Object with a template key in TypeScript

user_13502024 0
Viewed 36

I need to return a typed object from the function with a specific key in it. Like this:

function addNumber<Key extends string>(key : Key) {
    return { [key] : 5 }
}

const a = addNumber('x')

console.log(a.x) // OK
console.log(a.y) // compilation error: y doesn't exist in a

Or this:

function addNumber<key : string>() {
    return { [key] : 5 }
}

addNumber<'x'>() // { x : 5 }

I don't need a key to be dynamic, like any possible runtime dependent string. A compile-time literal should be enough. But I need the function to return an object that can be type checked.

Is this possible in TypeScript?

typescript templates
2 Answers

You're almost there by defining your generic and using the inference to type the result !

function addNumber<Key extends string>(key: Key): { [k in Key]: number; } {
  return { [key]: 5 } as { [k in Key]: number; }
}

const foo = addNumber('x')
foo.x // OK
foo.y // KO

Playground

user_8841230

I got u

const addNumber = <K extends string>(k: K) => ({ [k]: 5 } as  { [k in K]: number; });

const a = addNumber('x'); // a: { x: number }
console.log(a.x); // ok
console.log(a.y); // Property 'y' does not exist on type '{ x: number; }'.(2339)
user_81868980
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