What causes a compile-time error in visual code when using generics with multiple types?

I'm attempting to develop a function with a generic type that takes one parameter with multiple types.

It seems straightforward, but when I combine certain types, I encounter compile-time issues in Visual Studio Code.

Check out my example below:

This code snippet works fine:

class Test<T>{
    GetValue(value: string|boolean|number|undefined) {
        return value;
    }
}

new Test<Item>().getValue(4)
new Test<Item>().getValue(true)

This second piece of code also functions correctly:

    class Test<T>{
        GetValue<B>(value: (x: T) => B) {
            return value;
        }
    }

    new Test<Item>().getValue(x=> x.name)

However, the following code does not work. Any thoughts on why?

    class Test<T>{
        GetValue<B>(value: (x: T) => B|string|boolean|number|undefined) {
            return value;
        }
    }
    // this works
    new Test<Item>().getValue(x=> x.name)
    
    // this doesn't work - any ideas why?
    new Test<Item>().getValue(true)

Answer №1

Issue with operator precedence:

(x: T) => B|string|boolean|number|undefined

The correct format should be:

((x: T) => B)|string|boolean|number|undefined

If not corrected, the parameter value will be interpreted as a function that returns

B|string|boolean|number|undefined

Try it on playground

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