How can I iterate up and down a large javascript array in chunks?

I'm trying to write a function that takes a large array and iterates up and down it in a set number of chunks via a previous and next button. I have the next button working fine but cannot get it to reverse the array the same way I go forward. Here's what I have:

javscript:
          success: function(data) {
                  //alert(data);
                  var body = data; 
                  console.log(body.length);
                  //body/data is a string
                   var text = body.split(' ');
                  text.chunk = 0; text.chunkSize = 15;
                  // var text = body.split(' ');
                   var next = true;
                  console.log(text.length);

                   var increment = function(array,next) {
      if (array.chunk < array.length) {
             var slice = array.slice(array.chunk,Math.min(array.chunk+array.chunkSize, array.length));
                 var chunk = slice.join(" ");
                 if (next) {   
                      array.chunk += array.chunkSize; 
                        $( '#test' ).html('<p>' + chunk + '</p>');
                                 console.log(chunk);
                       }else  {
                           var slice = array.slice(array.chunk,Math.min(array.chunk+array.chunkSize, array.length));

                           array.chunk -= array.chunkSize;

                          //chunk = array.chunk - array.chunkSize;
                         $( '#test' ).html(chunk);
                                 console.log(chunk);
                      }

                          }



    }
        $("#prev").click(function() {
      increment(text);
       });
      $("#button").click(function() {
      increment(text, next);
      });