CODEWITHSUNDEEP
typescript
Natalia Davydova
Natalia Davydova

Reputation: 748

TypeScript assert function

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?

Upvotes: 2

Views: 684

Answers (1)

jonrsharpe
jonrsharpe

Reputation: 122116

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.

Upvotes: 3

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