How can I check if element is an instanceof U?

Question

I want to add elements of type any to an array and later get the elements from this array that are numbers:

function OfType<T, U>(list: T[]) : U[]
{
    var result: U[] = [];

    list.forEach(e => {
        // I want to check if e is of type U
        //if (typeof(e) === typeof(U)) // ERROR: doesn't work
            result.push(<U><any>e);
    });

    return <any[]>result;
}


var list: any[] = [];
list.push("A");
list.push(2);

var result = OfType<any, number>(list);

alert(result.toString());

But it doesn't allow me to check the type of the elements against a generic type.

Is there a way to accomplish this?

Answer 1

You can currently do it much better this way (TypeScript 3.9):

// tslint:disable-next-line: no-any
type Constructor<T> = new (...args: any[]) => T;

export function ofType<TElements, TFilter extends TElements>(array: TElements[], filterType: Constructor<TFilter>): TFilter[] {
    return <TFilter[]>array.filter(e => e instanceof filterType);
}

Example usage:

class ClassA { }
class ClassB { }

const list: ClassA[] = [new ClassA(), new ClassB()];
const filteredList = ofType(list,  ClassB);

Answer 2

As Judah pointed out it is unlikely to be possible using generic types alone. I found a workaround where I send in one more parameter with the type...

function OfType<T, U>(list: T[], arg: Function) : U[]
{
    var result: U[] = [];

    list.forEach(e => {
        // extract the name of the class
        // used to match primitive types
        var typeName = /function\s*([^(]*)/i.exec(arg+"")[1].toLocaleLowerCase();

        var isOfType = typeof(e) === typeName;

        // if it is not primitive or didn't match the type
        // try to check if it is an instanceof
        if (!isOfType)
        {
            try {
                isOfType = (e instanceof arg)
            }
            catch (ex) { }
        }

        if (isOfType)
            result.push(<U><any>e);
    });

    return <any[]>result;
}

Usage:

var numbers = OfType<any, number>(list, Number);
var foos = OfType<any, Foo>(list, Foo);

alert("Numbers: " + numbers);
alert("Foos: " + foos);

Little redundancy, if someone know a way to remove this redundancy please leave a comment or edit this code.

Or, for primitive types only I could use filter as Judah mentioned.