ensured

Tiny runtime assertions that never drop your error messages.

Why

tiny-invariant removes all error messages in production builds to save a few bytes. That might be okay for internal functions, but it's terrible for applications. You lose context when something goes wrong.

ensured keeps the bundle small, preserves your messages in all environments, and provides a consistent set of helpers for runtime guarantees.

Install

npm install ensured

Usage

import { ensure, reject, expect, unreachable } from 'ensured';

// ensure — checks truthiness
ensure(count > 0, 'count must be positive');

// reject — asserts falsiness
reject(isAdmin, 'admins are not allowed here');

// expect — checks non-nullishness
const user = expect(currentUser, 'no current user');

// unreachable — for exhaustive switches
switch (status) {
	case 'idle':
	case 'loading':
	case 'done':
		break;
	default:
		unreachable(status);
}

All functions throw Error instances and preserve your message. If you pass an existing Error, it will be rethrown unchanged.

API

ensure(condition: unknown, error?: ErrorParam): asserts condition

Throws if condition is falsy.

ensure(result.ok, new Error('request failed'));

reject(condition: unknown, error?: ErrorParam): asserts condition is false

Throws if condition is truthy. This is the inverse of ensure and is handy when something must not happen.

reject(user.isBanned, 'banned users cannot perform this action');

expect<Value>(value: Value | null | undefined, error?: ErrorParam): NonNullable<Value>

Throws if value is null or undefined, otherwise returns it.

const user = expect(findUser(id), 'user not found');

unreachable(x?: never | ErrorParam, error?: ErrorParam): never

Marks code that should never be reached. Supports both unreachable() and unreachable(value) forms.

unreachable(); // unconditional
unreachable(kind); // with value

ErrorParam

A flexible input type for all helpers:

type ErrorParam = Error | string | (() => Error | string);

This allows lazy construction for expensive messages:

ensure(isValid, () => new Error(`Invalid item: ${JSON.stringify(item)}`));

Why not strip messages?

Because your production logs matter more than a few kilobytes of bundle size. ensured keeps errors readable and actionable everywhere. No hidden conditions, no dead-code tricks, no silent failures.