Set-error-props NPM

Properly update an error's properties.

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Features

Prevents overriding error core properties (name, message, etc.)
Protects against prototype pollution
Prevents overriding existing properties
Copies another error's properties
Can set properties as non-enumerable
Preserves properties descriptors (enumerable, writable, configurable, get/set)
Exception-safe: this only throws syntax errors
Strict TypeScript typing of the return value

Example

import setErrorProps from 'set-error-props'

const error = new Error('one')
setErrorProps(error, { prop: true, message: 'two' })
console.log(error.prop) // true
console.log(error.message) // 'one': message is readonly

Install

npm install set-error-props

This package works in both Node.js >=18.18.0 and browsers.

This is an ES module. It must be loaded using an import or import() statement, not require(). If TypeScript is used, it must be configured to output ES modules, not CommonJS.

API

setErrorProps(error, props, options?)

error Error | object\ props Error | object\ options Options?\ Return value: Error

Assigns props to error, then returns error.

Options

soft

Type: boolean\ Default: false

Prevents overriding existing properties.

Usage

Error core properties

const error = new Error('one')
setErrorProps(error, { message: 'two' })
console.log(error.message) // 'one'

Prototype pollution

const error = new Error('one')
setErrorProps(error, { toString: () => 'injected' })
console.log(error.toString()) // 'Error: one'
console.log(Error.prototype.toString()) // 'Error'

Overriding protection

const error = new Error('message')
error.one = true
setErrorProps(error, { one: false, two: true }, { soft: true })
console.log(error.one) // true
console.log(error.two) // true

Error copy

const error = new Error('one')
const secondError = new Error('two')
secondError.prop = true
setErrorProps(error, secondError)
console.log(error.message) // 'one'
console.log(error.prop) // true

Non-enumerable properties

const error = new Error('message')

// Properties that start with `_` are not enumerable
setErrorProps(error, { _one: true, two: true })

console.log(error._one) // true
console.log(error.two) // true
console.log(Object.keys(error)) // ['two']
console.log(error) // Prints `two` but not `_one`

Descriptors

const error = new Error('message')
Object.defineProperty(error, 'prop', {
  value: false,
  enumerable: false,
  writable: true,
  configurable: true,
})
setErrorProps(error, { prop: true })
console.log(error.prop) // true
console.log(Object.getOwnPropertyDescriptor(error, 'prop').enumerable) // false

Exception safety

const error = new Proxy(new Error('message'), {
  set: () => {
    throw new Error('example')
  },
  defineProperty: () => {
    throw new Error('example')
  },
})
setErrorProps(error, { prop: true }) // This does not throw