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?

CodePudding user response：

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);