# TypeScript Dependent Types

Some examples of type-dependent and value-dependent types in TypeScript.
The nicest way to interact with these examples is to open this repo in an editor with TypeScript
support so you can hover your cursor over a term to view its computed type. Failing that, you can
compile the examples using `pnpm build`.

Many TypeScript values can be lifted as specific types. For example:

```ts
const value = {pi: 3.14}
type ValueType1 = {pi: 3.14}
type ValueType2 = typeof value
```

Here, `value` has the literal type `ValueType1`, which can be captured by the definition of `ValueType2`.

We can make use of this fact to make TypeScript type definitions dependent on specific values:

```ts
type PiOrTau<Num extends 'pi'|'tau'> = Num extends 'pi' ? {pi: 3.14} : {tau: 6.28}
```

This repo includes a few examples using this technique:

- `first.ts` - the simplest example, an implementation of `first(...)` that fails type-checking when applied to empty arrays.
- `overloading.ts` - a more interesting example that uses type inference to determine the signature and return type of a function.
- `routes.ts` - a more complex example, a webserver route definition library that supports parameter middleware.
  - The signature of the route handler function is dependent on the types of any parameter middleware defined on the route.