JavaScript cheque if adaptable exists (is outlined/initialized)

Which methodology of checking if a adaptable has been initialized is amended/accurate?(Assuming the adaptable may clasp thing (drawstring, int, entity, relation, and many others.))

if (elem) { // or !elem

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

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if (elem != null) {

You privation the typeof function. Particularly:

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

The typeof function volition cheque if the adaptable is truly undefined.

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

The typeof function, dissimilar the another operators, doesn't propulsion a ReferenceError objection once utilized with an undeclared adaptable.

Nevertheless, bash line that typeof null volition instrument "object". We person to beryllium cautious to debar the error of initializing a adaptable to null. To beryllium harmless, this is what we may usage alternatively:

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

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For much information connected utilizing strict examination === alternatively of elemental equality ==, seat:
Which equals function (== vs ===) ought to beryllium utilized successful JavaScript comparisons?

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Checking if an Element of a List Exists Between Two Values in JavaScript

Checking if an Element of a List Exists Between Two Values in JavaScript from Youtube.com