How to define the return type of a function according to the input parameters with Typescript?

Question

I have a function with an object as argument:

// Types
type Params = {
  first: string;
  second: number;
  third?: boolean;
};
type FunctionA = (params: Params) => { [key: string]: any } | void;

// Function
const functionA: FunctionA = params => {
  // ... do something
  if (params.third) {
    return {one: "object"}
  }
};

So basically, what I want to achieve is that if third has been submitted, I want that the function return type is {[key: string]: any} . If third is not submitted, I want it to be never .

I have found solutions where the return type of a function changes according to the argument which has been returned but nothing, if the object is an argument and with a return type which is different from the argument type.

Is this even possible?

Thanks! Any help appreciated!

Answer 1

You could use function overloads .

LE: After reading the question better I think this is what you want:

type Params = {
  first: string;
  second: number;
  third?: boolean;
};

function A(params: Omit<Params, "third">): void
function A(params: Required<Params>): { [key: string]: any }
function A(params: Params): { [key: string]: any } | void {
  if (params.third === undefined) {
    return
  }

  return {...params}
}

Again, you are overloading the function, but this time you are declaring one without third in the params and another where all fields are required, including third .

You can see this in action in this playground .

You can declare 2 functions with 2 and 3 params and then implement it. Something similar to this:

function A (first: string, second: string, third: boolean): {[key: string]: any}
function A (first: string, second: string): never
function A (first: string, second: string, third?: boolean) {
  if (third === undefined) {
    throw new Error("error message")
  }

  return {first, second, third}
}

Now your IDE or any code helper should know that if you pass 2 params you have a return type and if you pass 3 another.

Note: I don't see a valid reason for returning never when not passing the third argument. Is almost like you want to make the 3rd one mandatory and not optional.