prefer-const-assertions.md

Expected Behavior: prefer-const-assertions Rule

This rule enforces the use of const assertions (as const) on array and object literals for better type inference, but only when they are used in top-level contexts like variable declarations, return statements, and assignments.

Examples

Basic Array Literal

- const colors = ['red', 'green', 'blue']
+ const colors = ['red', 'green', 'blue'] as const

Basic Object Literal

- const config = { apiUrl: 'https://api.example.com', timeout: 5000 }
+ const config = { apiUrl: 'https://api.example.com', timeout: 5000 } as const

Return Statement Array

  function getColors() {
-   return ['red', 'green', 'blue']
+   return ['red', 'green', 'blue'] as const
  }

Return Statement Object

  function getConfig() {
-   return { apiUrl: 'https://api.example.com', timeout: 5000 }
+   return { apiUrl: 'https://api.example.com', timeout: 5000 } as const
  }

Arrow Function Return

- const getData = () => ({ id: 1, name: 'test' })
+ const getData = () => ({ id: 1, name: 'test' } as const)

Rule Scope

This rule only applies to:

• ✅ Array literals ([]) in top-level contexts:

  • Variable declarations (const arr = [...])
  • Return statements (return [...])
  • Assignment expressions (arr = [...])
  • Function expressions (() => [...])

• ✅ Object literals ({}) in top-level contexts:

  • Variable declarations (const obj = {...})
  • Return statements (return {...})
  • Assignment expressions (obj = {...})
  • Function expressions (() => ({...}))

This rule does NOT apply to:

• ❌ Empty arrays ([]) or objects ({})
• ❌ Arrays/objects in function parameters (function test(arr = [...]))
• ❌ Arrays/objects in arrow function parameters (const test = (arr = [...]) => ...)
• ❌ Arrays/objects in nested contexts (inside other arrays/objects)
• ❌ Arrays/objects that already have const assertion ([...] as const)

Auto-fix Behavior

The rule provides auto-fix suggestions that add as const to array and object literals:

• [1, 2, 3] → [1, 2, 3] as const
• { name: 'test' } → { name: 'test' } as const
• ['a', 'b', 'c'] → ['a', 'b', 'c'] as const
• { id: 1, active: true } → { id: 1, active: true } as const