CODEWITHSUNDEEP
typescriptgenerics
Sandeep
Sandeep

Reputation: 11

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

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?

Upvotes: 0

Views: 198

Answers (2)

Pouria Rezaei
Pouria Rezaei

Reputation: 98

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;
  }

Upvotes: 0

astroide
astroide

Reputation: 817

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})

Upvotes: 1

Related Questions