1. Null indicates an object that does not exist, and 0 when it is converted to a value. It means "no object", that is, it should not have a value and is commonly used to indicate that a function is attempting to return an object that does not exist. Null is a special kind of object (reference type), which represents an indeterminate, but from null itself, it is neither an object nor a type. 1. Undefined is a primitive value that represents "none" and is Nan when converted to a value. When the declared variable has not been initialized, the default value for the variable is undefined. Undefined means "missing value", that is, there should be a value here, but there is no definition. 2. When the operation is null and undefined can be converted to false by type, but not equivalent to false. The undefined is equal to NULL, but not fully equal. 3. Nan is a special Number,nan that is unequal to any value and not equal to itself. 4. Typeof and instanceof are often used to determine whether a variable is empty, or what type it is. Typeof is a unary operation, and before a certain operand, the operand can be any type, and the unknown is judged. Instanceof is used to determine whether a variable is an instance of an object, a known match, such as a null instanceof object, and a return value of false, because NULL itself is not an object, nor is it an instance of objects.
Understand Null,undefined,nan.
