CODEWITHSUNDEEP
javascriptflowtype
yachaka
yachaka

Reputation: 5569

FlowType : return type of function (k) => obj[k]

I have a User object:

type User = {
  name: string,
};

That has a get() function, that takes in parameter the key of an attribute on User and returns this attribute. The function get is

User.prototype.get = (prop) => {
  return this[prop];
};

How can I write this fonction definition ? Here's what I got so far:

type User = {
  name: string,
  get: (k: $Keys<User>) => any, // How can I change any to the correct property type ?
};

Upvotes: 1

Views: 160

Answers (1)

yachaka
yachaka

Reputation: 5569

Seems like you can use now $ElementType<T, K>. To be confirmed !

Source: https://github.com/facebook/flow/issues/4122#issuecomment-314700605

EDIT: Working example

/* @flow */

type User = {
  name: string,
  age: number,
}

const user: User = {
  name: 'Ilyes',
  age: 21,
}

function get<K: string>(key: K): $ElementType<User, K> {
  return user[key];
}

const number: number = get('name'); // error
const number2: number = get('age'); // works
const string: string = get('name'); // works
const string2: string = get('age'); // error

Upvotes: 1

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