TypeScript throws an error, overload signature is incompatible with implementation signature. How to fix the code?

Question

typescript

interface MyPosition {
  x: number | undefined
  y: number | undefined
}

interface MyPositionWithDefault extends MyPosition {
  default: string
}

function position(): MyPosition
function position(a: number): MyPositionWithDefault
function position(a: number, b: number): MyPosition

function position(a?: number, b?: number) {
  if (!a && !b) {
    return {x: undefined, y: undefined}
  }

  if (a && !b) {
    return {x: a, y: undefined, default: a.toString()}
  }

  return {x: a, y: b}
}

console.log('Empty: ', position())
console.log('One param: ', position(42))
console.log('Two params: ', position(10, 15))

Answer 1

Xs why it gives such an error, but if you describe the return type of the function implementation, then the error will disappear:

interface MyPosition {
  x: number | undefined
  y: number | undefined
}

interface MyPositionWithDefault extends MyPosition {
  default: string
}

function position(): MyPosition;
function position(a: number, b: number): MyPosition;

function position(a: number): MyPositionWithDefault;

function position(a?: number, b?: number): MyPosition | MyPositionWithDefault { // вот тут
  if (a===undefined && b===undefined) {
    return {x: undefined, y: undefined}
  }

  if (a!==undefined && b===undefined) {
    return {x: a, y: undefined, default: a.toString()}
  }

  return {x: a, y: b}
}

console.log('Empty: ', position())
console.log('One param: ', position(42))
console.log('Two params: ', position(10, 15))

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