shifts.js

var data  = [
      {
        "player": "Joe",
        "shifts": [
          { "shift_start": 3, "shift_end": 5 }, // Joe started playing at shift #3, and stopped after shift #5
          { "shift_start": 7, "shift_end": 11 }
        ]
      },
      {
        "player": "Marc",
        "shifts": [
          { "shift_start": 1, "shift_end": 4 },
          { "shift_start": 9, "shift_end": 13 }
        ]
      },
      {
        "player": "John",
        "shifts": [
          { "shift_start": 4, "shift_end": 8 },
          { "shift_start": 10, "shift_end": 12 }
        ]
      }
    ];

  var shifts = {};

  var i = data.length;
  while (i>0) {
    data.forEach(function(player) {
      
      for ( var e = player.shifts[0].shift_start; e <= player.shifts[0].shift_end; e++ ) {
        if ( !shifts[e] ) shifts[ e ] = {};
        shifts[ e ][ player.player ] = player.player;
      }
      for ( var e = player.shifts[1].shift_start; e <= player.shifts[1].shift_end; e++ ) {
        if ( !shifts[e] ) shifts[ e ] = {};
        shifts[ e ][ player.player ] = player.player;
      }

    });

    i--
  }

  console.log('shifts', shifts);