How the line number 4 of below code is taking the input

  angular.module('myReverseFilterApp', [])
    .filter('reverse', function() { 
           return function(input, uppercase) {
             input = input || ''; //  declaring the variable
                var out = '';
               for (var i = 0; i < input.length; i++) {
              out = input.charAt(i) + out;
            }
          // if condition for uppercase
            if (uppercase) {
              out = out.toUpperCase();
            }
            return out;//return statement
          };
        });

The code defines a filter for AngularJS that takes a string as input and returns the string in reverse order as well as uppercase.

I couldn't understand the code that is written in line number 4 that takes the input:

input = input || ''; //  declaring the variable

It will be helpful if any one can describe me how line number 4 is taking input.