FrontendHaltTypeScript·Intermediate·Coding·1 min read
What is type narrowing?
Short interview answer
Narrowing is TypeScript refining a value from a broader static type to a more specific type based on control-flow evidence. typeof, equality checks, in, instanceof, discriminants, and user-defined type predicates can all contribute evidence.
Example
function format(x: string | number) { if (typeof x === 'string') { return x.trim(); // x is string here } return x.toFixed(2); // x is number here}Key takeaway
Explain the underlying mental model clearly, then support it with a concrete example and its trade-offs.