Copy a object properties to another in TypeScript, cause error TS2339

Question

I use a extend() function to extend a object:

function extend(obj: Object, extension: Object) {
    for (var key in obj) {
        try {
            extension[key] = obj[key];
        } catch (e) {
            console.error(e);
        }
    }
}

And there is a class Observer() hava a method Update() and a object check which is HTMLInputElement type.

class Observer {
    Update(value) { }
}
var check:HTMLInputElement = document.createElement("input");

I use extend() function to extend checkbox,so it would have the method Update().

extend(new Observer(), check);
check.Update = function(value) {
    this.checked = value;
}

And then cause error TS2339:Property 'Update' does not exist on type 'HTMLInputElement'

How to Fixed this error? Change the extend() function?

Answer 1

Might be a good scenario for an intersection type?

function extend<T, U>(obj: T, extension: U) {
    Object.keys(obj).forEach((key) => {
        extension[key] = obj[key];
    });

    return extension as T & U;
}

var check: HTMLInputElement;
var extended = extend(new Observer(), check);

// example HTMLInputElement property use
extended.attributes;
// no compile error here either:
extended.Update = function(value) {
    this.checked = value;
};