Requirement: use default parameter values

[BK239]

Use default parameters, instead of checking if they're undefined. For example: Instead of:

function (x) {
       if (x === undefined)
              return 7;
       return x;
}

do this:

function (x=7) {
        return x;
}

See http://es6-features.org/#DefaultParameterValues for an explanation. Keep in mind this is one of the more difficult ones since it requires more knowledge about a function's use cases.

[RickardBjorklund]

No longer needed since assignment 3 is done!