# Don't leak. Export.

If you export a function, the types of its arguments leak.

Even if your language of choice doesn't allow you to write
`ReturnType<typeof f>` and you can't refer to them, their _concept_ leaks and
you need to be somewhat aware of it when using this function.

```diff
- type Props = {
+ export interface ChatProps {

- export function Chat(props: Props) {
+ export function Chat(props: ChatProps) {
```

This is super important when you're building a library: the consumer can't
trivially export your types, and on top of that there's a risk of divergence
whenever you change them and they bump a version.

But even if you're building an app, you can save your coworkers some time and
frustration if you explicitly export everything that's getting leaked anyway.