How I learned from a crazy idea / The 'old' way
HTML
<script src="https://arnaudbuchholz.github.io/blog/jsfiddle-assert.js"></script>
JavaScript
function OldA () {
this._a = "A";
}
OldA.prototype.a = function () {
return this._a;
};
var a = new OldA();
assert(() => a instanceof OldA);
assert(() => a.a() === "A");