prefer-promise-reject-errors.md

Expected Behavior: prefer-promise-reject-errors Rule

This rule enforces the use of Error objects in Promise.reject() calls instead of primitive values.

Examples

String Values

  function fetchUser(id: string) {
    if (!id) {
-     return Promise.reject('Invalid user ID')
+     return Promise.reject(new Error('Invalid user ID'))
    }
    if (id === 'admin') {
-     return Promise.reject('Access denied')
+     return Promise.reject(new Error('Access denied'))
    }
    return Promise.resolve({ id, name: 'John' })
  }

Number Values

  function validateInput(input: any) {
    if (typeof input !== 'string') {
-     return Promise.reject(400)
+     return Promise.reject(new Error('400'))
    }
    if (input.length < 3) {
-     return Promise.reject(422)
+     return Promise.reject(new Error('422'))
    }
    return Promise.resolve(input)
  }

Boolean Values

  function processRequest(data: any) {
    if (!data) {
-     return Promise.reject(false)
+     return Promise.reject(new Error('No data provided'))
    }
    if (data.invalid) {
-     return Promise.reject(true)
+     return Promise.reject(new Error('Invalid data format'))
    }
    return Promise.resolve(data)
  }

Null/Undefined Values

  function getConfig() {
    const config = loadConfig()
    if (!config) {
-     return Promise.reject(null)
+     return Promise.reject(new Error('Configuration not found'))
    }
    if (config.error) {
-     return Promise.reject(undefined)
+     return Promise.reject(new Error('Configuration error'))
    }
    return Promise.resolve(config)
  }

Arrow Functions

  const validateUser = (user: any) => {
    if (!user.name) {
-     return Promise.reject('Name is required')
+     return Promise.reject(new Error('Name is required'))
    }
    if (user.age < 18) {
-     return Promise.reject('User must be 18 or older')
+     return Promise.reject(new Error('User must be 18 or older'))
    }
    return Promise.resolve(user)
  }

Rule Scope

This rule only applies to:

• ✅ Promise.reject() calls with primitive values:
  • String literals ('error message')
  • Number literals (404, 500)
  • Boolean literals (true, false)
  • null and undefined
  • Variables containing primitive values
  • Expressions that evaluate to primitive values

This rule does NOT apply to:

• ❌ Promise.reject() with Error objects (new Error(), new TypeError())
• ❌ Promise.reject() with Error subclasses (new RangeError(), new SyntaxError())
• ❌ Promise.reject() with custom Error classes
• ❌ Promise.resolve() calls
• ❌ Other Promise methods (.then(), .catch(), .finally())
• ❌ throw statements
• ❌ Non-Promise rejections

Auto-fix Behavior

The rule provides auto-fix suggestions that wrap primitive values with new Error():

• Promise.reject('error') → Promise.reject(new Error('error'))
• Promise.reject(404) → Promise.reject(new Error(404))
• Promise.reject(true) → Promise.reject(new Error(true))
• Promise.reject(null) → Promise.reject(new Error(null))
• Promise.reject(variable) → Promise.reject(new Error(variable))
• Promise.reject(expression) → Promise.reject(new Error(expression))