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

(name is undefined)

You: What is name? ^(*)
JavaScript: name? What's a name? I don't know what you're talking about. You haven't ever mentioned any name before. Are you seeing some other scripting language on the (client-)side?

name = null;

You: What is name?
JavaScript: I don't know.

In short; undefined is where no notion of the thing exists; it has no type, and it's never been referenced before in that scope; null is where the thing is known to exist, but it's not known what the value is.

One thing to remember is that null is not, conceptually, the same as false or "" or such, even if they equate after type casting, i.e.

name = false;

You: What is name?
JavaScript: Boolean false.

name = '';

You: What is name?
JavaScript: Empty string

^{*: name in this context is meant as a variable which has never been defined. It could be any undefined variable, however, name is a property of just about any HTML form element. It goes way, way back and was instituted well before id. It is useful because ids must be unique but names do not have to be.}

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The difference can be summarized into this snippet:

alert(typeof(null));      // object
alert(typeof(undefined)); // undefined

alert(null !== undefined) //true
alert(null == undefined)  //true

Checking

object == null is different to check if ( !object ).

The latter is equal to ! Boolean(object), because the unary ! operator automatically cast the right operand into a Boolean.

Since Boolean(null) equals false then !false === true.

So if your object is not null, but false or 0 or "", the check will pass because:

alert(Boolean(null)) //false
alert(Boolean(0))    //false
alert(Boolean(""))   //false

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null is not an object, it is a primitive value. For example, you cannot add properties to it. Sometimes people wrongly assume that it is an object, because typeof null returns "object". But that is actually a bug (that might even be fixed in ECMAScript 6).

The difference between null and undefined is as follows:

undefined: used by JavaScript and means “no value”. Uninitialized variables, missing parameters and unknown variables have that value. > var noValueYet; > console.log(noValueYet); undefined > function foo(x) { console.log(x) } > foo() undefined > var obj = {}; > console.log(obj.unknownProperty) undefined Accessing unknown variables, however, produces an exception: > unknownVariable ReferenceError: unknownVariable is not defined

null: used by programmers to indicate “no value”, e.g. as a parameter to a function.

Examining a variable:

console.log(typeof unknownVariable === "undefined"); // true

var foo;
console.log(typeof foo === "undefined"); // true
console.log(foo === undefined); // true

var bar = null;
console.log(bar === null); // true

As a general rule, you should always use === and never == in JavaScript (== performs all kinds of conversions that can produce unexpected results). The check x == null is an edge case, because it works for both null and undefined:

> null == null
true
> undefined == null
true

A common way of checking whether a variable has a value is to convert it to boolean and see whether it is true. That conversion is performed by the if statement and the boolean operator ! (“not”).

function foo(param) {
    if (param) {
        // ...
    }
}
function foo(param) {
    if (! param) param = "abc";
}
function foo(param) {
    // || returns first operand that can't be converted to false
    param = param || "abc";
}

Drawback of this approach: All of the following values evaluate to false, so you have to be careful (e.g., the above checks can’t distinguish between undefined and 0).

undefined, null

Booleans: false

Numbers: +0, -0, NaN

Strings: ""

You can test the conversion to boolean by using Boolean as a function (normally it is a constructor, to be used with new):

> Boolean(null)
false
> Boolean("")
false
> Boolean(3-3)
false
> Boolean({})
true
> Boolean([])
true

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What is the difference between null and undefined??

A property when it has no definition is undefined. a null is an object. Its type is object. null is a special value meaning "no value. undefined is not an object, its type is undefined.

You can declare a variable, set it to null, and the behavior is identical except that you'll see "null" printed out versus "undefined". You can even compare a variable that is undefined to null or vice versa, and the condition will be true:

 undefined == null
 null == undefined

Refer to JavaScript Difference between null and undefined for more detail.

and with your new edit yes

if (object == null)  does mean the same  if(!object)

when testing if object is false, they both only meet the condition when testing if false, but not when true

Check here: Javascript gotcha

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First part of the question:

Why is null considered an object in JavaScript?

It is a JavaScript design error they can't fix now. It should have been type null, not type object, or not have it at all. It necessitates an extra check (sometimes forgotten) when detecting real objects and is source of bugs.

Second part of the question:

Is checking if (object == null) Do something the same as if (!object) Do something

The two checks are always both false except for:

object is undefined or null: both true.

object is primitive, and 0, "", or false: first check false, second true.

