How to write TypeScript `get` function for Map object?

Question

Is it possible to do something like this in TypeScript?

interface IPerson {
  name: string;
  age: number;
}

declare class Map<T> {
  get(name: keyof T): any // ? return type
}

Result I need

const map = new Map<IPerson>();
map.get("name"); // string
map.get("age"); // number

Your map isn't well-defined. It should be Map<string, IPerson>. — Ian MacDonald
– Ian MacDonald, Commented Oct 11, 2018 at 20:56
In my example Map is my own class. It's not standard Map class. It can contains only Keys like keyof T, and each Value has type defined in incoming object type and depending of Key — Microshine
– Microshine, Commented Oct 11, 2018 at 21:04

CRice · Accepted Answer · 2018-10-11 21:06:42Z

2

Let the name parameter be a generic that extends keyof T, and then your return type can use that to index T to get the right type. EG:

interface IPerson {
  name: string;
  age: number;
}

declare class MyMap<T> {
  get<K extends keyof T>(name: K): T[K]
}

// Result I need

const map = new MyMap<IPerson>();
map.get("name"); // string
map.get("age"); // number

P.S. Consider naming your class something other than Map, since it will be easily confused with the built-in javascript Map object, and will conflict with TypeScript's built-in types for Map if present.

answered Oct 11, 2018 at 21:06

CRice

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1 Comment

Microshine Over a year ago

Thank you very much! It does what I need

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