^^^^^^^^^^
  > 2 │   console.log(i, array[i]);
  > 3 │ }
      │ ^
    4 │ 
  
  ℹ For-in loops are confusing and easy to misuse. You likely want to use a regular loop, for-of loop or forEach instead.

Valid

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for (const value of array) {
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  console.log(value);
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}

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for (let i = 0; i < array.length; i += 1) {
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  console.log(i, array[i]);
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}

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array.forEach((value, i) => {
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  console.log(i, value);
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});

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for (const [i, value] of array.entries()) {
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  console.log(i, value);
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}