If the object is not a primitive, but a real Object, like new Number(0), new String(""), or new Boolean(false), then both checks are false.

So if 'object' is interpreted to mean a real Object then both checks are always the same. If primitives are allowed then the checks are different for 0, "", and false.

In cases like object==null, the unobvious results could be a source of bugs. Use of == is not recommended ever, use === instead.

Third part of the question:

And also: What is the difference between null and undefined?

In JavaScript, one difference is that null is of type object and undefined is of type undefined.

In JavaScript, null==undefined is true, and considered equal if type is ignored. Why they decided that, but 0, "" and false aren't equal, I don't know. It seems to be an arbitrary opinion.

In JavaScript, null===undefined is not true since the type must be the same in ===.

In reality, null and undefined are identical, since they both represent non-existence. So do 0, and "" for that matter too, and maybe the empty containers [] and {}. So many types of the same nothing are a recipe for bugs. One type or none at all is better. I would try to use as few as possible.

'false', 'true', and '!' are another bag of worms that could be simplified, for example, if(!x) and if(x) alone are sufficient, you don't need true and false.

A declared var x is type undefined if no value is given, but it should be the same as if x was never declared at all. Another bug source is an empty nothing container. So it is best to declare and define it together, like var x=1.

People are going round and round in circles trying to figure out all these various types of nothing, but it's all just the same thing in complicated different clothes. The reality is

undefined===undeclared===null===0===""===[]==={}===nothing

And maybe all should throw exceptions.

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typeof null;      // object
typeof undefined; // undefined

The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values and is treated as falsy for boolean operations.

var x = null;
var y;

x is declared & defined as null

y is declared but not defined. It is declared with no value so it is undefined.

z is not declared so would also be undefined if you attempted to use z.

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TLDR

undefined is a primitive value in JavaScript that indicates the implicit absence of a value. Uninitialized variables automatically have this value, and functions without an explicit return statement, return undefined.

null is also a primitive value in JavaScript. It indicates the intentional absence of an object value. null in JavaScript was designed to enable interoperability with Java.

typeof null returns "object" because of a peculiarity in the design of the language, stemming from the demand that JavaScript be interoperable with Java. It does not mean null is an instance of an object. It means: given the tree of primitive types in JavaScript, null is part of the "object-type primitive" subtree. This is explained more fully below.

Details

undefined is a primitive value that represents the implicit absence of a value. Note that undefined was not directly accessible until JavaScript 1.3 in 1998. This tells us that null was intended to be the value used by programmers when explicitly indicating the absence of a value. Uninitialized variables automatically have the value undefined. undefined is a one-of-a-kind type in the ECMAScript specification.

null is a primitive value that represents the intentional absence of an object value. null is also a one-of-a-kind type in the ECMAScript specification.

null in JavaScript was designed with a view to enable interoperability with Java, both from a "look" perspective, and from a programmatic perspective (eg the LiveConnect Java/JS bridge planned for 1996). Both Brendan Eich and others have since expressed distaste at the inclusion of two "absence of value" values, but in 1995 Eich was under orders to "make [JavaScript] look like Java".

Brendan Eich:

If I didn't have "Make it look like Java" as an order from management, and I had more time (hard to unconfound these two causal factors), then I would have preferred a Self-like "everything's an object" approach: no Boolean, Number, String wrappers. No undefined and null. Sigh.

In order to accommodate Java's concept of null which, due to the strongly-typed nature of Java, can only be assigned to variables typed to a reference type (rather primitives), Eich chose to position the special null value at the top of the object prototype chain (i.e. the top of the reference types), and to include the null type as part of the set of "object-type primitives".

The typeof operator was added shortly thereafter in JavaScript 1.1, released on 19th August 1996.

From the V8 blog:

typeof null returns object, and not null, despite null being a type of its own. To understand why, consider that the set of all JavaScript types is divided into two groups: objects (i.e. the Object type) primitives (i.e. any non-object value) As such, null means “no object value”, whereas undefined means “no value”.

Following this line of thought, Brendan Eich designed JavaScript to make typeof return 'object' for all values on the right-hand side, i.e. all objects and null values, in the spirit of Java. That’s why typeof null === 'object' despite the spec having a separate null type.

