Typescript map a list to function arguments

Question

I have a list of arguments looking a little something like this:

const args: FeatureEventArg[] = [
  {
    name: 'username',
    type: 'string',
  },
  {
    name: 'message',
    type: 'string',
  },
  {
    name: 'totalMessagesSent',
    type: 'number',
  },
];

And my goal is to take that list and with some type, like FeatreEventArgs<typeof args> or something, to get arguments for a callback function that would end up looking something like this:

function callback(username: string, message: string, totalMessagesSent: number) {
  // Other stuff
}

I've managed to get the types part with a lot of fidelling and a lot of extends "string" ? string kinda thing. But the names are just arg_0 instead of the name in the object above.

Anyways, if you have any tips or ideas of how I can achieve this or any other solutions to the problem please let me know.

I do not understand, do you need to create a type from the type of the list? — Nullndr
– Nullndr, Commented Mar 23, 2022 at 16:45
AFAIK this is not possible; I have an issue about this still open — k.tten
– k.tten, Commented Mar 23, 2022 at 18:36
Tuple labels and function parameter names are essentially unobservable from the type system; they are only for documentation. The types (username: string, message: string, totalMessagesSent: number)=>void and (x: string, y: string, z: number)=>void are identical. Also, what exactly do you mean by "get arguments for a callback function"? Is this in a type? Or an actual function implementation? — jcalz
– jcalz, Commented Mar 23, 2022 at 23:16

CertainPerformance · Accepted Answer · 2022-03-23 16:46:06Z

1

You'll need to drop the : FeatureEventArg[] and instead interpret the array of objects as const so that the exact string types get preserved. Then you can map over the [number] values of the array, extracting the name as the key and the type as the value (through a helper type that transforms the string into the corresponding type).

const args = [
  {
    name: 'username',
    type: 'string',
  },
  {
    name: 'message',
    type: 'string',
  },
  {
    name: 'totalMessagesSent',
    type: 'number',
  },
] as const;
type Args = typeof args;
type ToPrimitive<T> =
    T extends 'string' ? string
  : T extends 'number' ? number
  : never;

type ArgsObj = {
  [T in Args[number] as T["name"]]: ToPrimitive<T["type"]>
}

answered Mar 23, 2022 at 16:46

CertainPerformance

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1 Comment

JanuZz_dk Over a year ago

Thanks for answering so quickly, but I have come to the conclusion that my wording in the question was incorrect. I am trying to use it as arguments in a callback function, and I am afraid that that might require a different solution. I updated the question with a more accurate description of my goal if you want to see it

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