What is the difference between null and undefined in JavaScript

Question

I want to know what the difference is between null and undefined in JavaScript

User · Answer

I picked this from here

The undefined value is a primitive value used when a variable has not been assigned a value.

The null value is a primitive value that represents the null, empty, or non-existent reference.

When you declare a variable through var and do not give it a value, it will have the value undefined. By itself, if you try to WScript.Echo() or alert() this value, you won't see anything. However, if you append a blank string to it then suddenly it'll appear:

var s;
WScript.Echo(s);
WScript.Echo("" + s);

You can declare a variable, set it to null, and the behavior is identical except that you'll see "null" printed out versus "undefined". This is a small difference indeed.

You can even compare a variable that is undefined to null or vice versa, and the condition will be true:

undefined == null
null == undefined

They are, however, considered to be two different types. While undefined is a type all to itself, null is considered to be a special object value. You can see this by using typeof() which returns a string representing the general type of a variable:

var a;
WScript.Echo(typeof(a));
var b = null;
WScript.Echo(typeof(b));

Running the above script will result in the following output:

undefined
object

Regardless of their being different types, they will still act the same if you try to access a member of either one, e.g. that is to say they will throw an exception. With WSH you will see the dreaded "'varname' is null or not an object" and that's if you're lucky (but that's a topic for another article).

You can explicitely set a variable to be undefined, but I highly advise against it. I recommend only setting variables to null and leave undefined the value for things you forgot to set. At the same time, I really encourage you to always set every variable. JavaScript has a scope chain different than that of C-style languages, easily confusing even veteran programmers, and setting variables to null is the best way to prevent bugs based on it.

Another instance where you will see undefined pop up is when using the delete operator. Those of us from a C-world might incorrectly interpret this as destroying an object, but it is not so. What this operation does is remove a subscript from an Array or a member from an Object. For Arrays it does not effect the length, but rather that subscript is now considered undefined.

var a = [ 'a', 'b', 'c' ];
delete a[1];
for (var i = 0; i < a.length; i++)
WScript.Echo((i+".) "+a[i]);

The result of the above script is:

0.) a
1.) undefined
2.) c

You will also get undefined returned when reading a subscript or member that never existed.

The difference between null and undefined is: JavaScript will never set anything to null, that's usually what we do. While we can set variables to undefined, we prefer null because it's not something that is ever done for us. When you're debugging this means that anything set to null is of your own doing and not JavaScript. Beyond that, these two special values are nearly equivalent.

User · Answer

null - It is an assignment value  which is used with variable to represent no value  it s an object    undefined - It is a variable which does not have any value assigned to it  so JavaScript will assign an undefined to it  it s a data type    undeclared - If a variable is not created at all  it is known as undeclared

User · Answer

adding characters due to lack of characters  so I m allowed to post this

User · Answer

In Javascript null is an empty or non-existent value and it must be assigned  But Undefined means a variable has been declared  but not value has not been defined  let a   null    console log a         null let b           console log b         undefined  In JS both null and undefined are primitive values  Also you can look the following lines of code console log typeof null     Object console log typeof undefined     undefined console log 10 null      10 console log 10 undefined     NaN

User · Answer

I want to add a very subtle difference between null and undefined which is good to know when you are trying to learn Vanilla JavaScript JS  from ground up    null is a reserved keyword in JS while undefined is a variable on the global object of the run-time environment you re in    While writing code  this difference is not identifiable as both null and undefined are always used in RHS of a JavaScript statement  But when you use them in LHS of an expression then you can observe this difference easily  So JS interpreter interprets the below code as error   var null    foo    It gives below error      Uncaught SyntaxError  Unexpected token null   While below code runs successfully although I won t recommend doing so in real life   var undefined    bar    This works because undefined is a variable on the global object  browser window object in case of client-side JS

