ES5 classes, compatible with ES6 classes

class.js

function A() {}

A.prototype.thing = function () {
  return "A";
};

function B() {}
Object.setPrototypeOf(B, A)
B.prototype = Object.create(A.prototype)
B.prototype.constructor = B;

B.prototype.thing = function () {
  return A.prototype.thing.call(this) + "B";
};

// ---

function assert(ok) { if (!ok) throw new Error("Assertion failed") };

var a = new A();
var b = new B();
assert(a.constructor === A);
assert(b.constructor === B);
assert(a.thing() === "A");
assert(b.thing() === "AB");
assert(b instanceof A);