How can you define a function type for a rest parameter in Typescript?

At this point, I have a function that takes in two parameters: param 'a' as a string and 'b' as a function that returns a string. My intention is to call it using a rest parameter and specify the types accordingly.

However, on line 10 (the last line), the TypeScript compiler is generating an error message saying "Argument of type '"a"' is not assignable to parameter of type 'TFoo'.ts(2345)".

type TFoo = () => string | string;

function foo(...args: TFoo[]) {
  let [a, b] = args;

  if (typeof b === 'function') {
    b = b();
  }  
}

foo('a', () => 'baz');

My expectation is for TypeScript to allow strings or functions as arguments for these function parameters.

Is there a way to resolve this issue?

Answer №1

There seems to be a conflict in the declaration order of types, as Ahmed pointed out in his comment.

An alternate solution could be:

type FooFunc = () => string
type FooInput = string | FooFunc

function foo (...args: FooInput[]) {

Alternatively:

type FooInput = string | (() => string)

function foo (...args: FooInput[]) {

In my opinion, the function should look like this:

function foo (a: FooInput, b: FooInput) {

However, there might be a valid reason for expecting an unknown number of arguments.

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