JS: null

What is null

• null is a literal expression, e.g. you can write let x = null;
• null is one of the type of JavaScript values, and it is the only value of that type. 〔see Value Types〕

typeof null

typeof null return "object". (this is a historical bug. typeof null should return "null" )

typeof null === "object"

Why “typeof null” return "object"

typeof null return "object" is a historical implementation bug. Now we are stuck with it. See: 〔The history of “typeof null” 2013-11-05 By Dr Axel Rauschmayer. At http://www.2ality.com/2013/10/typeof-null.html , accessed on 2015-10-28〕

null is not a object, because It doesn't have the defining quality of JavaScript objects, namely, it's not a collection of key/value pairs; you cannot add properties to null.

// null.p = 4;
// TypeError: Cannot set property 'p' of null

〔see Object Type〕

What is the Use of null

There are 2 major uses of null.

• you can set a object's prototype (parent) to null, to make it having no prototype. 〔see Get Set Prototype〕 Or, use Object.create with null to create a object with no parent.
• You can set a variable to null as default initial value. (unassigned variable has default value of undefined. 〔see undefined〕).