FrontendHaltTypeScript·Intermediate·Coding·1 min read
What is a type predicate?
Short interview answer
A return type such as value is User tells TypeScript that a true result narrows the argument. The implementation must actually validate that claim; the compiler trusts it, so an incorrect predicate creates unsoundness similar to a bad assertion.
Example
function isUser(v: unknown): v is User { return typeof v === 'object' && v !== null && 'id' in v;}
if (isUser(payload)) { payload.id; // narrowed to User}Key takeaway
Explain the underlying mental model clearly, then support it with a concrete example and its trade-offs.