TypeScript assert function

Question

I want to create an assert function with TypeScript which will tell me if an element is an instance of, for example, an HTMLElement.

I'm trying to build it like this:

function assertIntanceOf(
  element: unknown,
  prototype: ??? // which type should I place here?
): boolean {
  return element instanceof prototype;
}

and it must work like this:

const elem = document.querySelector('.someClass'); // returns an elem
assertInstanceOf(elem, HTMLElement); // returns true

const elem2 = document.querySelector('.someClassWhichNotExists'); // returns null cause elem with class .someClassWhichNotExists doesn't exist
assertInstanceOf(elem, HTMLElement); // returns false


const someVar = 123; // simply a number
assertInstanceOf(someVar, HTMLElement); //returns false

const someObj = { //just an object
 a: 'abc'
} 
assertInstanceOf(someVar, HTMLElement); //returns false

Which type should I place for prototype argument?

@VLAZ typeof elem in this case will return an object simply. It's not really compatible to TypeScript — Natalia Davydova
– Natalia Davydova, Commented Feb 9, 2021 at 12:21
Sorry, I meant the instanceof operator. I'm not sure why I said typeof. — VLAZ
– VLAZ, Commented Feb 9, 2021 at 12:22
Why not simply instanceof then? It seems to me you are trying to rewrite instanceof? For that matter, all your custom function does is return a instanceof b — Jeremy Thille
– Jeremy Thille, Commented Feb 9, 2021 at 12:23
@VLAZ I can use typeof but I'm interested in the type of argument here, as I've asked. I want to practice this case and find out how to create a function properly — Natalia Davydova
– Natalia Davydova, Commented Feb 9, 2021 at 12:24

jonrsharpe · Accepted Answer · 2021-02-09 12:24:32Z

3

I would make this generic, using the new() "construct signature" per the FAQ, and return a type predicate so that using it will actually narrow the type of the first argument:

function assertInstanceOf<T>(
  element: unknown,
  prototype: { new(): T },
): element is T {
  // ...
}

Playground

That said, TypeScript already knows about instanceof type guards, so you don't necessarily need to put this in a function.

answered Feb 9, 2021 at 12:24

jonrsharpe

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