TypeScript narrowing fundamentals
Runtime checks such as typeof, in, discriminants and predicates refine a broad static type inside a controlled branch.
How it works
A union describes several possible values, while the typeof check gives TypeScript evidence about the value in this branch; checking a tag on an object works in the same way, but casting untrusted JSON with as only changes what the compiler assumes and cannot replace runtime validation.
function label(value: string | number) {
if (typeof value === "string") {
return value.trim();
}
return value.toFixed(2);
}Common misconception
A type assertion does not validate data at runtime.
Quick practice
1. Which operator narrows primitives?
typeof
2. What identifies a discriminated union member?
A shared literal tag.
3. Does `as` validate input?
No.