User · Answer

Check this out  The output is worth thousand words    x000D   x000D  var b1   document getElementById  b1    x000D   x000D  checkif  1  no argument                            x000D  checkif  2  undefined explicitly       undefined   x000D  checkif  3  null explicitly                 null   x000D  checkif  4  the 0                              0   x000D  checkif  5  empty string                           x000D  checkif  6  string                       string    x000D  checkif  7  number                        123456   x000D   x000D  function checkif  a1  a2    x000D   print   ncheckif        a1         x000D   if  a2    undefined    x000D    print    undefined     YES    x000D     else   x000D    print    undefined     NO    x000D     x000D   if  a2     undefined    x000D    print     undefined    YES    x000D     else   x000D    print     undefined    NO    x000D     x000D   if  a2    null    x000D    print    null          YES    x000D     else   x000D    print    null          NO    x000D     x000D   if  a2     null    x000D    print     null         YES    x000D     else   x000D    print     null         NO    x000D     x000D   if  a2          x000D    print                  YES    x000D     else   x000D    print                  NO    x000D     x000D   if  a2           x000D    print                  YES    x000D     else   x000D    print                  NO    x000D     x000D   if  isNaN a2     x000D    print  isNaN           YES    x000D     else   x000D    print  isNaN           NO    x000D     x000D   if  a2    x000D    print  if-             YES    x000D     else   x000D    print  if-             NO    x000D     x000D    print  typeof              typeof a2    x000D    x000D   x000D  function print v    x000D   b1 innerHTML    v     n   x000D    x000D   lt  DOCTYPE html gt  x000D   lt html gt  x000D   lt body gt  x000D   lt pre id  b1  gt  lt  pre gt  x000D   lt  body gt  x000D   lt  html gt  x000D   x000D   x000D    See also    https   developer mozilla org en-US docs Web JavaScript Reference Global Objects undefined https   developer mozilla org en-US docs Web JavaScript Reference Global Objects NaN https   developer mozilla org en-US docs Web JavaScript Reference Global Objects null https   developer mozilla org en-US docs Web JavaScript Reference Operators Comparison Operators   Cheers

User · Answer

In javascript all variables are stored as key value pairs   Each variable is stored as variable name   variable value reference   undefined means a variable has been given a space in memory  but no value is assigned to it   As a best practice  you should not use this type as an assignment   In that case how to denote when you want a variable to be without value at a later point in the code   You can use the type null  which is also a type that is used to define the same thing  absence of a value  but it is not the same as undefined  as in this case you actually have the value in memory   That value is null  Both are similar but usage and meaning are different

User · Answer

Per Ryan Morr s thorough article on this subject      Generally  if you need to assign a non-value to a variable or property  pass it to a function  or return it from a function  null is almost always the best option  To put it simply  JavaScript uses undefined and programmers should use null    See Exploring the Eternal Abyss of Null and Undefined

User · Answer

If a variable is not initialized then it is undefined  undefined is not a object  Example   var MyName            console log typeof MyName    Check the console log in development tool  it will be printed as undefined   null is a a object If you want some variable to be null then null is used null variable exists but value is not known It should be assigned to a variable pro grammatically   null is not automatically initialized   Example   var MyName   null            console log typeof MyName   Check the csole log in development tool  it will be an object

User · Answer

The difference between undefined and null is minimal  but there is a difference  A variable whose value is undefined has never been initialized  A variable whose value is null was explicitly given a value of null  which means that the variable was explicitly set to have no value  If you compare undefined and null by using the null  undefined expression  they will be equal

User · Answer

null is a special keyword that indicates an absence of value    think about it as a value  like     foo   is string   true   is boolean    1234   is number   null   is undefined      undefined property indicates that a variable has not been assigned a value including null too   Like  var foo      defined empty variable is null of datatype undefined    Both of them are representing a  value of a variable with no value  AND null doesn t represent a string that has no value - empty string-    Like  var a        console log typeof a      string  console log a    null     false  console log a    undefined      false    Now if   var a  console log a    null     true console log a    undefined     true    BUT  var a   console log a     null     false  console log a     undefined      true   SO each one has it own way to use   undefined use it to compare the variable data type   null use it to empty a value of a variable   var a    javascript   a   null      will change the type of variable  a  from string to object

User · Answer

Both Null and undefined in JavaScript indicate absence of value   var a   null    variable assigned null value var b      undefined       Despite the fact both exist for absence of value but  Undefined   actually means the variable is not initialized  Functions that return   nothing and function parameters for which no value is supplied    undefined value is returned  Use strict equality operator     to   distinguish between null and undefined    Reference  http   www thesstech com javascript null-and-undefined

