Rover

gistfile1.js

// a answer to the rover interview question http://www.techinterviewpuzzles.com/2010/09/mars-rovers-thoughtworks-puzzles.html

describe("Rover", function() {

  function rover(startCords, movement) {
    var x = Number( startCords[0] );
    var y = Number( startCords[2] );
    var direction = startCords.charAt( startCords.length -1 );

    var steps = movement.split('');

    steps.forEach( function(step){
      if ( step === 'L' || step === 'R' ) {
        return direction = turn(direction, step);
      }

      return move(direction);
    });

    function move(dir){
      switch (dir) {
        case 'N':
          y += 1
          break;
        case 'E':
          x += 1
          break;
        case 'S':
          y += -1
          break;
        case 'W':
          x += -1
          break;
      }
    }

    return [x, y, direction].join(' ');
  }

  function turn(start, dir) {
    var directions = ['N', 'E', 'S', 'W'];
    var index = directions.indexOf(start);

    if (dir === 'R') {
      index = (index + 1 > 3) ? 0 : index + 1

      return directions[index];
    }

    if (dir === 'L') {
      index = (index - 1 < 0) ? 3 : index - 1

      return directions[index];
    }
  }


  describe("turn() truns around the weather compass", function(){
    it("turns right", function(){
      var expected = 'E';

      expect( turn('N', 'R') ).to.eq(expected);
    });

    it("turns left", function(){
      var expected = 'W';

      expect( turn('N', 'L') ).to.eq(expected);
    })
  });

  it("moves the rover by passing args", function(){
    var expected = '1 3 N';

    expect( rover('1 2 N', 'LMLMLMLMM') ).to.eq(expected);
  });

  it("moves the rover by passing args", function(){
    var expected = '5 1 E';

    expect( rover('3 3 E', 'MMRMMRMRRM') ).to.eq(expected);
  });

});

I like how you calculated the turn, using an array and quick indexing hopping.

If this was ruby I would want the code to be cleaned/tightened up a bit more.

Lin 14 and 17, I like how move and turn are two different functions. One suggestion would be that I think it's a bit odd that move will change the variable for you and turn returns a value that you then set. It would be less confusing if they matched in that regards.

Not sure why you did: '
var direction = startCords.charAt( startCords.length -1 );
instead of:
var direction = startCords.charAt( 4 );
to match how you were grabbing chars before.

Overall, it looks pretty good. As mentioned above, I would tighten the code up if it were in ruby but not sure about js. Also, a part of this exercise is working through it with the person but the code side of it looks good.