TypeScript 3.7 will support an asserts Identifier is Type syntax in function annotation positions (much like with type predicates).

declare function assertIsFoo(x): asserts x is Foo;

function assertIsDefined<T>(x: T): asserts x is NonNullable<T> {
    if (x == null) throw "oh no";
}

See more at the original pull request: microsoft/TypeScript#32695