ES6function f (x, y, ...a) {    return (x + y) * a.length}f(1, 2, "hello", true, 7) === 9ES5function f (x, y) {    var a = Array.prototype.slice.call(arguments, 2);    return (x + y) * a.length;};f(1, 2, "hello", true, 7) === 9;