CODEWITHSUNDEEP
typescriptassert
Itay Ganor
Itay Ganor

Reputation: 4205

Typescript Wrap "function asserts foo is type"

Let's create two dummy classes and some example code:

class A {
    constructor() {}
}

class B extends A {
    value: number;

    constructor(value: number) {
        super();
        this.value = value;
    }
}


const values: A[] = [new A(), new A(), new B(123)];
const oneOfThem = values[2];

validateB(oneOfThem);

console.log(oneOfThem.value);


function validateB(item: A): asserts item is B {
    if (!(item instanceof B)) {
        throw new Error('wrong type!');
    }
}

This code runs perfectly.

The thing is I need to wrap validateB with another function:

function validateBWrapper(item: A) {
    return validateB(item);
}

But as soon as I use this function instead of validateB in the code above, it loses the asserts ... info:

error

I don't want to copy asserts item is B to the wrapper function. In my actual usage, the wrapping function wraps a function it gets a parameter.

I'm trying to find something similar to ReturnType<>, but with asserts.

Is there any way to retrieve asserts info from a given function?

Thanks!

Playground

Upvotes: 1

Views: 111

Answers (1)

satanTime
satanTime

Reputation: 13574

Unfortunately, you are right, asserts and is are special keywords that help typescript parser to understand our goals when ReturnType<> only extracts a type, in case of asserts it is void and in case of is it is boolean.

and without saying explicitly that function asserts or value is T there's no way to inherit it.

The way we have - to open an issue on github and to wait until they add a new utility type for it here https://www.typescriptlang.org/docs/handbook/utility-types.html

Upvotes: 1

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