Is JavaScript a pass-by-reference or pass-by-value language

Question

The primitive types  number  string  etc   are passed by value  but objects are unknown  because they can be both passed-by-value  in case we consider that a variable holding an object is in fact a reference to the object  and passed-by-reference  when we consider that the variable to the object holds the object itself    Although it doesn t really matter at the end  I want to know what is the correct way to present the arguments passing conventions  Is there an excerpt from JavaScript specification  which defines what should be the semantics regarding this

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Simple values inside functions will not change those values outside of the function  they are passed by value   whereas complex ones will  they are passed by reference    function willNotChange x         x   1     var x   1000   willNotChange x    document write  After function call  x       x     lt br gt        Still 1000  function willChange y         y num   2     var y    num  2000    willChange y   document write  After function call y num       y num     lt br gt        Now 2  not 2000

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For programming language lawyers  I ve went through the following sections of ECMAScript 5 1  which is easier to read than the latest edition   and go as far as asking it on the ECMAScript mailing list  TL DR  Everythings re passed by value  but properties of Objects are references  and the definition of Object is creepily lacking in the standard  Construction of Argument Lists Section 11 2 4  quot Argument Lists quot  says the following on producing a argument list consisting of only 1 argument   The production ArgumentList   AssignmentExpression is evaluated as follows   Let ref be the result of evaluating AssignmentExpression  Let arg be GetValue ref   Return a List whose sole item is arg    The section also enumerate cases where argument list has 0 or  gt 1 arguments  Thus  everything s are passed by reference  Access of Object Properties Section 11 2 1  quot Property Accessors quot   The production MemberExpression   MemberExpression   Expression   is evaluated as follows   Let baseReference be the result of evaluating MemberExpression  Let baseValue be GetValue baseReference   Let propertyNameReference be the result of evaluating Expression  Let propertyNameValue be GetValue propertyNameReference   Call CheckObjectCoercible baseValue   Let propertyNameString be ToString propertyNameValue   If the syntactic production that is being evaluated is contained in strict mode code  let strict be true  else let strict be false  Return a value of type Reference whose base value is baseValue and whose referenced name is propertyNameString  and whose strict mode flag is strict    Thus  properties of Objects are always available as reference  On Reference It is described in section 8 7  quot The Reference Specification Type quot   that references are not real types in the language - they re only used to describe the behavior of the delete  the typeof  and the assignment operators  Definition of  Object  It is defined in 5 1 edition that  quot An Object is a collection of properties quot   Therefore  we can infer  that the value of the object is the collection  but as to what is the value of the collection is poorly defined in the spec  and requires a bit of effort to understand

