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The arguments object

Inside every function an arguments object is accessible. It is an Array-like object that contains the values of the arguments passed to the function.
var add = function() {
  var arg0 = arguments[0];
  var arg1 = arguments[1];
  return arg0 + arg1;
};

var sum = add(1, 2);
add is called with arguments 1 and 2. So arguments[0] contains the value 1 and arguments[1] contains the value 2.

With the arguments object you can access the passed arguments without parameters. This is useful, for example, if you want to write a function that processes an arbitrary number of arguments.
var add = function() {
  var sum = 0;
  for (var i = 0; i < arguments.length; i++) {
    sum = sum + arguments[i];
  }
  return sum;
};

var s1 = add(2, 4);
var s2 = add(2, 3, 5, 7);
We iterate over all passed arguments and sum them up. Thereby arguments.length returns the number of passed arguments. Now we can call add with an arbitrary number of arguments.

Exercise

Write a function max that calculates the maximum of an arbitrary number of numbers. max(1, 2) should return 2 and max(2, 3, 1) should return 3.