typescript assert non null
interface Foo { bar(): void }
declare function getFoo(): Foo | undefined;
function assert(value: unknown): asserts value {
if (value === undefined) {
throw new Error('value must be defined');
}
}
function test() {
const foo = getFoo();
// foo is Foo | undefined here
assert(foo);
// foo narrowed to Foo
foo.bar();
}