Typescript function return type depending on number or type of arguments

Question

Let's say I have a function that should return different types depending on the number and type of arguments. How would I describe this in TypeScript?

function foo(t: number): string
function foo(t: number, s: string): boolean {
   return typeof s === 'string' ? true : 'false';
}

In words: the function foo returns a string when invoked with a number argument and returns a boolean when invoked with a number and a string parameter.

Answer 1

You can define the overload of your function + change the implementation, like:

Playground

function foo(t: number, s: string): boolean;

function foo(t: number): string;

function foo(t: number, s?: string): string | boolean {
   return s !== void 0 ? true : 'false';
}

const var1 = foo(50);
const var2 = foo(50, 'str');

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In this way, TypeScript will be able to define which is the correct type of your variable depending on the number of parameter of your function.

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Answer 2

TypeScript doesn't have that level of granularity. You can say:

function foo(t: number, s?: string): string | boolean

…in which it takes an optional second argument and returns either a string or a boolean, but if you want to make the return value type conditional on the arguments then you would need to define two seperate functions.

Answer 3

Interesting question. TypeScript don't support this as far as my experience go. What you could do is to set the return type to a this | that this | that type. This means "return type is this or that "

function foo(t: number, s: string): string | boolean {}

In my team at work we generally try to avoid the this | that this | that approach because it can be an added source of bugs.

My recommendation is two functions with specific return types, if this is possible in your application.

function getFoo(t: number): string {}

function isFoo(t: number, s: string): boolean {}