A declared variable without assigning any value to it.
Implicit returns of functions due to missing return statements.
return statements that do not explicitly return anything.
Lookups of non-existent properties in an object.
Function parameters that have not passed.
Anything that has been set to the value of undefined.
Any expression in the form of void(expression)
The value of the global variable undefined
null means empty or non-existent value which is used by programmers to indicate no value. null is a primitive value and you can assign null to any variable. null is not an object, it is a primitive value. For example, you cannot add properties to it. Sometimes people wrongly assume that it is an object, because typeof null returns "object".