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My two cents    This is the way I understand it   Feel free to correct me if I m wrong  It s time to throw out everything you know about pass by value   reference  Because in JavaScript  it doesn t matter whether it s passed by value or by reference or whatever  What matters is mutation vs assignment of the parameters passed into a function  OK  let me do my best to explain what I mean  Let s say you have a few objects  var object1       var object2        What we have done is  quot assignment quot     We ve assigned 2 separate empty objects to the variables  quot object1 quot  and  quot object2 quot   Now  let s say that we like object1 better    So  we  quot assign quot  a new variable  var favoriteObject   object1   Next  for whatever reason  we decide that we like object 2 better  So  we do a little re-assignment  favoriteObject   object2   Nothing happened to object1 or to object2  We haven t changed any data at all  All we did was re-assign what our favorite object is  It is important to know that object2 and favoriteObject are both assigned to the same object  We can change that object via either of those variables  object2 name    Fred   console log favoriteObject name     Logs Fred favoriteObject name    Joe   console log object2 name      Logs Joe  OK  now let s look at primitives like strings for example var string1    Hello world   var string2    Goodbye world    Again  we pick a favorite  var favoriteString   string1   Both our favoriteString and string1 variables are assigned to  Hello world   Now  what if we want to change our favoriteString    What will happen    favoriteString    Hello everyone   console log favoriteString      Logs  Hello everyone  console log string1      Logs  Hello world   Uh oh     What has happened  We couldn t change string1 by changing favoriteString    Why   Because we didn t change our string object  All we did was  quot RE ASSIGN quot  the favoriteString variable to a new string  This essentially disconnected it from string1  In the previous example  when we renamed our object  we didn t assign anything   Well  not to the variable itself      we did  however  assign the name property to a new string   Instead  we mutated the object which keeps the connections between the 2 variables and the underlying objects   Even if we had wanted to modify or mutate the string object itself  we couldn t have  because strings are actually immutable in JavaScript   Now  on to functions and passing parameters     When you call a function  and pass a parameter  what you are essentially doing is an  quot assignment quot  to a new variable  and it works exactly the same as if you assigned using the equal     sign  Take these examples  var myString    hello       Assign to a new variable  just like when you pass to a function  var param1   myString  param1    world      Re assignment  console log myString      Logs  hello  console log param1        Logs  world   Now  the same thing  but with a function function myFunc param1        param1    world        console log param1        Logs  world     var myString    hello      Calls myFunc and assigns param1 to myString just like param1   myString myFunc myString    console log myString      logs  hello   OK  now let   s give a few examples using objects instead    first  without the function  var myObject         firstName   Joe       lastName   Smith         Assign to a new variable  just like when you pass to a function  var otherObj   myObject      Let s mutate our object otherObj firstName    Sue      I guess Joe decided to be a girl  console log myObject firstName      Logs  Sue  console log otherObj firstName      Logs  Sue      Now  let s reassign the variable otherObj         firstName   Jack       lastName   Frost         Now  otherObj and myObject are assigned to 2 very different objects    And mutating one object has no influence on the other console log myObject firstName      Logs  Sue  console log otherObj firstName      Logs  Jack    Now  the same thing  but with a function call function myFunc otherObj            Let s mutate our object     otherObj firstName    Sue       console log otherObj firstName      Logs  Sue          Now let s re-assign     otherObj             firstName   Jack           lastName   Frost             console log otherObj firstName      Logs  Jack          Again  otherObj and myObject are assigned to 2 very different objects        And mutating one object doesn t magically mutate the other    var myObject         firstName   Joe       lastName   Smith         Calls myFunc and assigns otherObj to myObject just like otherObj   myObject myFunc myObject    console log myObject firstName      Logs  Sue   just like before  OK  if you read through this entire post  perhaps you now have a better understanding of how function calls work in JavaScript  It doesn t matter whether something is passed by reference or by value    What matters is assignment vs mutation  Every time you pass a variable to a function  you are  quot Assigning quot  to whatever the name of the parameter variable is  just like if you used the equal     sign  Always remember that the equals sign     means assignment  Always remember that passing a parameter to a function in JavaScript also means assignment  They are the same and the 2 variables are connected in exactly the same way  which is to say they aren t  unless you count that they are assigned to the same object   The only time that  quot modifying a variable quot  affects a different variable is when the underlying object is mutated  in which case you haven t modified the variable  but the object itself  There is no point in making a distinction between objects and primitives  because it works the same exact way as if you didn t have a function and just used the equal sign to assign to a new variable  The only gotcha is when the name of the variable you pass into the function is the same as the name of the function parameter  When this happens  you have to treat the parameter inside the function as if it was a whole new variable private to the function  because it is  function myFunc myString           myString is private and does not affect the outer variable     myString    hello      var myString    test   myString   myString     Does nothing  myString is still  test    myFunc myString   console log myString      Logs  test

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I have found the extend method of the Underscore js library very useful when I want to pass in an object as a parameter which may either be modified or replaced entirely   function replaceOrModify aObj      if  modify         aObj setNewValue  foo         else       var newObj   new MyObject            extend destination   sources        extend newObj  aObj

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JavaScript passes primitive types by value and object types by reference     Now  people like to bicker endlessly about whether  pass by reference    is the correct way to describe what Java et al  actually do  The point   is this          Passing an object does not copy the object    An object passed to a function can have its members modified by the function    A primitive value passed to a function cannot be modified by the function  A copy is made          In my book that s called passing by reference      8212  Brian Bi - Which programming languages are pass by reference     Update  Here is a rebuttal to this   There is no  pass by reference  available in JavaScript

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sharing what I know of references in JavaScript   In JavaScript  when assigning an object to a variable  the value assigned to the variable is a reference to the object    x000D   x000D  var a     x000D    a  1  x000D    b  2  x000D    c  3 x000D     x000D  var b   a  x000D   x000D     b c is referencing to a c value x000D  console log b c     Output  3 x000D     Changing value of b c x000D  b c   4 x000D     Also changes the value of a c x000D  console log a c     Output  4 x000D   x000D   x000D

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Primitives  number  Boolean  etc   are passed by value    Strings are immutable  so it doesn t really matter for them   Objects are passed by reference  the reference is passed by value

