Operators

typeof

The bridge from a value to its type.

What it is

Types and values live in separate namespaces, and typeof is the only way across in one direction. In a type position it means "the type of that value", which is unrelated to the JavaScript typeof operator that returns a string at run time — same spelling, different worlds, and one of the few genuine puns in the language.

Examples

```
typeof config
```
→{ retries: number; url: string; }
```
typeof send
```
→(to: string) => void
```
keyof typeof config
```
→"retries" | "url"
The pairing you will write most often: cross into the type world, then take the keys.

Each resolved type above was printed by TypeScript 6.0.3, not written by hand.

What it does not do

It does not accept an expression. typeof foo.bar works because that is a reference; typeof foo() does not, because calling something is not a value you can name.
It does not narrow. typeof in a type position reads the declared type, not whatever the variable has been narrowed to at that line.

Takeaway

When a type is already described perfectly by a value you have, do not write it twice. typeof keeps the two from drifting apart.