想在jQuery中检查null值

Question

$('#test').click(function() {
    var modal_class = $("#test").attr('data-code');
    $('.sign').each(function() {
        var sign = $(this).attr("href");
        if (sign.split("&")[0] != NULL) {
            var sign1 = sign.split("&")[0];
        } else {
            sign1 = $(this).attr("href");
        }
        $(this).attr("href", sign1 + "&test_carrer=" + modal_class);
    });
});

Now these sign and sign1 variables both are href I want to check value of sign.split("&")[0] which shows error NULL is not defined.. Above code is to check a variable on which basis url is generated.

if(sign.split("&")[0] != NULL){  

line is showing the following error in console

Uncaught ReferenceError: NULL is not defined

Answer 1

NULL and null are two different things and you're most probably looking to check against the latter. That thing aside, a better practice would be to test if the length of the resulting array is larger than 1 , so try to replace

if(sign.split("&")[0] != NULL) {...}

with

if (sign.split("&").length > 1) {...}

If separator is not found or is omitted, the array contains one element consisting of the entire string.

Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split

Answer 2

Try any of these..

<script>       
$('#test').click(function(){
 var modal_class = $("#test").attr('data-code');
 $('.sign').each(function() {
 var sign = $(this).attr("href");
 if(sign.split("&")[0] !== null)
 //OR
 if(sign.split("&")[0]) // This will work for null, 0, false
 //OR
 if(sign.split("&")[0] != null)
 { 
  var sign1=sign.split("&")[0]; 
   }else{
  sign1= $(this).attr("href");
  }
    $(this).attr("href", sign1+"&test_carrer="+modal_class);
 });
 });
 </script>