TypeScriptAssertion functions có return type asserts condition hoặc asserts param is Type.
Khi function return (không throw), TS assume assertion đúng và narrow type trong phần code tiếp theo.
typescript
function assert(cond: unknown, msg = 'assertion failed'): asserts cond {
if (!cond) throw new Error(msg);
}
function assertIsString(val: unknown): asserts val is string {
if (typeof val !== 'string') throw new TypeError();
}
const val: string | undefined = getValue();
assert(val !== undefined, 'val must exist');
val.toUpperCase(); // OK — TS biết val là string (không còn undefined)Assertion functions have a return type of asserts condition or asserts param is Type.
When the function returns (without throwing), TS assumes the assertion holds and narrows the type in the following code.
typescript
function assert(cond: unknown, msg = 'assertion failed'): asserts cond {
if (!cond) throw new Error(msg);
}
function assertIsString(val: unknown): asserts val is string {
if (typeof val !== 'string') throw new TypeError();
}
const val: string | undefined = getValue();
assert(val !== undefined, 'val must exist');
val.toUpperCase(); // OK — TS knows val is string (no longer undefined)