Enforce that all keys in a map are a key of a certain type (Typescript)

If you’re instantiating a new Map data structure

const myMap = new Map()

and you want to ensure that every key in this object belongs to a certain type where the key will always be a string (as a Map allows you to set other types as the key as well, for eg: a number)

function foo(someKey: string) {
  const myMap = new Map()
  const myMap.set(someKey, 'some_value')
}

We can use the Typescript Extract keyword like so:

type KeyOf<T> = Extract<keyof T, string>

And our function will now look like so:

function foo<T>(someKey: KeyOf<T>) {
  const myMap = new Map()
  myMap.set(someKey, 'some_value')
}

If in the example above, 'foo' isn’t a key of T , you will get a transpile time error.

Here’s an example that will get caught if you try to use it.

type Transaction = {
  merchantName: string,
  transactionAmount: number,
}
foo<Transaction>('merchantId') 
// Error: Argument of type '"merchantId"' is not assignable to parameter of type 'keyof Transaction'

This will ensure you’re only setting keys of a certain type as the key in a map.