Narrowing mapped types to exclude undefined values

[jg]

Problem

TypeScript Version: 2.1.4

Code

(--strictNullChecks)

interface Store {
  value: string | undefined
}

export function get<K extends keyof Store>(
  store: Store,
  fieldName: K
): Store[K] {
  if (store[fieldName] == undefined) throw(fieldName + " is undefined!");

  return store[fieldName]
}

const store: Store = { value: "stuff" }

const result = get(store, "value").replace("stuff", "otherstuff")

Expected behavior:

(It's possible to narrow the resulting type of get to exclude undefined values)

Actual behavior:

Type error is reported: get(a, "value") is possibly undefined.

Proposal

I'd propose an operator which could be used to express that the mapped type does not contain undefined. Consider the use of bang (!) operator below:

export function get<K extends keyof Store>(
  store: Store,
  fieldName: K
): Store[K]! {
  if (store[fieldName] == undefined) throw(fieldName + " is undefined!");

  return store[fieldName]
}

[aluanhaddad]

I believe this is what you are looking for #12578

[jg]

Yep