CODEWITHSUNDEEP
typescript
VityaSchel
VityaSchel

Reputation: 708

TypeScript overload function signature with boolean parameter with dependant return type

I would like to call a function with return type based on input boolean parameter, so it returns some type if parameter is false and some other type if parameter is true. I thought overloads are perfect for this case, but TypeScript won't let me use them:

  hello(foo: false): ''
  hello(foo: true): 'bar' {
    if(foo) {
      return 'bar'
    } else {
      return ''
    }
  }

Because I get This overload signature is not compatible with its implementation signature.

Should I use something else, modify this code or just switch to multiple functions with different names and similar behaviour?

Upvotes: 1

Views: 987

Answers (1)

Lesiak
Lesiak

Reputation: 26006

Your attempt to create an overload function is incorrect. Each variant must be compatible with the underlying implementation

In your code:

  • underlying implementation accepts only true and returns bar
  • thus hello(foo: false): '' is not compatible with it
function hello(foo: true): 'bar'
function hello(foo: false): ''
function hello(foo: boolean): '' | 'bar' {
  if(foo) {
    return 'bar'
  } else {
    return ''
  }
}

const aFoo = hello(true);
const anEmptyString = hello(false);

Upvotes: 2

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