[javascript] Checking if a variable exists in javascript

I know there are two methods to determine if a variable exists and not null(false, empty) in javascript:

1) if ( typeof variableName !== 'undefined' && variableName )

2) if ( window.variableName )

which one is more preferred and why?

This question is related to javascript

The answer is


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).


if ( typeof variableName !== 'undefined' && 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 => see what works for your case

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.


If you want to check if a variable (say v) has been defined and is not null:

if (typeof v !== 'undefined' && 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
}

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  && typeof variableName !== 'undefined' )

I found this shorter and much better:

    if(varName !== (undefined || null)) { //do something }