So Eich designed the hierarchy of primitive types to enable interoperability with Java. This led to him positioning null along with the "object-type primitives" on the hierarchy. To reflect this, when typeof was added to the language shortly thereafter, he chose typeof null to return "object".

The surprise expressed by JavaScript developers at typeof null === "object" is the result of an impedance mismatch (or abstraction leak) between a weakly-typed language (JavaScript) that has both null and undefined, and another, strongly-typed language (Java) that only has null, and in which null is strictly defined to refer to a reference type (not a primitive type).

Note that this is all logical, reasonable and defensible. typeof null === "object" is not a bug, but a second-order effect of having to accommodate Java interoperability.

A number of imperfect backwards rationalisations and/or conventions have emerged, including that undefined indicates implicit absence of a value, and that null indicates intentional absence of a value; or that undefined is the absence of a value, and null is specifically the absence of an object value.

A relevant conversation with Brendan Eich, screenshotted for posterity:

https://i.stack.imgur.com/EVwWS.png

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One way to make sense of null and undefined is to understand where each occurs.

Expect a null return value in the following situations:

Methods that query the DOM console.log(window.document.getElementById("nonExistentElement")); //Prints: null

JSON responses received from an Ajax request

    {
      name: "Bob",
      address: null
    }

RegEx.exec.

New functionality that is in a state of flux. The following returns null:

        var proto = Object.getPrototypeOf(Object.getPrototypeOf({}));

       // But this returns undefined:

        Object.getOwnPropertyDescriptor({}, "a");

All other cases of non-existence are denoted by undefined (as noted by @Axel). Each of the following prints "undefined":

    var uninitalised;
    console.log(uninitalised);

    var obj = {};
    console.log(obj.nonExistent);

    function missingParam(missing){
        console.log(missing);
    }

    missingParam();

    var arr = [];
    console.log(arr.pop());        

Of course if you decide to write var unitialised = null; or return null from a method yourself then you have null occurring in other situations. But that should be pretty obvious.

A third case is when you want to access a variable but you don't even know if it has been declared. For that case use typeof to avoid a reference error:

if(typeof unknown !== "undefined"){
    //use unknown
}

In summary check for null when you are manipulating the DOM, dealing with Ajax, or using certain ECMAScript 5 features. For all other cases it is safe to check for undefined with strict equality:

if(value === undefined){
  // stuff
}

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Comparison of many different null checks in JavaScript:

http://jsfiddle.net/aaronhoffman/DdRHB/5/

// Variables to test
var myNull = null;
var myObject = {};
var myStringEmpty = "";
var myStringWhiteSpace = " ";
var myStringHello = "hello";
var myIntZero = 0;
var myIntOne = 1;
var myBoolTrue = true;
var myBoolFalse = false;
var myUndefined;

...trim...

http://aaron-hoffman.blogspot.com/2013/04/javascript-null-checking-undefined-and.html

https://i.stack.imgur.com/ThA4b.png

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To add to the answer of What is the differrence between undefined and null, from JavaScript Definitive Guide 6th Edition, p.41 on this page:

You might consider undefined to represent system-level, unexpected, or error-like absense of value and null to represent program-level, normal, or expected absence of value. If you need to assign one of these values to a variable or property or pass one of these values to a function, null is almost always the right choice.

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null and undefined are both false for value equality (null==undefined): they both collapse to boolean false. They are not the same object (null!==undefined).

undefined is a property of the global object ("window" in browsers), but is a primitive type and not an object itself. It's the default value for uninitialized variables and functions ending without a return statement.

null is an instance of Object. null is used for DOM methods that return collection objects to indicate an empty result, which provides a false value without indicating an error.

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Some precisions:

null and undefined are two different values. One is representing the absence of a value for a name and the other is representing the absence of a name.

What happens in an if goes as follows for if( o ):

The expression in the parentheses o is evaluated, and then the if kicks in type-coercing the value of the expression in the parentheses - in our case o.

Falsy (that will get coerced to false) values in JavaScript are: '', null, undefined, 0, and false.

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The following function shows why and is capable for working out the difference:

function test() {
        var myObj = {};
        console.log(myObj.myProperty);
        myObj.myProperty = null;
        console.log(myObj.myProperty);
}

If you call

test();

You're getting

undefined null

The first console.log(...) tries to get myProperty from myObj while it is not yet defined - so it gets back "undefined". After assigning null to it, the second console.log(...) returns obviously "null" because myProperty exists, but it has the value null assigned to it.

