Typescript enforce function accepts first argument

Question

I would like to enforce that a particular function accepts a first argument of a particular type

type MyFunc = <Args extends []>(channelId: string, ...args: Args) => string

const doThing: MyFunc = (channelId: string, arg1: number, arg2: string) => arg1 + arg2

I get the following error

Type '(channelId: string, arg1: number, arg2: string) => string' is not assignable to type 'MyFunc'.
  Types of parameters 'arg1' and 'args' are incompatible.
    Type 'Args' is not assignable to type '[arg1: number, arg2: string]'.
      Type '[]' is not assignable to type '[arg1: number, arg2: string]'.
        Source has 0 element(s) but target requires 2.

Playground link

jsejcksn · Accepted Answer · 2021-12-28 00:51:23Z

1

This is how you'd do it, but...

Are you sure you want to concatenate a number and a string (and not use the channelId argument)?

TS Playground

type MyFunc<Params extends readonly unknown[]> = (channelId: string, ...params: Params) => string;

const doThing: MyFunc<[n: number, str: string]> = (channelId, arg1, arg2) => arg1 + arg2;

const result = doThing('my_channel_id', 42, 'hello world');
console.log(result) // "42hello world"

answered Dec 28, 2021 at 0:51

jsejcksn

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4 Comments

david_adler Over a year ago

Oh you can ignore the implementation it's just an example, thanks!

jsejcksn Over a year ago

@david_adler So this answered the question?

david_adler Over a year ago

still checking, as I might need to infer the generic parameter you've statically set so will circle back to the answer

jsejcksn Over a year ago

If it turns out that I misunderstood a detail in the question, just let me know.

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