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Think of it like this  It s always pass by value  However  the value of an object is not the object itself  but a reference to that object    Here is an example  passing a number  a primitive type   function changePrimitive val           At this point there are two  10 s in memory         Changing one won t affect the other     val   val   10    var x   10  changePrimitive x      x     10   Repeating this with an object yields different results   function changeObject obj           At this point there are two references  x and obj  in memory         but these both point to the same object         changing the object will change the underlying object that        x and obj both hold a reference to      obj val   obj val   10    var x     val  10    changeObject x      x       val  100     One more example   function changeObject obj           Again there are two references  x and obj  in memory         these both point to the same object         now we create a completely new object and assign it         obj s reference now points to the new object         x s reference doesn t change      obj     val  100      var x     val  10    changeObject x      x       val  10

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This is little more explanation for pass by value and pass by reference  JavaScript   In this concept  they are talking about passing the variable by reference and passing the variable by reference   Pass by value  primitive type   var a   3  var b   a   console log a      a   3 console log b      b   3  a 4  console log a      a   4 console log b      b   3    applies to all primitive type in JavaScript  string  number  Boolean  undefined  and null   a is allocated a memory  say 0x001  and b creates a copy of the value in memory  say 0x002   So changing the value of a variable doesn t affect the other  as they both reside in two different locations      Pass by reference  objects   var c      name     john     var d   c   console log c         name     john    console log d         name     john     c name    doe    console log c         name     doe    console log d         name     doe       The JavaScript engine assigns the object to the variable c  and it points to some memory  say  0x012   When d c  in this step d points to the same location  0x012   Changing the value of any changes value for both the variable  Functions are objects     Special case  pass by reference  objects   c     name     jane    console log c         name     jane    console log d         name     doe       The equal    operator sets up new memory space or address

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An easy way to determine whether something is  pass by reference  is whether you can write a  swap  function  For example  in C  you can do   void swap int  i  int  j        int t      t    i       i    j       j   t      If you can t do the equivalent of that in JavaScript  it is not  pass by reference

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Semantics   Setting concrete definitions will necessarily make some answers and comments incompatible since they are not describing the same thing even when using the same words and phrases  but it is critical to get past the confusion  especially for new programmers    First of all  there are multiple levels of abstraction that not everyone seems to grasp   Newer programmers who have learned on 4th or 5th generation languages may have difficulty wrapping their mind around concepts familiar to assembly or C programmers not phased by pointers to pointers to pointers   Pass-by-reference does not simply mean the ability to change a referenced object using a function parameter variable   Variable  Combined concept of a symbol which references a value at a particular location in memory   This term is usually too loaded to be used alone in discussing details   Symbol   Text string used to refer to variable  i e  variable s name    Value   Particular bits stored in memory and referenced using variable s symbol   Memory location   Where a variable s value is stored    The location itself is represented by a number separate from the value stored at the location    Function parameter   Variable declared in a function definition  used for referencing variables passed to the function   Function argument  Variable outside the function which is passed to the function by the caller   Object variable   Variable whose basic underlying value is not the  object  itself  rather its value is a pointer  memory location value  to another location in memory where the object s actual data is stored   In most higher-generation languages  the  pointer  aspect is effectively hidden by automatic de-referencing in various contexts   Primitive variable  Variable whose value IS the actual value   Even this concept can be complicated by auto-boxing and object-like contexts of various languages  but the general ideas is that the variable s value IS the actual value represented by the variable s symbol rather than a pointer to another memory location   Function arguments and parameters are not the same thing   Also  a variable s value is not the variable s object  as already pointed out by various people  but apparently ignored    These distinctions are critical to proper understanding   Pass-by-value or Call-by-sharing  for objects    The function argument s value is COPIED to another memory location which is referenced by the function s parameter symbol  regardless of whether it s on the stack or heap    In other words  the function parameter received a copy of the passed argument s value    AND  critical  the argument s value IS NEVER UPDATED   ALTERED   CHANGED by the calling function   Remember  an object variable s value is NOT the object itself  rather it is the pointer to the object  so passing an object variable by value copies the pointer to the function parameter variable   The function parameter s value points to the exact same object in memory   The object data itself can be altered directly via the function parameter  BUT the function argument s value IS NEVER UPDATED  so it will continue to point to the same object throughout and even after the function call  even if its object s data was altered or if the function parameter is assigned a different object altogether    It is incorrect to conclude that the function argument was passed by reference just because the referenced object is updatable via the function parameter variable   Call   Pass-by-reference   The function argument s value can will be updated directly by the corresponding function parameter   If it helps  the function parameter becomes an effective  alias  for the argument--they effectively refer to the same value at the same memory location   If a function argument is an object variable  the ability to change the object s data is no different than the pass-by-value case since the function parameter will still point to the same object as the argument   But in the object variable case  if the function parameter is set to a completely different object  then the argument will likewise also point to the different object--this does not happen in the pass-by-value case   JavaScript does not pass by reference   If you read closely  you will realize that all contrary opinions misunderstand what is meant by pass-by-value and they falsely conclude that the ability to update an object s data via the function parameter is synonymous to  pass-by-value    Object clone copy  A new object is created and the original object s data is copied   This can be a deep copy or shallow copy  but the point is that a new object is created   Creating a copy of an object is a separate concept from pass-by-value   Some languages distinguish between class object and structs  or the like   and may have different behavior for passing variables of the different types   But JavaScript does not do anything like this automatically when passing object variables   But the absence of automatic object cloning does not translate to pass-by-reference

