How to compare variables to undefined if I don t know whether they exist

Question

In JavaScript you can declare a variable and if it   s undefined  you can check variable    undefined  I know that  but how can you compare a value that you don   t know yet if it   s in memory   For example  I have a class which is created when the user clicks a button  Before this  the class is undefined     it doesn   t exist anywhere  how can I compare it   Is there a way without using try   catch

User · Answer

if   obj           object  not class   doesn t exist yet   else

User · Answer

undefined is true in javascript  so if you want to know whether your variable or object is undefined and want to take actions  you could do something like this   if  lt object or variable gt            take actions if object is not undefined   else          take actions if object is undefined

User · Answer

if  document getElementById  theElement       do whatever after this   For undefined things that throw errors  test the property name of the parent object instead of just the variable name - so instead of   if  blah        do   if  window blah

User · Answer

The best way is to check the type  because undefined null false are a tricky thing in JS  So   if typeof obj      undefined            obj is a valid variable  do something here      Note that typeof always returns a string  and doesn t generate an error if the variable doesn t exist at all

User · Answer

if  obj     undefined           Create obj     If you are doing extensive javascript programming you should get in the habit of using     and     when you want to make a type specific check    Also if you are going to be doing a fair amount of javascript  I suggest running code through JSLint http   www jslint com it might seem a bit draconian at first  but most of the things JSLint warns you about will eventually come back to bite you

[javascript] How to compare variables to undefined, if I don’t know whether they exist?

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