Checking if a variable exists in javascript

Question

I know there are two methods to determine if a variable exists and not null false  empty  in javascript   1  if   typeof variableName      undefined   amp  amp  variableName     2  if   window variableName    which one is more preferred and why

User · Answer

if  variable  can be used if variable is guaranteed to be an object  or if false  0  etc  are considered  default  values  hence equivalent to undefined or null    typeof variable     undefined  can be used in cases where a specified null has a distinct meaning to an uninitialised variable or property  This check will not throw and error is variable is not declared

User · Answer

if   typeof variableName      undefined   amp  amp  variableName        could throw an error if var doesnt exist at all  if   window variableName        could be true if var    0      further on it depends on what is stored into that var    if you expect an object to be stored in that var maybe if     window variableName     could be the right way  best way   gt  see what works for your case

User · Answer

I found this shorter and much better       if varName      undefined    null       do something

User · Answer

I m writing an answer only because I do not have enough reputations to comment the accepted answer from apsillers  I agree with his answer  but     If you really want to test if a variable is undeclared  you ll need to   catch the ReferenceError       is not the only way  One can do just   this hasOwnProperty  bar     to check if there is a variable bar declared in the current context   I m not sure  but calling the hasOwnProperty could also be more fast effective than raising an exception  This works only for the current context  not for the whole current scope

User · Answer

A variable is declared if accessing the variable name will not produce a ReferenceError  The expression typeof variableName      undefined  will be  false in only one of two cases    the variable is not declared  i e   there is no var variableName in scope   or the variable is declared and its value is undefined  i e   the variable s value is not defined    Otherwise  the comparison evaluates to true   If you really want to test if a variable is declared or not  you ll need to catch any ReferenceError produced by attempts to reference it   var barIsDeclared   true   try  bar    catch e        if e name     ReferenceError             barIsDeclared   false            If you merely want to test if a declared variable s value is neither undefined nor null  you can simply test for it   if  variableName     undefined  amp  amp  variableName     null            Or equivalently  with a non-strict equality check against null   if  variableName    null            Both your second example and your right-hand expression in the  amp  amp  operation tests if the value is  falsey   i e   if it coerces to false in a boolean context  Such values include null  false  0  and the empty string  not all of which you may want to discard

User · Answer

It is important to note that  undefined  is a perfectly valid value for a variable to hold   If you want to check if the variable exists at all    if  window variableName    is a more complete check  since it is verifying that the variable has actually been defined   However  this is only useful if the variable is guaranteed to be an object  In addition  as others have pointed out  this could also return false if the value of variableName is false  0      or null   That said  that is usually not enough for our everyday purposes  since we often don t want to have an undefined value   As such  you should first check to see that the variable is defined  and then assert that it is not undefined using the typeof operator which  as Adam has pointed out  will not return undefined unless the variable truly is undefined   if   variableName   amp  amp  typeof variableName      undefined

User · Answer

If you want to check if a variable  say v  has been defined and is not null   if  typeof v      undefined   amp  amp  v     null           Do some operation       If you want to check for all falsy values such as  undefined  null      0  false   if  v          Do some operation

[javascript] Checking if a variable exists in javascript

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