Why is null considered an object in JavaScript?  Is checking   if ( object == null )       Do something   the same as   if ( !object )       Do something   ?  And also:  What is the difference between null and undefined?...

How can I check for null values in JavaScript? I wrote the code below but it didn't work.  if (pass == null || cpass == null || email == null || cemail == null || user == null) {            alert("fill all columns");     return false;...

How do I verify the existence of an object in JavaScript?  The following works:  if (!null)    alert("GOT HERE");   But this throws an Error:  if (!maybeObject)    alert("GOT HERE");   The Error:      maybeObject is not defined....

I've come across the following code:  function test(data) {     if (data != null && data !== undefined) {         // some code here     } }   I'm somewhat new to JavaScript, but, from other questions I've been reading here, I'...

I'm using a system to alert users when a major update has happened to a site, and I do it with LocalStorage, when I made the system, I made the system check if tip was "null", then set "tip" to true when they got the alert. Now, I...

While trying to debug I am get the 'length' null error with this line.    It is written just like the book instructed, so I don't understand why it is giving me the error? Thanks, =)  if (capital.length < 1) {   ( here is the full code...

null and undefined don't have a toString or valueOf method. Afaik using String calls the toString method of its parameter (e.g. String({}) => [object Object]).   Why do String(null) or String(undefined work then? It doesn't implicitly do O...

[Bounty Edit]  I'm looking for a good explanation when you should set/use null or undefined and where you need to check for it. Basically what are common practices for these two and is really possible to treat them separately in generic maintaina...

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I often see in third party JavaScript code that after:  var el = document.getElementById(elementId);   object is often nulled and comment along this operation says that it is done for IE:  el = null; // IE   What's the real purpose? Any resource...