Edit in JSFiddle

function multiply(a, b) {
  return a * b;
}

const handler = {
  apply: function(target, thisArg, argumentsList) {
    return target(argumentsList[0], argumentsList[1]) + 1;
  }
};

var proxy = new Proxy(multiply, handler);

document.write(multiply(2, 5), "<br>"); // Result -> 10
document.write(proxy(2, 5)); // Result -> 11