Function overloading in typescript throw error

Question

I'm trying to understand how function overloading in typescript works.

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');
}

The code above throws the error:

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 ensured that I resolve the parameter tag before returning the appropriate type for a given tag type. Any ideas why this doesn't work?

Wouldn't CreateElement be type CreateElement = HTMLAnchorElement | HTMLCanvasElement | HTMLTableElement? — Brandon Dyer
– Brandon Dyer, Commented Jan 27, 2020 at 22:40
Not sure what you mean, I'm overloading the function by taking union of function signature but you're referring to union of objects. — quantdaddy
– quantdaddy, Commented Jan 27, 2020 at 22:45

ford04 · Accepted Answer · 2020-01-27 23:16:37Z

1

Function overloads can only be used with function declarations.

Workarounds with "overloaded" (arrow) function expressions may work in a limited, weak typed way. But unlike the former, the compiler does not have a special treatment for these expressions. So they will not behave in the manner you expect it (your code error is already a good example).

Your rewritten example with proper function overloads looks like this:

function createElement(tag: 'a'): HTMLAnchorElement // overload 1
function createElement(tag: 'canvas'): HTMLCanvasElement // 2
function createElement(tag: 'table'): HTMLTableElement // 3 
function createElement(tag: 'a' | 'canvas' | 'table') { // fn 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") // HTMLAnchorElement

answered Jan 27, 2020 at 23:16

ford04

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2 Comments

quantdaddy Over a year ago

I see...this works. Only drawback is the need to repeat the function name compared to function expression.

ford04 Over a year ago

yeah, I also like the leaner arrow function syntax a bit more (and you can declare function interface types for them). But this one will provide stronger types for the function implementation. So I would prefer it anytime.

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