Typeof

While Javascript already has typeof that can be used in an expression context, Typescript adds a typeof that can be used in a type context.

• that is, we can use it to refer to the type of a property/variable.
• it’s only legal to use typeof on identifiers (i.e. variable names) or their properties

Like keyof, typeof is used to create new types

let s = "hello";
let n: typeof s;

typeof can be used to conveniently express many patterns.

• imagine we want to grab the return type of function f() for usage elsewhere:

function f() {
  return { x: 10, y: 3 };
}
// note: ReturnType is a built-in, and gives us the return type of a given function
type P = ReturnType<typeof f>;

typeof lets us achieve narrowing and type guarding in Typescript

• https://www.typescriptlang.org/docs/handbook/2/narrowing.html

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Backlinks

• Typescript