check if given argument is ES6 class or ES5 function based class-like

isES6ClassBased.js

// we can determine whether a given
// function based implementation of a "class" has "writable" attribute set to true.
// ES6 class based implementation has "writable" attribute set to false.
//
// Given any object: it does not matter if it was made from a function or a class,
// there always is the constructor and based on this we get to the same results
// like testing on the function/class itself.
//
// So simply give the isClassBasedFunction any instance or function/class and 
// it will tell you if it was made from a ES6 class or not.
function isClassBased(theClassToTest) {
    theClassToTest = (typeof theClassToTest === 'object') && ('constructor' in theClassToTest) && theClassToTest.constructor || theClassToTest;

    return !Object.getOwnPropertyDescriptor(theClassToTest, 'prototype').writable;
}

isES6ClassBasedTests.js

// These are some tests run to make sure the isClassBased function works correctly
class ClassBasedClass {
    test() {
        console.log('INST C');
    }

    static test2() {
        console.log('STATIC C');
    }
}

function FunctionBasedClass() {
    this.test = function() {
        console.log('INST F');
    }
}
FunctionBasedClass.test2 = function() {
    console.log('STATIC F');
}

var instanceToTest = new FunctionBasedClass();
var instance2ToTest = new ClassBasedClass();
var toTest = [
    instanceToTest,
    FunctionBasedClass,
    instance2ToTest,
    ClassBasedClass
];

toTest.map(isClassBased);