JQuery Javascript check if var exists

Question

Possible Duplicate    How can I check whether a variable is defined in JavaScript    Is there a standard function to check for null  undefined  or blank variables in JavaScript        I have a script that occurs in two parts   The first part sets up a var   var pagetype    textpage     The 2nd part is a simple if statement   if pagetype     textpage       do something      Now the 2nd part  the if statement  appears on all pages of my site  But the first part  where the var is declared  only appears on some of my pages   On the pages without the var I naturally get this error   Uncaught ReferenceError  pagetype is not defined   So my question is  is there a way with JavaScript or JQ to detect if a var exists at all  not just if it has data assigned to it    I am imagining I would just use another if statment  eg   if   a var called pagetypes exists

User · Accepted Answer

I suspect there are many answers like this on SO but here you go   if   typeof pagetype      undefined   amp  amp  pagetype     textpage

User · Answer

You can use typeof   if  typeof pagetype      undefined            pagetype doesn t exist

User · Answer

Before each of your conditional statements  you could do something like this   var pagetype   pagetype    false  if  pagetype      something           do stuff

User · Answer

For your case  and 99 9  of all others elclanrs answer is correct   But because undefined is a valid value  if someone were to test for an uninitialized variable  var pagetype       undefined if  typeof pagetype      undefined     true   the only 100  reliable way to determine if a var exists is to catch the exception   var exists   false  try   pagetype  exists   true   catch e     if  exists  amp  amp            But I would never write it this way

User · Answer

To test for existence there are two methods   a   property  in object  This method checks the prototype chain for existence of the property   b  object hasOwnProperty   property     This method does not go up the prototype chain to check existence of the property  it must exist in the object you are calling the method on   var x     variable declared in global scope and now exists   x  in window     true window hasOwnProperty   x       true   If we were testing using the following expression then it would return false  typeof x      undefined      false

User · Answer

It is impossible to determine whether a variable has been declared or not other than using try  catch to cause an error if it hasn t been declared  Test like   if  typeof varName     undefined      do not tell you if varName is a variable in scope  only that testing with typeof returned undefined  e g   var foo  typeof foo     undefined      true typeof bar     undefined      true   In the above  you can t tell that foo was declared but bar wasn t  You can test for global variables using in   var global   this       bar  in global      false   But the global object is the only variable object  you can access  you can t access the variable object of any other execution context   The solution is to always declare variables in an appropriate context    The global object isn t really a variable object  it just has properties that match global variables and provide access to them so it just appears to be one

[javascript] JQuery/Javascript: check if var exists

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