User · Answer

When you declare a variable in javascript  it is assigned the value undefined  This means the variable is untouched and can be assigned any value in future  It also implies that you don t know the value that this variable is going to hold at the time of declaration   Now you can explicitly assign a variable null  It means that the variable does not have any value  For example - Some people don t have a middle name  So in such a case its better to assign the value null to the middlename variable of a person object   Now suppose that someone is accessing the middlename variable of your person object and it has the value undefined  He wouldn t know if the developer forgot to initialize this variable or if it didn t have any value  If it has the value null  then the user can easily infer that middlename doesn t have any value and it is not an untouched variable

User · Answer

Please read the following carefully  It should remove all your doubts regarding the difference between null and undefined in JavaScript  Also  you can use the utility function at the end of this answer to get more specific types of variables  In JavaScript we can have the following types of variables   Undeclared Variables Declared but Unassigned Variables Variables assigned with literal undefined Variables assigned with literal null Variables assigned with anything other than undefined or null  The following explains each of these cases one by one   Undeclared Variables  Can only be checked with the typeof operator which returns string  undefined  Cannot be checked with the loose equality operator      undefined    let alone the strict equality operator       undefined    as well as if-statements and ternary operators             these throw Reference Errors   Declared but Unassigned Variables  typeof returns string  undefined     check with null returns true    check with undefined returns true     check with null returns false     check with undefined returns true Is falsy to if-statements and ternary operators           Variables assigned with literal undefined These variables are treated exactly the same as Declared But Unassigned Variables   Variables assigned with literal null  typeof returns string  object     check with null returns true    check with undefined returns true     check with null returns true     check with undefined returns false Is falsy to if-statements and ternary operators           Variables assigned with anything other than undefined or null  typeof returns one of the following strings   bigint    boolean    function    number    object    string    symbol     Following provides the algorithm for correct type checking of a variable   Get the typeof our variable and return it if it isn t  object  Check for null  as typeof null returns  object  as well Evaluate Object prototype toString call o  with a switch statement to return a more precise value  Object s toString method returns strings that look like   object ConstructorName   for native host objects  For all other objects  user-defined objects   it always returns   object Object   If that last part is the case  the stringified version of the variable being   object Object    and the parameter returnConstructorBoolean is true  it will try to get the name of the constructor by toString-ing it and extracting the name from there  If the constructor can t be reached   object  is returned as usual  If the string doesn t contain its name   anonymous  is returned   supports all types up to ECMAScript 2020  function TypeOf o  returnConstructorBoolean      const type   typeof o    if  type      object   return type   if  o     null         return  null     const toString   Object prototype toString call o     switch  toString           Value types  6     case   object BigInt               return  bigint      case   object Boolean              return  boolean      case   object Date                 return  date      case   object Number               return  number      case   object String               return  string      case   object Symbol               return  symbol          Error types  7     case   object Error                return  error      case   object EvalError            return  evalerror      case   object RangeError           return  rangeerror      case   object ReferenceError       return  referenceerror      case   object SyntaxError          return  syntaxerror      case   object TypeError            return  typeerror      case   object URIError             return  urierror          Indexed Collection and Helper types  13     case   object Array                return  array      case   object Int8Array            return  int8array      case   object Uint8Array           return  uint8array      case   object Uint8ClampedArray    return  uint8clampedarray      case   object Int16Array           return  int16array      case   object Uint16Array          return  uint16array      case   object Int32Array           return  int32array      case   object Uint32Array          return  uint32array      case   object Float32Array         return  float32array      case   object Float64Array         return  float64array      case   object ArrayBuffer          return  arraybuffer      case   object SharedArrayBuffer    return  sharedarraybuffer      case   object DataView             return  dataview          Keyed Collection types  2     case   object Map                  return  map      case   object WeakMap              return  weakmap          Set types  2     case   object Set                  return  set      case   object WeakSet              return  weakset          Operation types  3     case   object RegExp               return  regexp      case   object Proxy                return  proxy      case   object Promise              return  promise          Plain objects     case   object Object          if   returnConstructorBoolean          return type        const  prototype   Object getPrototypeOf o        if    prototype                        return type        const  constructor    prototype constructor       if    constructor                      return type        const matches   Function prototype toString call  constructor  match   function s     s               return matches   matches 1     anonymous       default  return toString split      1  slice 0  -1

