Show loading image while $.ajax is performed

You can, of course, show it before making the request, and hide it after it completes:

$('#loading-image').show();
$.ajax({
      url: uri,
      cache: false,
      success: function(html){
        $('.info').append(html);
      },
      complete: function(){
        $('#loading-image').hide();
      }
    });

I usually prefer the more general solution of binding it to the global ajaxStart and ajaxStop events, that way it shows up for all ajax events:

$('#loading-image').bind('ajaxStart', function(){
    $(this).show();
}).bind('ajaxStop', function(){
    $(this).hide();
});

Use the ajax object's beforeSend and complete functions. It's better to show the gif from inside beforeSend so that all the behavior is encapsulated in a single object. Be careful about hiding the gif using the success function. If the request fails, you'll probably still want to hide the gif. To do this use the Complete function. It would look like this:

$.ajax({
    url: uri,
    cache: false,
    beforeSend: function(){
        $('#image').show();
    },
    complete: function(){
        $('#image').hide();
    },
    success: function(html){
       $('.info').append(html);
    }
});

HTML Code :

<div class="ajax-loader">
  <img src="{{ url('guest/images/ajax-loader.gif') }}" class="img-responsive" />
</div>

CSS Code:

.ajax-loader {
  visibility: hidden;
  background-color: rgba(255,255,255,0.7);
  position: absolute;
  z-index: +100 !important;
  width: 100%;
  height:100%;
}

.ajax-loader img {
  position: relative;
  top:50%;
  left:50%;
}

JQUERY Code:

$.ajax({
  type:'POST',
  beforeSend: function(){
    $('.ajax-loader').css("visibility", "visible");
  },
  url:'/quantityPlus',
  data: {
   'productId':p1,
   'quantity':p2,
   'productPrice':p3},
   success:function(data){
     $('#'+p1+'value').text(data.newProductQuantity);
     $('#'+p1+'amount').text("₹ "+data.productAmount);
     $('#totalUnits').text(data.newNoOfUnits);
     $('#totalAmount').text("₹ "+data.newTotalAmount);
  },
  complete: function(){
    $('.ajax-loader').css("visibility", "hidden");
  }
});

}

I think this might be better if you have tons of $.ajax calls

$(document).ajaxSend(function(){
    $(AnyElementYouWantToShowOnAjaxSend).fadeIn(250);
});
$(document).ajaxComplete(function(){
    $(AnyElementYouWantToShowOnAjaxSend).fadeOut(250);
});

NOTE:

If you use CSS. The element you want to shown while ajax is fetching data from your back-end code must be like this.

AnyElementYouWantToShowOnAjaxSend {
    position: fixed;
    top: 0;
    left: 0;
    height: 100vh; /* to make it responsive */
    width: 100vw; /* to make it responsive */
    overflow: hidden; /*to remove scrollbars */
    z-index: 99999; /*to make it appear on topmost part of the page */
    display: none; /*to make it visible only on fadeIn() function */
}