How to check if a variable is not null

Question

I know that below are the two ways in JavaScript to check whether a variable is not null  but I   m confused which is the best practice to use   Should I do   if  myVar          or  if  myVar     null

User · Answer

code inside your if myVar    code   will be NOT executed only when myVar is equal to  false  0   quot  quot   null  undefined  NaN or you never defined variable myVar  then additionally code stop execution and throw exception   code inside your if myVar     null   code  will be NOT executed only when myVar is equal to null or you never defined it  throws exception    Here you have all  src  if      its negation           its negation

User · Answer

Have a read at this post  http   enterprisejquery com 2010 10 how-good-c-habits-can-encourage-bad-javascript-habits-part-2   It has some nice tips for JavaScript in general but one thing it does mention is that you should check for null like   if myvar        It also mentions what s considered  falsey  that you might not realise

User · Answer

Sometimes if it was not even defined is better to be prepared  For this I used typeof  if typeof variable       undefined           it exist     if variable     null              and is not null           else             but is null         else         it doesn t

User · Answer

Here is how you can test if a variable is not NULL   if  myVar     null         the block will be executed if myVar is not null   it will be executed if myVar is undefined or false or 0 or NaN or anything else

User · Answer

The two conditional statements you list here are not better than one another  Your usage depends on the situation  You have a typo by the way in the 2nd example  There should be only one equals sign after the exclamation mark   The 1st example determines if the value in myVar is true and executes the code inside of the        The 2nd example evaluates if myVar does not equal null and if that case is true it will execute your code inside of the        I suggest taking a look into conditional statements for more techniques  Once you are familiar with them  you can decide when you need them

User · Answer

They are not equivalent  The first will execute the block following the if statement if myVar is truthy  i e  evaluates to true in a conditional   while the second will execute the block if myVar is any value other than null   The only values that are not truthy in JavaScript are the following  a k a  falsy values     null undefined 0     the empty string  false NaN

User · Answer

if  0  means false  if  -1  or any other number than 0  means true  following value are not truthy  null  undefined  0   quot  quot empty string  false  NaN never use number type like id as       if  id      for id type with possible value 0  we can not use if  id      because  if  0  will means false  invalid  which we want it means valid as true id number  So for id type  we must use following     if   Id     undefined   amp  amp   Id     null   amp  amp   Id      quot  quot                                                                                                                                                                    else                                                                                                                                                                for other string type  we can use if  string       because null  undefined  empty string all will evaluate at false  which is correct         if  string type variable

User · Answer

if myVar is null then if block not execute other-wise it will execute   if  myVar    null

User · Answer

There is another possible scenario I have just come across   I did an ajax call and got data back as null  in a string format  I had to check it like this   if value     null       So  null was a string which read  null  rather than really being null   EDIT  It should be understood that I m not selling this as the way it should be done   I had a scenario where this was the only way it could be done   I m not sure why    perhaps the guy who wrote the back-end was presenting the data incorrectly  but regardless  this is real life   It s frustrating to see this down-voted by someone who understands that it s not quite right  and then up-voted by someone it actually helps

[javascript] How to check if a variable is not null?

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