JavaScript: checking for null and undefined [duplicate]

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
– canon, Commented Apr 30, 2016 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
– CupawnTae, Commented Apr 30, 2016 at 15:58
do you want if((value.technology7)){ instead of if(!(value.technology7)){ no "!" — adrianj98
– adrianj98, Commented Apr 30, 2016 at 16:03

user3786755 · Accepted Answer · 2016-04-30 18:52:14Z

0

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

answered Apr 30, 2016 at 18:52

user3786755

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Comments

Rishi · Accepted Answer · 2016-04-30 16:04:26Z

0

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

answered Apr 30, 2016 at 16:04

Rishi

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