User · Answer

Quote from  quot Eloquent Javascript quot  3rd edition by Marijn Haverbeke   The difference in meaning between undefined and null is an accident of Javascript s design  and it doesn t matter most of the time  In cases where you actually have to concern yourself with these values  I recommend treating them as mostly interchangeable  Honestly  at first  I am a bit skeptical about this advice  However  in my own interpretation  it is a lazy  vs eager  way to deal with their differences  Maybe  we don t have to deal with the differences at all  If we have to  we can delay our concern  util we have to  and not hyperactively defensively worry about it every step of the way as those values  null and undefined  flow through our code  PS  This is not a direct answer to your question  This is just a related opinion

User · Answer

Undefined means a variable has been declared but has no value   var var1  alert var1     undefined alert typeof var1     undefined   Null is an assignment   var var2  null  alert var2     null alert typeof var2     object

User · Answer

For the undefined type  there is one and only one value  undefined    For the null type  there is one and only one value  null    So for both of them  the label is both its type and its value    The difference between them  For example    null is an empty value undefined is a missing value   Or    undefined hasn t had a value yet null had a value and doesn t anymore   Actually  null is a special keyword  not an identifier  and thus you cannot treat it as a variable to assign to   However  undefined is an identifier  In both non-strict mode and strict mode  however  you can create a local variable of the name undefined  But this is one terrible idea   function foo         undefined   2     bad idea     foo     function foo          use strict       undefined   2     TypeError     foo

User · Answer

In JavaScript  undefined means a variable has been declared but has not yet been assigned a value  such as   var TestVar  alert TestVar     shows undefined alert typeof TestVar     shows undefined   null is an assignment value  It can be assigned to a variable as a representation of no value   var TestVar   null  alert TestVar     shows null alert typeof TestVar     shows object   From the preceding examples  it is clear that undefined and null are two distinct types  undefined is a type itself  undefined  while null is an object   null     undefined    false null    undefined    true null     null    true   and  null    value     ReferenceError undefined    value      value

User · Answer

null is a special value meaning  no value   null is a special object because typeof null returns  object    On the other hand  undefined means that the variable has not been declared  or has not been given a value

User · Answer

In JavasSript there are 5 primitive data types String   Number   Boolean   null and undefined  I will try to explain with some simple example  lets say we have a simple function   function test a          if a    null           alert  a is null           else           alert  The value of a is     a                also in above function if a    null  is same as if  a   now when we call this function without passing the parameter a     test    it will alert  a is null      test 4   it will alert  The value of a is     4    also  var a  alert typeof a      this will give undefined  we have declared a variable but we have not asigned any value to this variable   but if we write  var a   null  alert typeof a   will give alert as object   so null is an object  in a way we have assigned a value null to  a

User · Answer

Null is ALWAYS an unknown object that exists in memory  whereas undefined is not

User · Answer

null and undefined are both are used to represent the absence of some value   var a   null    a is initialized and defined   typeof a    object   null is an object in JavaScript  Object prototype toString call a      object Object   var b    b is undefined and uninitialized  undefined object properties are also undefined  For example  x  is not defined on object c and if you try to access c x  it will return undefined   Generally we assign null to variables not undefined

User · Answer

Just to add my views -   A variable that is declared only  and never used anywhere  is removed off by an optimizing compiler in case of compiled languages like C   or a warning is flagged in the IDE   It ultimately means that the variable is non-existent because it s memory is never allocated   In case of javascript interpreter   I guess  a variable is treated as existing only from the point onwards where it is given a value  Before that point  it s type is  undefined   and no memory is allocated for it  And  so its type is undefined   A null in javascript is a value that represents an address  but that address points to nothing yet non-existent reference   Nevertheless  its a value

User · Answer

I ll explain undefined  null and Uncaught ReferenceError   1 - Uncaught ReferenceError   variable has not been declared in your script  there is no reference to this varaible  2 - undefined  Variable declared but does not initialised  3 - null   Variable declared and is an empty value

