Superseded by Emery.

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.

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

Note: The narrow type of boolean is an intentional design decision.

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.

The assert function has a default message:

let falsyValue = '';
assert(typeof falsyValue === 'string' && falsyValue.length > 0);
// → TypeError: Assert failed

Or you can provide a custom message:

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

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);
  }
}

Warning: Regardless of the condition, this function always throws.

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

In development both assert and assertNever will include a debugger statement, which will pause execution to aid debugging.