In order to be able to query this difference, JavaScript has null and undefined: While null is - just like in other languages an object, undefined cannot be an object because there is no instance (even not a null instance) available.

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For example window.someWeirdProperty is undefined, so

"window.someWeirdProperty === null" evaluates to false while

"window.someWeirdProperty === undefined" evaluates to true.

Moreover checkif if (!o) is not the same as checking if (o == null) for o being false.

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In Javascript null is not an object type it is a primitave type.

What is the difference? Undefined refers to a pointer that has not been set. Null refers to the null pointer for example something has manually set a variable to be of type null

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Look at this:

   <script>
function f(a){
  alert(typeof(a));
  if (a==null) alert('null');
  a?alert(true):alert(false);
}
</script>
                                          //return:
<button onclick="f()">nothing</button>    //undefined    null    false
<button onclick="f(null)">null</button>   //object       null    false
<button onclick="f('')">empty</button>    //string               false
<button onclick="f(0)">zero</button>      //number               false
<button onclick="f(1)">int</button>       //number               true
<button onclick="f('x')">str</button>     //string               true

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From "The Principles of Object-Oriented Javascript" by Nicholas C. Zakas

But why an object when the type is null? (In fact, this has been acknowledged as an error by TC39, the committee that designs and maintains JavaScript. You could reason that null is an empty object pointer, making "object" a logical return value, but that’s still confusing.)

Zakas, Nicholas C. (2014-02-07). The Principles of Object-Oriented JavaScript (Kindle Locations 226-227). No Starch Press. Kindle Edition.

That said:

var game = null; //typeof(game) is "object"

game.score = 100;//null is not an object, what the heck!?
game instanceof Object; //false, so it's not an instance but it's type is object
//let's make this primitive variable an object;
game = {}; 
typeof(game);//it is an object
game instanceof Object; //true, yay!!!
game.score = 100;

Undefined case:

var score; //at this point 'score' is undefined
typeof(score); //'undefined'
var score.player = "felix"; //'undefined' is not an object
score instanceof Object; //false, oh I already knew that.

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null is an object. Its type is null. undefined is not an object; its type is undefined.

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The best way to think about 'null' is to recall how the similar concept is used in databases, where it indicates that a field contains "no value at all."

Yes, the item's value is known; it is 'defined.' It has been initialized.

The item's value is: "there is no value."

This is a very useful technique for writing programs that are more-easily debugged. An 'undefined' variable might be the result of a bug ... (how would you know?) ... but if the variable contains the value 'null,' you know that "someone, somewhere in this program, set it to 'null.'" Therefore, I suggest that, when you need to get rid of the value of a variable, don't "delete" ... set it to 'null.' The old value will be orphaned and soon will be garbage-collected; the new value is, "there is no value (now)." In both cases, the variable's state is certain: "it obviously, deliberately, got that way."

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The other fun thing about null, compared to undefined, is that it can be incremented.

x = undefined x++ y = null y++ console.log(x) // NaN console.log(y) // 0

This is useful for setting default numerical values for counters. How many times have you set a variable to -1 in its declaration?

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Undefined means a variable has been declared but it has not been assigned any value while Null can be assigned to a variable representing "no value".(Null is an assignment operator)

2.Undefined is a type itself while Null is an object.

3.Javascript can itself initialize any unassigned variable to undefined but it can never set value of a variable to null. This has to be done programatically.

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Use null to define something as having no value, use undefined when you expect something might not be defined at all.

For example, if a variable has no value, assign it as null.

var weDontHaveAValue = null;

If you expect that something might be not defined at all, e.g. an optional options argument, use undefined.

if (typeof args.optionalParam !== 'undefined') { }

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What is a type?

A type is a way to categorize values. Here is a table with the types in question and their typeof result.

Type Values type contains typeof result Is typeof result a lie? Undefined Only: undefined "undefined" No Null Only: null "object" Yes Object Infinite amount of values: {}, {a: "b"}, ... "object" No

null is not an object, its a value of type Null.

The typeof operator is lying! It returning "object" for null is a mistake in the JavaScript language.

I wrote a chapter about this in my open-source e-book. You can read it here https://github.com/carltheperson/advanced-js-objects

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