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assert ​

Problem ​

You need to validate a runtime condition and have TypeScript narrow the type when it passes — for example asserting a parsed value matches an expected shape.

Solution ​

Use assert(condition, message?, options?) to throw if the condition is falsy. TypeScript narrows the type after the assertion via asserts condition.

ts
import { assert } from '@vielzeug/arsenal';

function process(id: string | undefined) {
  assert(id !== undefined, 'id is required');
  id; // string — undefined narrowed away
}

Custom error class ​

ts
import { assert } from '@vielzeug/arsenal';

assert(value >= 0, 'value must be non-negative', { type: RangeError });
// throws RangeError('value must be non-negative') when condition is false

Pitfalls ​

  • assert throws synchronously — it cannot be used inside async validators as a replacement for await.
  • The type option accepts any ErrorConstructor; it does not wrap the error.