JavaScript: checking for null and undefined

Question

This question has been answered earlier. I checked multiple question in SO but i am not able to understand that properly. Apologies for that.

I am using ajax and data is coming from database.

if(data.count > 0 ){
    $.each(data.content, function(key, value ){
if((value.technology) != ''){
 html+='<button class="form-control" disabled>'+value.technology+'</button>&nbsp;';
                            }

i also tried this

if((value.technology6 !== 'null')){
     alert(value.technology6);
  html+='<button class="testsss testss" disabled>'+value.technology6+'</button>&nbsp;';
                            }

and tried this as well

if(!(value.technology7)){
 html+='<button class="testsss testss" disabled>'+value.technology7+'</button>&nbsp;';
  }

problem is i am getting null as output. i am not sure what am i doing wrong and what is the proper way to check null and undefined.

Thanks for your advise.

Have you tried logging `value` in your loop? What does `data.content` look like? — canon, Apr 30 '16 at 15:57
`if (value.technology)` will skip both, but it will also skip `false`, `NaN`, `0` and `''` so it depends on what you want to do with those — CupawnTae, Apr 30 '16 at 15:58
do you want `if((value.technology7)){` instead of `if(!(value.technology7)){` no "!" — adrianj98, Apr 30 '16 at 16:03
`null` is a null-value `'null'` is a String containing the word "null". `null !== 'null'`. Fix your condition! — Thomas, Apr 30 '16 at 16:35

score 0 · Answer 1 · answered Apr 30 '16 at 16:04

please check if value and data are not null or undefined , then write them in alert() function to see what alert you are getting like alert(data.content); or alert(value) ..

else check of null is correct. to check undefined you have to check like

if(data.content != undefined){
// your code here 
}

if you can more elaborate what is in there data and value , might help to solve your problem more appropriately

score 0 · Accepted Answer · answered Apr 30 '16 at 18:52

Check Undefined

if(x != undefined)
{
    //x is defined
} else {
    //x is not defined
}

Check Null

if(y != null)
{
    //y is not null, we can do stuff
} else {
    //uh-oh, y is null
}

Also:

z = null;
w = undefined;
console.log(z == w); //Returns true

JavaScript: checking for null and undefined

2 Answers2