Using function overloading in TypeScript causes an error

I'm currently exploring the concept of function overloading in TypeScript and how it functions. 

type CreateElement = {
  (tag: 'a'): HTMLAnchorElement
  (tag: 'canvas'): HTMLCanvasElement
  (tag: 'table'): HTMLTableElement
}

let createElement: CreateElement = (tag: 'a' | 'canvas' | 'table') => {
  if (tag === 'a') return new HTMLAnchorElement()
  if (tag === 'canvas') return new HTMLCanvasElement()
  if (tag === 'table') return new HTMLTableElement()
  throw new Error('wrong tag');
}

An issue arises from the code above:

Type 'HTMLAnchorElement | HTMLCanvasElement | HTMLTableElement' is not assignable to type 'HTMLAnchorElement'.
    Type 'HTMLCanvasElement' is missing the following properties from type 'HTMLAnchorElement': charset, coords, download, hreflang, and 21 more.

I have made sure that I properly handle the parameter tag before returning the corresponding type based on the tag. Can anyone provide insight into why this implementation is failing?

typescript

Answer №1

Function overloads are specifically designed to be used with function declarations.

While there are workarounds using "overloaded" (arrow) function expressions, these may only work in a limited and weakly typed manner. Unlike true function overloads, the compiler does not treat these expressions specially, leading to unexpected behavior as seen in examples of code errors.

To properly write an example with function overloads, it should look like this:

function createElement(tag: 'a'): HTMLAnchorElement // overload 1
function createElement(tag: 'canvas'): HTMLCanvasElement // overload 2
function createElement(tag: 'table'): HTMLTableElement // overload 3 
function createElement(tag: 'a' | 'canvas' | 'table') { // function implementation
    if (tag === 'a') return new HTMLAnchorElement()
    if (tag === 'canvas') return new HTMLCanvasElement()
    if (tag === 'table') return new HTMLTableElement()
    throw new Error('wrong tag');
}

const ret = createElement("a") // returns HTMLAnchorElement

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