jQuery push multiple values in input tag

Just set value to input store it in some temporary variable, append the value you want to store and set the value of input to this temporary variable eg.

<input id="user" type="text" name="user" class="form-control" />
<input id="user_id" type="hidden" name="user_id" value="" />

var oldValue = $("#user_id").val();
var arr = oldValue === "" ? [] : oldValue.split(',');
arr.push(yardval);
var newValue = arr.join(',');
$("#user_id").val(newValue);