@philiprehberger/ts-invariant

Tiny runtime assertion with TypeScript narrowing.

Installation

npm install @philiprehberger/ts-invariant

Usage

import { invariant, assertNever, unreachable } from '@philiprehberger/ts-invariant';

const user = getUser(id);
invariant(user, 'User %s not found', id);
user.name; // narrowed to User

// Lazy message
invariant(data, () => `Expected data, got ${typeof data}`);

// Exhaustiveness checking
type Shape = 'circle' | 'square';
switch (shape) {
  case 'circle': return handleCircle();
  case 'square': return handleSquare();
  default: assertNever(shape);
}

Custom Error Classes

import { invariantAs } from '@philiprehberger/ts-invariant';

class NotFoundError extends Error {
  constructor(message?: string) {
    super(message);
    this.name = 'NotFoundError';
  }
}

invariantAs(NotFoundError, user, 'User not found');

API

| Function | Description | |----------|-------------| | invariant(condition, message?, ...args) | Assert condition is truthy, narrows type | | invariantAs(ErrorClass, condition, message?) | Assert with custom error type | | assertNever(value) | Exhaustiveness checking for switch/if chains | | unreachable(message?) | Mark unreachable code paths | | InvariantError | Error class thrown by invariant() |

Development

npm install
npm run build
npm test

License

MIT