Test if something is not undefined in JavaScript

Question

I m checking if response 0  title     undefined   but I get the error      Uncaught TypeError  Cannot read property  title  of undefined

User · Answer

typeof   var foo  if  typeof foo     undefined        do stuff

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response 0  is not defined  check if it is defined and then check for its property title   if typeof response 0       undefined   amp  amp  typeof response 0  title      undefined          Do something

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I had trouble with all of the other code examples above  In Chrome  this was the condition that worked for me   typeof possiblyUndefinedVariable      undefined    I will have to test that in other browsers and see how things go I suppose

User · Answer

You must first check whether response 0  is undefined  and only if it s not  check for the rest  That means that in your case  response 0  is undefined

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Check if you re response 0  actually exists  the error seems to suggest it doesn t

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Check if condition    null   It will resolve the problem

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Actually you must surround it with an Try Catch block so your code won t stop from working  Like this   try      if typeof response 0  title      undefined             doSomething             catch e       console log  responde 0  title is undefined

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It ll be because response 0  itself is undefined

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I know i went here 7 months late  but I found this questions and it looks interesting  I tried this on my browser console   try x true catch e  false    If variable x is undefined  error is catched and it will be false  if not  it will return true  So you can use eval function to set the value to a variable  var isxdefined   eval  try x true catch e  false

User · Answer

Just check if response 0  is undefined   if response 0      undefined            If you still need to explicitly check the title  do so after the initial check   if response 0      undefined  amp  amp  response 0  title     undefined

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In some of these answers there is a fundamental misunderstanding about how to use typeof   Incorrect  if  typeof myVar     undefined      Correct  if  typeof myVar      undefined       The reason is that typeof returns a string  Therefore  you should be checking that it returned the string  undefined  rather than undefined  not enclosed in quotation marks   which is itself one of JavaScript s primitive types  The typeof operator will never return a value of type undefined     Addendum  Your code might technically work if you use the incorrect comparison  but probably not for the reason you think  There is no preexisting undefined variable in JavaScript - it s not some sort of magic keyword you can compare things to  You can actually create a variable called undefined and give it any value you like   let undefined   42    And here is an example of how you can use this to prove the first method is incorrect   https   jsfiddle net p6zha5dm

[javascript] Test if something is not undefined in JavaScript

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