Assertions

An assertion declares that a condition be true before executing subsequent code.

• If the condition resolves to true the code continues running.
• If the condition resolves to false an error will be thrown.

Errors

assert

Asserts that a condition is true, ensuring that whatever condition is being checked must be true for the remainder of the containing scope.

function assert(condition: boolean, message?: string): asserts condition;

Other assertion functions may accept "truthy" and "falsy" conditions, while assert only accepts conditions that resolve to boolean.

The goal is to promote consideration from consumers when dealing with potentially ambiguous values like 0 or '', which can introduce subtle bugs.

Messages

The assert function has a default message:

assert(false);
// → TypeError: Assert failed

Or you can provide a custom message:

let invalidValue = -1;
assert(invalidValue >= 0, `Expected a non-negative number, but received: ${invalidValue}`);
// → TypeError: Expected a non-negative number, but received: -1

assertNever

Asserts that allegedly unreachable code has been executed.

function assertNever(condition: never): never;

Use assertNever to catch logic forks that shouldn't be possible.

function doThing(type: 'draft' | 'published') {
  switch (type) {
    case 'draft':
      return; /*...*/
    case 'published':
      return; /*...*/

    default:
      assertNever(type);
  }
}

doThing('archived');
// → Error: Unexpected call to assertNever: 'archived'

Logs

warning

Similar to assert but only logs a warning if the condition is not met.

function warning(condition: boolean, message: string): void;

Use warning to let consumers know about potentially hazardous circumstances.

warning(options.scrollEventThrottle === 0, 'Unthrottled scroll handler may harm performance.');
// → console.warn('Warning: Unthrottled scroll handler may harm performance.');