asserts condition — functions that narrow types by throwing, for fail-fast validation patterns.
Five passes over the same idea, each from a different angle. Do them in order, or jump to whichever you need.
Assertion functions use the "asserts" return type to narrow types after the function call. Syntax: asserts value is Type or asserts value. If the function returns normally, TypeScript narrows the type. Ideal for validation at function boundaries, invariant checks, and fail-fast patterns.
