JavaScript check if variable exists (is defined/initialized)

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

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?