Strings

Lowercase

Lower-cases a string literal type.

What it is

The mirror of `Uppercase`, and `intrinsic` in the same way. Its most common real use is not transformation but comparison: pairing it with `Uppercase` is how you ask whether a character is a letter at all.

Examples

```
Lowercase<'HELLO'>
```
→"hello"
```
Lowercase<'MiXeD'>
```
→"mixed"

Uppercase<'7'> extends Lowercase<'7'> ? true : false

→true

Each resolved type above was printed by TypeScript 5.9.3, not written by hand.

What it does not do

It does not tell you a character is a letter on its own. `Lowercase<"7">` is `"7"`, and only comparing against `Uppercase` reveals that there was no case to change.
It does not distribute the way you might hope over a template. Applying it to a whole string lower-cases everything, not the first character.

Takeaway

`Uppercase<C> extends Lowercase<C>` is true exactly when `C` has no case — the cleanest letter test the type system offers.