User · Answer

Both special values imply an empty state   The main difference is that undefined represents the value of a variable that wasn   t yet initialized  while null represents an intentional absence of an object      The variable number is defined  however  is not assigned with an initial value    let number  number       gt  undefined      number variable is undefined  which clearly indicates an uninitialized variable      The same uninitialized concept happens when a non-existing object property is accessed    const obj     firstName   Dmitri     obj lastName       gt  undefined      Because lastName property does not exist in obj  JavaScript correctly evaluates obj lastName to undefined    In other cases  you know that a variable expects to hold an object or a function to return an object  But for some reason  you can   t instantiate the object  In such a case null is a meaningful indicator of a missing object   For example  clone   is a function that clones a plain JavaScript object  The function is expected to return an object   function clone obj      if  typeof obj      object   amp  amp  obj     null        return Object assign     obj         return null    clone  name   John          gt   name   John   clone 15                    gt  null clone null                  gt  null   However  clone   might be invoked with a non-object argument  15 or null  or generally a primitive value  null or undefined   In such case  the function cannot create a clone  so it returns null - the indicator of a missing object      typeof operator makes the distinction between the two values    typeof undefined       gt   undefined  typeof null            gt   object       The strict quality operator     correctly differentiates undefined from null    let nothing   undefined  let missingObject   null  nothing     missingObject       gt  false

User · Answer

null  absence of value for a variable  undefined  absence of variable itself     where variable is a symbolic name associated with a value   JS could be kind enough to implicitly init newly declared variables with null  but it does not

User · Answer

The best way to understand the difference is to first clear your mind of the inner workings of JavaScript and just understand the differences in meaning between   let supervisor    None         I have a supervisor named  None   let supervisor   null        I do NOT have a supervisor  It is a FACT that I do not   let supervisor   undefined        I may or may not have a supervisor  I either don t know        if I do or not  or I am choosing not to tell you  It is        irrelevant or none of your business    There is a difference in meaning between these three cases  and JavaScript distinguishes the latter two cases with two different values  null and undefined  You are free to use those values explicitly to convey those meanings   So what are some of the JavaScript-specific issues that arise due to this philosophical basis    A declared variable without an initializer gets the value undefined because you never said anything about the what the intended value was   let supervisor  assert supervisor     undefined    A property of an object that has never been set evaluates to undefined because no one ever said anything about that property   const dog     name   Sparky   age  2    assert dog breed     undefined    null and undefined are  similar  to each other because Brendan Eich said so  But they are emphatically not equal to each other   assert null    undefined   assert null     undefined    null and undefined thankfully have different types  null belongs to the type Null and undefined to the type Undefined  This is in the spec  but you would never know this because of the typeof weirdness which I will not repeat here  A function reaching the end of its body without an explicit return statement returns undefined since you don t know anything about what it returned    By the way  there are other forms of  nothingness  in JavaScript  it s good to have studied Philosophy        NaN Using a variable that has never been declared and receiving a ReferenceError Using a let or const defined local variable in its temporal dead zone and receiving a ReferenceError Empty cells in sparse arrays  Yes these are not even undefined although they compare     to undefined     node  gt  const a    1  undefined  2   gt  const b    1    2   gt  a   1  undefined  2    gt  b   1   lt 1 empty item gt   2

User · Answer

tl dr  Use null for set a variable you know it is an Object   Use undefined for set a variable whose type is mixed     This is my usage of both 5 primitives and Object type  and that explain the difference between    use case    of undefined or null   String  If you know a variable is only a string while all lifecycle  by convention  you could initialize it  to             true   false     false typeof         string     Hello World     true   false     true typeof  Hello World       string    Number  If you know a variable is only a number while all lifecycle  by convention  you could initialize it  to 0  or NaN if 0 is an important value in your usage     0    true   false     false typeof 0      number    16    true   false     true typeof 16      number    or   NaN    true   false     false typeof NaN      number    16    true   false     true typeof 16      number    Boolean  If you know a variable is only a boolean while all lifecycle  by convention  you could initialize it  to false    false    true   false     false typeof false      boolean    true    true   false     true typeof true      boolean    Object  If you know a variable is only an Object while all lifecycle  by convention  you could initialize it  to null    null    true   false     false typeof null      object          true   false     true typeof         object    Note  the smart usage off null is to be the falsy version of an Object because an Object is always true  and because typeof null return object  That means typeof myVarObject return consistent value for both Object and null type   All  If you know a variable has a mixed type  any type while all lifecycle   by convention  you could initialize it  to undefined

User · Answer

As typeof returns undefined  undefined is a type where as null is an initializer indicates the variable points to no object virtually everything in Javascript is an object

