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

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Why is null considered an object in JavaScript?

Is checking

if ( object == null )
      Do something

the same as

if ( !object )
      Do something

?

And also:

What is the difference between null and undefined?

First answer

(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.

Second answer

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

Third answer

What is the difference between null and undefined??

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

Reprint:https://stackoverflow.com/questions/801032/why-is-null-an-object-and-whats-the-difference-between-null-and-undefined
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