Type Narrowing

Part of the TypeScript course

A union says "this value is one of several types". Narrowing is the compiler's process of figuring out which one, based on the runtime checks you write. You barely write any new syntax for this lesson, the work is in reading code like the compiler does.

The simplest case: a parameter that is string | number. Inside an if (typeof x === "string") branch, the compiler knows x is a string and lets you call string methods on it. ...