How can I check whether a variable is defined in JavaScript? [duplicate]

Question

Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.))

if (elem) { // or !elem 

or

if (typeof elem !== 'undefined') { 

or

if (elem != null) { 

Answer 1

You want the typeof operator. Specifically:

if (typeof variable !== 'undefined') {     // the variable is defined } 

Answer 2

The typeof operator will check if the variable is really undefined.

if (typeof variable === 'undefined') {     // variable is undefined } 

The typeof operator, unlike the other operators, doesn't throw a ReferenceError exception when used with an undeclared variable.

However, do note that typeof null will return "object". We have to be careful to avoid the mistake of initializing a variable to null. To be safe, this is what we could use instead:

if (typeof variable === 'undefined' || variable === null) {     // variable is undefined or null } 

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For more info on using strict comparison === instead of simple equality ==, see:
Which equals operator (== vs ===) should be used in JavaScript comparisons?