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Primitives are passed by value and objects are passed by reference  This is quite different from other languages like C  Visual Basic  or Delphi  I can t say how they handle objects and primitives exactly  but I know of Visual Basic and Delphi that it can  and should  be specified   PHP does something similar since version 5  all objects are passed by reference  but all primitives may be passed by reference  if preceded by an ampersand   amp    Otherwise primitives are passed by value   So in JavaScript  if I pass an object X into a function via a parameter  it will still be X  If you are changing data inside the function  or any other object  but that s not important  that new value is also available outside the function

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Mostly simplest way     Copy JS object without reference var first     a   value1   b   value2    var clone   JSON parse  JSON stringify  first        var second     a   b   c    var clone   JSON parse  JSON stringify  second

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I would say it is pass-by-copy -  Consider arguments and variable objects are objects created during the execution context created in the beginning of function invocation - and your actual value reference passed into the function just get stored in this arguments   variable objects   Simply speaking  for primitive types  the values get copied in the beginning of function call  for object type  the reference get copied

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primitive type variable like string number are always pass as  pass by value  Array and Object is passed as  pass by reference or  pass by value based on these two condition    if you are changing value of that Object or array with new Object or Array then it is pass by Value   object1    item   car      array1  1 2 3     here you are assigning new object or array to old one you are not changing the value of property  of old object so it is pass by value    if you are changing a property value of an object or array then it is pass by Reference   object1 key1   car     array1 0  9    here you are changing a property value of old object you are not assigning new object or array to old one so it is pass by reference    Code      function passVar object1  object2  number1             object1 key1   laptop           object2                 key2   computer                     number1   number1   1             var object1             key1   car             var object2             key2   bike             var number1   10       passVar object1  object2  number1       console log object1 key1       console log object2 key2       console log number1    Output  -     laptop     bike     10

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Passing arguments to a function in JavaScript is analogous to passing parameters by pointer value in C      The following C program demonstrates how arguments to JavaScript functions are passed in a way analogous to pass-by-pointer-value in C  The original JavaScript test case by  Shog9 follows with the translation of the code into C  This should make things clear to those transitioning from C to JavaScript   function changeStuff num  obj1  obj2        num   num   10      obj1 item    changed       obj2    item   changed       var num   10  var obj1    item   unchanged    var obj2    item   unchanged    changeStuff num  obj1  obj2   console log num   console log obj1 item       console log obj2 item    This produces the output   10 changed unchanged      include  lt stdio h gt   include  lt stdlib h gt   struct obj       char  item      void changeStuff int  num  struct obj  obj1  struct obj  obj2           make pointer point to a new memory location        holding the new integer value     int  old num   num      num   malloc sizeof int         num    old num   10         make property of structure pointed to by pointer        point to the new value     obj1- gt item    changed          make pointer point to a new memory location        holding the new structure value     obj2   malloc sizeof struct obj        obj2- gt item    changed       free num      end of scope     free obj2      end of scope    int num   10  struct obj obj1      unchanged     struct obj obj2      unchanged      int main            pass pointers by value  the pointers        will be copied into the argument list        of the called function and the copied        pointers will point to the same values        as the original pointers     changeStuff  amp num   amp obj1   amp obj2       printf   d n   num       puts obj1 item       puts obj2 item       return 0

