Generic constraints

⬅️ Execute Program Advanced TypeScript

Generic constraints

Constraints let us say “this can be any type… as long as it’s compatible with this other type”.
They are a way to voluntarily constrain ourselves, so that we can only see a small piece of a type inside of the function.
We do this by adding the generic constraint <T extends {age: number}>. In TypeScript, we can always add extends ... to a generic type parameter. Anywhere we can write <T>, we can also write <T extends ...>.

function filterBelowAge<T extends {age: number}>(
  things: Array<T>,
  limit: number
): Array<T> {
  return things.filter(thing => thing.age < limit);
}

Inside the function, T always has the property age: number. That forces us to write the function in a way that works for any object with an age. Using the type in any other way is a type error.
Generic constraints are a critical feature of TypeScript, but they show up primarily in library or framework code