Assert Overview

The assert package provides runtime assertion functions that work in any JavaScript runtime. The API is compatible with Node.js node:assert, so you can write assertions that run identically in Node.js, Deno, Bun, Cloudflare Workers, and browsers.

When to Use This Package

• Writing tests with the test package
• Adding runtime invariant checks to application code
• Validating preconditions in functions that should fail fast on bad input

Quick Example

import { assert, equal, deepEqual, throws } from 'remix/assert'

// Truthy assertion
assert(user != null, 'User must exist')

// Equality
equal(status, 200)
deepEqual(result, { name: 'Alice', age: 30 })

// Exception testing
throws(() => {
  JSON.parse('not json')
}, SyntaxError)

Available Assertions

Function	What It Checks
assert(value, message?)	Value is truthy
ok(value, message?)	Alias for assert
equal(actual, expected)	Loose equality (==)
notEqual(actual, expected)	Loose inequality (!=)
strictEqual(actual, expected)	Strict equality (===)
notStrictEqual(actual, expected)	Strict inequality (!==)
deepEqual(actual, expected)	Deep structural equality
notDeepEqual(actual, expected)	Deep structural inequality
throws(fn, expected?)	Function throws an error
doesNotThrow(fn)	Function does not throw
rejects(fn, expected?)	Async function rejects
doesNotReject(fn)	Async function does not reject
match(string, regexp)	String matches a regular expression
doesNotMatch(string, regexp)	String does not match
fail(message?)	Always throws -- marks unreachable code

Key Concepts

• AssertionError -- All assertion failures throw an AssertionError with a descriptive message. Test runners catch this to report failures.
• Cross-runtime -- Unlike node:assert, this package works in every JavaScript environment without polyfills.
• Compatible API -- If you already know node:assert, you know this package. The function signatures are the same.

Related

• test -- Test runner that uses these assertions via t.assert
• API Reference -- Full API documentation