Javascript混淆null，instanceof和typeof的语法不一致？

Question

var obj = {};
typeof obj; // returns "object"
obj instanceof Object // return true

typeof null // returns "object"
null instanceof Object // returns false

And how about

typeof undefined // return "undefined"
undefined instanceof undefined 
// ERROR:  Uncaught TypeError: Expecting a function in instanceof check, 
//         but got undefined

Why is this the case? I have read a lot about related topics on SO but still can't get this.

Understand that 'typeof' would return a String, so it pretty much reflects the rules in Javascript.(eg. null is a object... well fine..) But why "null instanceof Object" return false ?

"x instanceof y"

Does it mean 'x' has to be created by the 'y' constructor? And for null this is not the case ?

EDIT

Would really appreciate if you could explain the different intention behind instanceof and typeof otherthan then syntax and return value.

Difference between null and undefined

typeof null        // object (bug in ECMAScript, should be null)
typeof undefined   // undefined
null === undefined // false
null  == undefined // true

REF

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

Answer 1

It's just the design decision which might be contrived or weird. According to the typeof UnaryExpression if evaluated as the following. I've just included the poin that matters.

ECMA Spec: Return a String determined by Type(val) according to Table 20.

Table 20:
╔═════════════╦══════════╗
║ Type of val ║  Result  ║
╠═════════════╬══════════╣
║ null        ║ "object" ║
╚═════════════╩══════════╝

So, there's nothing we can do about it. It's status-by-design . But it's correct to return false because, there is a separate type for null called Null type

Null type: type whose sole value is the null value

null isn't an instance of Object, obviously, since it has got it's own type. It's just that typeof operator returns "object" . It's got to do with the design of javascript.

Why is it so? Will have to ask Brendan Eich(Founder of Javascript).

Answer 2

That's a common bug of ECMAScript.

null is not an object , it's a primitive value.(So you can't modify it like adding properties to it)

typeof null should return null

typeof null // object (bug in ECMAScript, should be null)

typeof undefined // undefined

null === undefined // false

null == undefined // true

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/null

EDITED

It's not available to see though

Changed 3 weeks ago by brendan

You know, this all came about because of rushing in early May 1995, which led to a leak of type tag representation shared by null and object types. But null means "no object", so it didn't raise hackles until it was too late to fix in Netscape 2, and after that we were loath to "fix" it and "break the web". That argument only applies more in degree of web population now. We have other fish to fry. This one was has been swallowed already. Let's not change typeof null for ES4 and work on more vital issues.

http://web.archive.org/web/20071110193102/http://bugs.ecmascript.org/ticket/250

also check this answer

Why is null an object and what's the difference between null and undefined?

Answer 3

instanceof more appealing, however you checked instaceof undefined going throw exception that because the undefined is not any singular type object name, It's may or may not sub method of JavaScript Object . Because undefined that type for null (Null Type).

typeof undefined // return "undefined"
undefined instanceof Object // return False 

You should check undefined is instaceof JS object, that give a no it's not the object. It's give a boolean result.