JS Wrap

wrap.js

function wrap(orig, newDef) {
  return function(...args) {
    return newDef.apply(this, [orig].concat(args));
  };
}

myLib.getTotal = wrap(myLib.getTotal, function(orig, ...args) {
  let total = orig.apply(this, args) * 0.8;
  return total + this.getTax();
});