Why null does not have a __proto__?

Question

null in JS is considered as an object. typeof null == "object" //true

Then why does null not have a __proto__ . All objects should have a __proto__ in JS

 var n = null; console.log(n.__proto__); //Cannot read property '__proto__' of null` 

Can someone provide a detailed answer?

Answer 1

null in JS is considered as an object. typeof null == "object" //true

As already mentioned, null is actually a primitive value (like undefined , a Boolean value, a String value, a Number value, or a symbol) . But the language spec dictates that typeof null returns the string "object" . (Similarly the specification dictates to return "function" for function objects instead of "object" , like it does for array objects )

But that's only one half of the story.

Then why does null not have a __proto__

You might be wondering:

Why can I access the protoype of a string value ( "foo".__proto__ ) but not null if both are primitive values ?

That's because Booleans, Strings and Numbers have object equivalents ( eg new String("foo") ) and when you try to access a property on a boolean/number/string primitive, they are internally converted to an object value

 console.log("foo".slice) // does `new String("foo").slice` internally 

However for undefined and null , no such object equivalents exist, hence the error.

 try { console.log(null.foo); } catch(e) { console.log(e.message); } try { console.log(undefined.foo); } catch(e) { console.log(e.message); } 

All objects should have a __proto__ in JS

Not necessarily. It is possible to create objects without a prototype via Object.create :

 const obj = Object.create(null); console.log(obj.__proto__); 

Answer 2

typeof null == "object" is a mistake, it's not actually an object. See why is typeof null "object"? for more details about why typeof considers it to be an object.

Answer 3

From MDN web docs:

A primitive (primitive value, primitive data type) is data that is not an object and has no methods. In JavaScript, there are 6 primitive data types: string, number, boolean, null , undefined, symbol (new in ECMAScript 2015).

Most of the time, a primitive value is represented directly at the lowest level of the language implementation.

All primitives are immutable, ie, they cannot be altered. It is important not to confuse a primitive itself with a variable assigned a primitive value. The variable may be reassigned a new value, but the existing value can not be changed in the ways that objects, arrays, and functions can be altered.

https://developer.mozilla.org/en-US/docs/Glossary/Primitive

Furthermore:

In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Consequently, null had 0 as type tag, hence the bogus typeof return value.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof#null