What method can be used to fetch generic type parameter in typescript?

Question

What method can be used to fetch generic type parameter in typescript?

I am having trouble finding the type of E within the code provided below.

class A<E> {
  getParameterType() {
    // I need to determine the type of E
  }
}

class B {
}

** Example **
new A<number>().getParameterType() // number
new A<B>().getParameterType() // B

typescript typescript-generics

Answer 1

Answer №1

This code snippet demonstrates a type-safe approach:

class A<E> {
    constructor(public generic: E) { }
    getParameterType() {
        return this.generic
    }
}

class B {
}

const result1 = new A(42).getParameterType() // number
const result2 = new A(new B()).getParameterType() // B

It is important to note that using explicit generics unrelated to any argument can lead to unsafe code:

function fn<Char extends "a">(): Char {
    return "a" // error
}

const result = fn<'a' & { hello: 42 }>()

const check = result.hello // 42, but undefined in runtime

Check out my article for more information on inference.

Answer 2

This code snippet demonstrates a type-safe approach:

class A<E> {
    constructor(public generic: E) { }
    getParameterType() {
        return this.generic
    }
}

class B {
}

const result1 = new A(42).getParameterType() // number
const result2 = new A(new B()).getParameterType() // B

It is important to note that using explicit generics unrelated to any argument can lead to unsafe code:

function fn<Char extends "a">(): Char {
    return "a" // error
}

const result = fn<'a' & { hello: 42 }>()

const check = result.hello // 42, but undefined in runtime

Check out my article for more information on inference.

What method can be used to fetch generic type parameter in typescript?

Answer №1

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