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

Question

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

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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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The other fun thing about null  compared to undefined  is that it can be incremented   x000D   x000D  x   undefined x000D  x   x000D  y   null x000D  y   x000D  console log x     NaN x000D  console log y     0 x000D   x000D   x000D    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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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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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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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    gt  var noValueYet   gt  console log noValueYet   undefined   gt  function foo x    console log x     gt  foo   undefined   gt  var obj        gt  console log obj unknownProperty  undefined   Accessing unknown variables  however  produces an exception    gt  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    gt  null    null true  gt  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     gt  Boolean null  false  gt  Boolean     false  gt  Boolean 3-3  false  gt  Boolean     true  gt  Boolean     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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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  quot undefined quot   After assigning null to it  the second console log      returns obviously  quot null quot  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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null is an object  Its type is null  undefined is not an object  its type is undefined

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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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Look at this       lt script gt  function f a     alert typeof a      if  a  null  alert  null      a alert true  alert false      lt  script gt                                              return   lt button onclick  f    gt nothing lt  button gt       undefined    null    false  lt button onclick  f null   gt null lt  button gt      object       null    false  lt button onclick  f      gt empty lt  button gt       string               false  lt button onclick  f 0   gt zero lt  button gt         number               false  lt button onclick  f 1   gt int lt  button gt          number               true  lt button onclick  f  x    gt str lt  button gt        string               true

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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  quot  quot  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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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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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

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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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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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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

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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  amp  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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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 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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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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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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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  quot object quot  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  quot object-type primitive quot  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  quot look quot  perspective  and from a programatic perspective  eg the LiveConnect Java JS bridge planned for 1996   Both Brendan Eich and others have since expressed distaste at the inclusion of two  quot absence of value quot  values  but in 1995 Eich was under orders to  quot make  JavaScript  look like Java quot   Brendan Eich   If I didn t have  quot Make it look like Java quot  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  quot everything s an object quot  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  ie  the top of the reference types   and to include the null type as part of the set of  quot object-type primitives quot   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 heirarchy of primitive types to enable interoperability with Java  This led to him positioning null along with the  quot object-type primitives quot  on the heirarchy  To refelct this  when typeof was added to the language shortly thereafter  he chose typeof null to return  quot object quot   The surprise expressed by JavaScript developers at typeof null      quot object quot  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      quot object quot  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

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

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