TypeScript: convert typeof back to instance

Question

I have function that creates instances of given class and I don't how to tell TypeScript that:
function builder returns instances of class passed in first argument

class A extends HTMLElement {
  color: 'white'
}

function builder<T extends typeof HTMLElement>(classParam: T) {
  let instance = new classParam()
  return instance
}

let instance = builder(A)
instance.color = 'black'
// Property 'color' does not exist on type 'HTMLElement'

Is it possible without using type assertion?

You can't new an HTMLElement though, I don't understand what it is you're trying to do. Are you looking for document.createElement() by any chance? — Madara's Ghost
– Madara's Ghost, Commented Jun 19, 2017 at 18:58
the code will later use document.createElement I have simplified the code — pravdomil
– pravdomil, Commented Jun 19, 2017 at 19:08

Julian · Accepted Answer · 2017-06-19 19:24:03Z

2

I've fixed and explained the problems:

class A extends HTMLElement {
  color: string = 'white' // you gave it the type "white", 
                          // but I assume you need the value "white" (and type `string`)
}


function builder<T extends HTMLElement>(classParam: { new(): T }) : T { 
  // previous line:
  // - `typeof` in the generic constraint is not valid
  // - the parameter could not be of type `T`, because you would not be 
  //   able to use `new`. It's now a parameter which has a constructor
  //    which creates a `T`
  // - we are now returning `T`, instead of (implicit) `any`
  let instance = new classParam()
  return instance
}

let instance = builder(A)
instance.color = 'black'

Link to playground

edited Jun 19, 2017 at 19:24

answered Jun 19, 2017 at 18:53

Julian

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2 Comments

pravdomil Over a year ago

I got Type 'HTMLElement' is not assignable to type 'T', try it in TypeScript playground

Julian Over a year ago

I assumed your code was valid, but there where some mistakes. Fixed it and tested in the playground

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