How to type a Generic Function in TypeScript with predefined key value?

Question

I am trying to type this function in TypeScript and hitting a dead end every time. It’s complaining about using any as a type of the passed in d object.

const current = (d: any): number => d.currentYear;

Basically, it is a function that takes in an object d and returns .curentYear property value out of it. The shape of the d object may change and hence it can be any object. I tried to look into Generic Types and this is the closest I could find:

const getProperty = <T, K extends keyof T>(obj: T, key: K) => obj[key];

The downside is that I have to pass in the property during the function call:

getProperty(d, 'currentYear')

Is there any better way to type this function signature?

not sure why sometimes people on stackoverflow, give down vote without telling the reason. It's really unfair. — Pouria Rezaei
– Pouria Rezaei, Commented May 30, 2021 at 16:30

astroide · Accepted Answer · 2021-05-27 13:57:29Z

1

Use an interface that has the property you need :

interface HasCurrentYear {
    currentYear: number;
}
const current = (d: HasCurrentYear): number => d.currentYear;

Then, you can call current with any object that has a currentYear property :

current({currentYear:2021})

answered May 27, 2021 at 13:57

astroide

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Comments

Pouria Rezaei · Accepted Answer · 2021-05-30 17:39:55Z

0

if your 'd' may change, using extends keyword is better solution. in this way you guarantee that your d object will have a currentYear prop, no matter what.

 interface HasCurrentYear = {
    currentYear: number;
  };
  
  function current<T extends HasCurrentYear>(obj: T) {
    return obj.currentYear;
  }

so if you pass a object without cureentYear prop, you will give an error and the good point is that if you pass a object with currentYear and some other props it still works. also you can wtite it inline

 function current<T extends { currentYear: number }>(obj: T) {
    return obj.currentYear;
  }

edited May 30, 2021 at 17:39

answered May 30, 2021 at 16:26

Pouria Rezaei

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