Assertion Functions

An assertion function checks a condition and throws when it fails. Its asserts annotation tells TypeScript what is established if the call returns normally.

What you will learn: the purpose of assertion functions, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Unlike the as assertion operator, an assertion function can perform real runtime verification. Design a useful error message and verify every condition that makes the asserted type safe.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

function assertString(value: unknown): asserts value is string {
  if (typeof value !== "string") throw new Error("Expected a string");
}
const incoming: unknown = "  ready ";
assertString(incoming);
console.log(incoming.trim().toUpperCase());
try { assertString(5); } catch (error) {
  console.log(error instanceof Error ? error.message : "Unknown error");
}

Verified output

READY
Expected a string

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

The first assertion returns normally because incoming is a string; later code can call trim. The second assertion receives 5, throws, and the catch prints the deliberate error message.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

An as string cast does not throw on wrong input. Use an assertion function if you need to enforce a runtime precondition.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Write assertPositive(n: number): asserts n is number that throws for n <= 0, and test the error branch.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Assertion Functions: An assertion function checks a condition and throws when it fails.

The first assertion returns normally because incoming is a string; later code can call trim. The second assertion receives 5, throws, and the catch prints the deliberate error message.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.