What is the mos reliable way if I want to check if the variable is null or is not present?.
There are diferent examples:
if (null == yourvar)
if (typeof yourvar != 'undefined')
if (undefined != yourvar)
None of the above.
You don't want to use ==, or a variety thereof, because it performs type coercion. If you really want to check whether something is explicitly null, use the === operator.
Then again, your question shows perhaps some lack of clarity of your requirements. Do you actually mean null specifically; or does undefined count too? myVar === null will certainly tell you if the variable is null, which is the question you asked, but is this really what you want?
Note that there's a lot more information in this SO question. It's not a direct duplicate, but it covers very similar principles.
