function add() {
  var args = Array.prototype.slice.call(arguments)
  return args.reduce(function(a, b) { return a + b })
}

console.log(add(1, 2)) // 3

console.log(add(1, 2, 3, 4)) // 10

See here.

You can access arguments of a function with arguments object. It returns an object.

function someFn() {
  console.log(arguments)
}

someFn(1, 2, 3)  // { 0: 1, 1: 2, 2: 3 }
someFn(1, 2)     // { 0: 1, 1: 2 }