prefer-array-includes.md

Expected Behavior: prefer-array-includes Rule

This rule enforces the use of Array.includes() over manual indexOf() !== -1 checks for better readability and performance.

Examples

Basic Array Search

- if (arr.indexOf(item) !== -1) {
+ if (arr.includes(item)) {
    console.log('Item found')
  }

Variable Assignment

- const found = arr.indexOf(item) !== -1
+ const found = arr.includes(item)

Function Returns

  function hasItem(arr, item) {
-   return arr.indexOf(item) !== -1
+   return arr.includes(item)
  }

Arrow Function Returns

- const checkExists = (items, target) => items.indexOf(target) !== -1
+ const checkExists = (items, target) => items.includes(target)

Method Calls

- if (getArray().indexOf(getItem()) !== -1) {
+ if (getArray().includes(getItem())) {
    console.log('Found')
  }

Simple Variable Assignment

- const exists = items.indexOf(item) !== -1
+ const exists = items.includes(item)

Property Access

- if (obj.items.indexOf(item) !== -1) {
+ if (obj.items.includes(item)) {
    console.log('Item in collection')
  }

- const inList = user.favorites.indexOf(book) !== -1
+ const inList = user.favorites.includes(book)

Conditional Expressions

- const result = arr.indexOf(item) !== -1 ? 'found' : 'not found'
+ const result = arr.includes(item) ? 'found' : 'not found'

- const status = items.indexOf(target) !== -1 && 'active'
+ const status = items.includes(target) && 'active'

Rule Scope

This rule only applies to:

• ✅ array.indexOf(item) !== -1 comparisons
• ✅ Variable assignments with indexOf() !== -1
• ✅ Return statements with indexOf() !== -1
• ✅ Conditional expressions with indexOf() !== -1

This rule does NOT apply to:

• ❌ array.indexOf(item) === -1 (not found checks)
• ❌ array.indexOf(item) > -1 or array.indexOf(item) >= 0
• ❌ Already using Array.includes()
• ❌ Other array methods (find, some, every, etc.)
• ❌ indexOf() with assignment or other operators

Covered Operations

This rule applies to the following operations:

• ✅ arr.indexOf(item) !== -1 → arr.includes(item)
• ✅ items.indexOf(target) !== -1 → items.includes(target)
• ✅ getArray().indexOf(getItem()) !== -1 → getArray().includes(getItem())

Auto-fix Behavior

The rule provides auto-fix suggestions that convert indexOf() !== -1 patterns to Array.includes():

• arr.indexOf(item) !== -1 → arr.includes(item)
• items.indexOf(target) !== -1 → items.includes(target)
• getArray().indexOf(getItem()) !== -1 → getArray().includes(getItem())