Finding a constructor name in JavaScript

Like many said in JavaScript almost any function object is a constructor (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function). However if you want to distinguish between the native functions that can't be constructors you can try:

function isConstructor(x) {
    return typeof x === 'function' && typeof x.prototype !== 'undefined';
}

Getting the name is definitely trickier, you can use the Function.prototype.toString.apply(x) and match the name, but for example with the jQuery object that would be an empty string, i.e. if it's anonymous function.

There's only an approximation to this supposed problem; whether a function can be invoked using new can't be reliably done without actually trying to perform such a call and check for a TypeError:

function isConstructor(fn)
{
    try {
        var r = new fn(); // attempt instantiation
    } catch (e) {
        if (e instanceof TypeError) {
            return false; // probably shouldn't be called as constructor
        }
        throw e; // something else went wrong
    }
    return true; // some constructors may not return an instance of itself
}

The problems with this:

1. Any function may choose to throw a TypeError for reasons other than the fact that it shouldn't be called as a constructor;
2. The constructor may require certain arguments to be passed;
3. The return value of a constructor doesn't have to meet the requirement that instanceof obj === func;
4. The call itself causes a side effect.

The definition of a constructor is an Object that implements an internal [[Construct]] method. The only way to test for that is to call it as a constructor and see what happens. You can't test for internal properties other than the [[Class]] property using Object.prototype.toString.

Any user defined ECMAScript function is a constructor by default, e.g.

function foo(){}

can be called as a constructor. A function is an object that implements an internal [[Call]] method and therefore typeof will return "function". But not all functions are constructors, e.g.

typeof document.getElementById  // function

but

new document.getElementById()

will throw a type error because it's not a constructor. There are also constructors that aren't functions:

typeof XMLHttpRequest  // object

XMLHttpRequest is a constructor, but it doesn't have [[Call]] so can't be called as a function:

var x = XMLHttpRequest() // TypeError: XMLHttpRequest isn't a function

Lastly, you could try testing for a prototype property that is an object or function, but that also isn't reliable since a prototype property could be added to pretty much any object, host or native.

So the bottom line is that try..catch is probably the only way.

If you don't mind creating a test instance of the function you could do the following:

function isConstructor(func) {
    var t = new func();
    return (t instanceof func);
}

BUT: this only workes if the constructor accepts no arguments. It might as well throw an exception. If it does you could try other (not so reliable) things, like checking if the name of the function starts with an uppercase Letter.

Thank you all for your very clear and helpful answers! I think I do understand the facts now, but I also got some helpful hints that pointed me to ECMAScript details I didn't know, yet. I need to dive into the literature a little more. Thank you, again! Tom