Usage of rest parameter and spread operator in javascript

Question

What's the usage of rest parameter that will be added in ECMAScript 6?

For example, in ECMAScript 5 you can do the following to get an array of parameters starting from the second element:

// ES 5
store('Joe', 'money');
store('Jane', 'letters', 'certificates');
function store(name) {
  var items = [].slice.call(arguments, 1); //['money'] in first case
  items.forEach(function (item) {
    vault.customer[name].push(item);
  });
}

and that will be equivalent to the following code in ECMAScript 6:

// ES 6
store('Joe', 'money');
store('Jane', 'letters', 'certificates');
function store(name, ...items) {
  items.forEach(function (item) {
    vault.customer[name].push(items)
  });
}

Is the difference between them is just syntax or there's a performance issue?

Also for spread operator (...)

//in ecmascript5
var max = Math.max.apply(null, [14, 3, 77]);
//but in ecmascript6
var max = Math.max(...[14, 3, 77]);

Is this just syntax change or performance issue?

Answer 1

Is the difference between them is just syntax or there's a performance issue?

Both, and more...

Rest parameters:

1. Are a known idiom in other languages (Ruby, Python).
2. Are esier to read and maintain (vs. slice).
3. Are easier to understand for beginners.
4. Can (and likely will) result in better performance, since engines can optimize.
5. Are tool friendlier, as they can be analyzed statically.