When is null or undefined used in JavaScript?

Question

I am confused as to when JavaScript returns null or undefined. Also different browsers seem to be returning these differently.

I am seeking some examples of null/undefined with the browsers that return them.

While I am now clear on the undefined aspect, I am still not 100% clear on null. Is it similar to a blank value?

E.g. You have a text box which does not have any value set. Now when you try to access its value, will it be null or undefined and are they similar?

Answer 1

The DOM methods getElementById(), nextSibling(), childNodes[n], parentNode() and so on return null (defined but having no value) when the call does not return a node object.

The property is defined, but the object it refers to does not exist.

This is one of the few times you may not want to test for equality-

if(x!==undefined) will be true for a null value

but if(x!= undefined) will be true (only) for values that are not either undefined or null.