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 6.0.3, 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.