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README.md
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:
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:
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 offirst(...)
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.