Generics keyof Constraint in TypeScript

keyof constraint in TypeScript generics make sure that the value passed is a key of another object.

Here is a simple function that returns the value of an object property passed:

function boom(obj: object, key: string) {
  return obj[key];
}

After that we are calling the function like below:

boom({}, "age");

Above code does not throw an error. But, it is not valid as we know that key age is not there in an empty object.

What if we need to make sure that the passed key is an existing key in the object? We can use keyof constraint in Generics.

function boom<T extends object, U extends keyof T>(obj: T, key: U) {
  return obj[key];
}

It says that, the second argument should be a key of first argument.