Whether a variable is undefined

Question

How do I find if a variable is undefined   I currently have   var page name       pageToEdit  selected   text    var table name       pageToEdit  selected   val    var optionResult       pageToEditOptions  selected   val     var string     z z   if   page name     undefined      string      amp page name     page name    if   table name     undefined      string      amp table name     table name    if   optionResult     undefined      string      amp optionResult     optionResult

User · Answer

jQuery val   and  text   will never return  undefined  for an empty selection   It always returns an empty string  i e         html   will return null if the element doesn t exist though You need to do   if page name          For other variables that don t come from something like jQuery val   you would do this though   if typeof page name     undefined     You just have to use the typeof operator

User · Answer

if  var     undefined   or more precisely  if  typeof var      undefined    Note the     is used

User · Answer

You can just check the variable directly  If not defined it will return a falsy value   var string     z z   if  page name    string      amp page name     page name    if  table name    string      amp table name     table name    if  optionResult    string      amp optionResult     optionResult

User · Answer

function my url  base  opt        var retval         base       retval push  opt page name     amp page name     opt page name            retval push  opt table name     amp table name     opt table name            retval push  opt optionResult     amp optionResult     opt optionResult            return retval join         my url   z z      page name    pageX     no table name and optionResult             Returns        z z amp page name pageX      This avoids using typeof whatever      undefined    Also  there isn t any string concatenation

User · Answer

http   constc blogspot com 2008 07 undeclared-undefined-null-in-javascript html  Depends on how specific you want the test to be  You could maybe get away with   if page name   string      amp page name     page name

[jquery] Whether a variable is undefined

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