User · Answer

null and undefined are two distinct object types which have the following in common    both can only hold a single value  null and undefined respectively  both have no properties or methods and an attempt to read any properties of either will result in a run-time error  for all other objects  you get value undefined if you try to read a non-existent property   values null and undefined are considered equal to each other and to nothing else by    and    operators    The similarities however end here  For once  there is a fundamental difference in the way how keywords null and undefined are implemented  This is not obvious  but consider the following example   var undefined    foo   WScript Echo undefined      This will print  foo   undefined  NaN and Infinity are just names of preinitialized  superglobal  variables - they are initialized at run-time and can be overridden by normal global or local variable with the same names    Now  let s try the same thing with null   var null    foo      This will cause a compile-time error WScript Echo null     Oops  null  true and false are reserved keywords - compiler won t let you use them as variable or property names  Another difference is that undefined is a primitive type  while null is an object type  indicating the absense of an object reference   Consider the following   WScript Echo typeof false      Will print  boolean WScript Echo typeof 0      Will print  number WScript Echo typeof         Will print  string WScript Echo typeof         Will print  object WScript Echo typeof undefined      Will print  undefined WScript Echo typeof null            Will print  object   Also  there is an important difference in the way null and undefined are treated in numeric context   var a     declared but uninitialized variables hold the value undefined WScript Echo a     undefined      Prints  -1  var b   null     the value null must be explicitly assigned  WScript Echo b     null      Prints  -1  WScript Echo a    b      Prints  -1  as expected  WScript Echo a  gt   b      Prints  0  WTF     WScript Echo a  gt   a      Prints  0          WScript Echo isNaN a       Prints  -1  a evaluates to NaN   WScript Echo 1 a      Prints  -1  IND  in Echo output this means NaN   WScript Echo b  gt   b      Prints  -1  as expected  WScript Echo isNaN b       Prints  0  b evaluates to a valid number  WScript Echo 1 b      Prints  0  b evaluates to 0   WScript Echo a  gt   0  amp  amp  a  lt   0      Prints  0  as expected  WScript Echo a    0      Prints  0  as expected  WScript Echo b  gt   0  amp  amp  b  lt   0      Prints  -1  as expected  WScript Echo b    0      Prints  0         null becomes 0 when used in arithmetic expressions or numeric comparisons - similarly to false  it is basically just a special kind of  zero   undefined  on the other hand  is a true  nothing  and becomes NaN   not a number   when you try to use it in numeric context   Note that null and undefined receive a special treatment from    and    operators  but you can test true numeric equality of a and b with the expression  a  gt   b  amp  amp  a  lt   b

User · Answer

OK  we may get confused when we hear about null and undefined  but let s start it simple  they both are falsy and similar in many ways  but weird part of JavaScript  make them a couple of significant differences  for example  typeof null is  object  while typeof undefined is  undefined    typeof null     object  typeof undefined     undefined     But if you check them with    as below  you see they are both falsy   null  undefined    true   Also you can assign null to an object property or to a primitive  while undefined can simply be achieved by not assigning to anything   I create a quick image to show the differences for you at a glance

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You might consider undefined to represent a system-level  unexpected  or error-like absence of value and null to represent program-level  normal  or expected absence of value    via JavaScript The Definitive Guide

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In addition to a different meaning there are other differences    Object destructuring works differently for these two values   const   a    default        a  undefined       a is  default  const   b    default        b  null            b is null  JSON stringify   keeps null but omits undefined  const json   JSON stringify   undefinedValue  undefined  nullValue  null     console log json      prints   nullValue  null   typeof operator  console log typeof undefined       undefined  console log typeof null            object  instead of  null

User · Answer

The difference in meaning between undefined and null is an accident of JavaScript   s design  and it doesn   t matter most of the time  In cases where you actually have to concern yourself with these values  I recommend treating them as mostly interchangeable   From the Eloquent Javascript book

User · Answer

Basically  Undefined is a global variable that javascript create at the run time whether null means that no value has assigned to the variable  actually null is itself an object    Let s take an example           var x     we declared a variable x  but no value has been assigned to it          document write x    let s print the variable x   Undefined that s what you will get as output   Now                  x 5          y null          z x y    and you will get 5 as output  That s the main difference between the Undefined and null

[javascript] What is the difference between null and undefined in JavaScript?

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