4.9. Null
Intellectually, the null type is nearly identical to the undefined type. Like the undefined datatype, the null datatype is used to represent a lack of data and has only one legal value, the primitive value null. The null value is not assigned by the interpreter automatically, but rather by us deliberately.
We assign null to a variable, array element, or object property to indicate that the specified data container does not contain a legal number, string, boolean, array, or object value.
Note that null only compares equal to itself and undefined:
null == undefined; // true null == null; // true
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| 4.8. Undefined |  | 4.10. Onward! |
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