Generic options parameter handling.

Usage no npm install needed!

<script type="module">
  import assertOptions from '';



Smart options handling, with one line of code:

  • throw detailed error on invalid options
  • set default values for missing options

Strongly-typed, built with TypeScript 4.x strict mode, for JavaScript clients.

Build Status Coverage Status


  • Passing in invalid or misspelled option names is one of the most common errors in JavaScript.
  • Assigning defaults is the most common operation for methods that take options.

This module automates proper options handling - parsing and setting defaults where needed.

Although this library is implemented in TypeScript, its objective is mainly to help JavaScript clients, because TypeScript itself can handle invalid options and defaults natively.


$ npm i assert-options


const { assertOptions } = require('assert-options');

function functionWithOptions(options) {
    options = assertOptions(options, {first: 123, second: null});
    // options is a safe object here, with all missing defaults set.

When default values are not needed, you can just use an array of strings:

function functionWithOptions(options) {
    options = assertOptions(options, ['first', 'second']);
    // the result is exactly the same as using the following:
    // options = assertOptions(options, {first: undefined, second: undefined});
    // options is a safe object here, without defaults.


assertOptions(options, defaults) => {}

  • When options is null/undefined, new {} is returned, applying defaults as specified.

  • When options contains an unknown property, Error Option "name" is not recognized. is thrown.

  • When a property in options is missing or undefined, its value is set from the defaults, provided it is available and its value is not undefined.

  • When options is not null/undefined, it must be of type object, or else TypeError is thrown: Invalid "options" parameter: value.

  • Parameter defaults is required, as a non-null object or an array of strings, or else TypeError is thrown: Invalid "defaults" parameter: value.