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Observation  If there isn t any way for an observer to examine the underlying memory of the engine  there is no way to determine whether an immutable value gets copied or a reference gets passed  JavaScript is more or less agnostic to the underlying memory model  There is no such thing as a reference    JavaScript has values  Two variables can hold the same value  or more accurate  two environment records can bind the same value   The only type of values that can be mutated are objects through their abstract   Get   and   Set   operations  If you forget about computers and memory  this is all you need to describe JavaScript s behaviour  and it allows you to understand the specification   let a     prop  1     let b   a     a and b hold the same value  a prop    quot test quot      The object gets mutated  can be observed through both a and b  b     prop  2       b holds now a different value  Now you might ask yourself how two variables can hold the same value on a computer  You might then look into the source code of a JavaScript engine and you ll most likely find something which a programmer of the language the engine was written in would call a reference  So in fact you can say that JavaScript is  quot pass by value quot   whereas the value can be shared  and you can say that JavaScript is  quot pass by reference quot   which might be a useful logical abstraction for programmers from low level languages  or you might call the behaviour  quot call by sharing quot   As there is no such thing as a reference in JavaScript  all of these are neither wrong nor on point  Therefore I don t think the answer is particularly useful to search for     The term Reference in the specification is not a reference in the traditional sense  It is a container for an object and the name of a property  and it is an intermediate value  e g   a b evaluates to Reference   value   a  name    quot b quot      The term reference also sometimes appears in the specification in unrelated sections

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I ve read through these answers multiple times  but didn t REALLY get it until I learned about the technical definition of  Call by sharing  as termed by Barbara Liskov     The semantics of call by sharing differ from call by reference in that assignments to function arguments within the function aren t visible to the caller  unlike by reference semantics  citation needed   so e g  if a variable was passed  it is not possible to simulate an assignment on that variable in the caller s scope  However  since the function has access to the same object as the caller  no copy is made   mutations to those objects  if the objects are mutable  within the function are visible to the caller  which may appear to differ from call by value semantics  Mutations of a mutable object within the function are visible to the caller because the object is not copied or cloned     it is shared    That is  parameter references are alterable if you go and access the parameter value itself  On the other hand  assignment to a parameter will disappear after evaluation  and is non-accessible to the function caller

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A very detailed explanation about copying  passing and comparing by value and by reference is in this chapter of the  quot JavaScript  The Definitive Guide quot  book   Before we leave the topic of manipulating objects and arrays by reference  we need to clear up a point of nomenclature  The phrase  quot pass by reference quot  can have several meanings  To some readers  the phrase refers to a function invocation technique that allows a function to assign new values to its arguments and to have those modified values visible outside the function  This is not the way the term is used in this book  Here  we mean simply that a reference to an object or array -- not the object itself -- is passed to a function  A function can use the reference to modify properties of the object or elements of the array  But if the function overwrites the reference with a reference to a new object or array  that modification is not visible outside of the function  Readers familiar with the other meaning of this term may prefer to say that objects and arrays are passed by value  but the value that is passed is actually a reference rather than the object itself

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In a low-level language  if you want to pass a variable by reference  you have to use a specific syntax in the creation of the function  int myAge   14  increaseAgeByRef myAge   function increaseAgeByRef int  amp age       age    age   1     The  amp age is a reference to myAge  but if you want the value you have to convert the reference  using  age  JavaScript is a high level language that does this conversion for you  So  although objects are passed by reference  the language converts the reference parameter to the value  You don t need to use  amp   on the function definition  to pass it by reference  neither    on the function body  to convert the reference to the value  JavaScript does it for you  That s why when you try to change an object inside a function  by replacing it s value  i e  age    value 5    the change doesn t persist  but if you change it s properties  i e  age value   5   it does  Learn more

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well  it s about  performance  and  speed  and in the simple word  memory management  in a programming language   in javascript we can put values in two layer  type1-objects and type2-all other types of value such as string  amp  boolean  amp  etc  if you imagine memory as below squares which in every one of them just one type2-value can be saved     every type2-value  green  is a single square while a type1-value  blue  is a group of them     the point is that if you want to indicate a type2-value  the address is plain but if you want to do the same thing for type1-value that s not easy at all       and in a more complicated story     so here references can rescue us    while the green arrow here is a typical variable  the purple one is an object variable  so because the green arrow typical variable  has just one task  and that is indicating a typical value  we don t need to separate it s value from it so we move the green arrow with the value of that wherever it goes and in all assignments  functions and so on      but we cant do the same thing with the purple arrow  we may want to move  john  cell here or many other things     so the purple arrow will stick to its place and just typical arrows that were assigned to it will move      a very confusing situation is where you can t realize how your referenced variable changes  let s take a look at a very good example   let arr    1  2  3  4  5     arr is an object now and a purple arrow is indicating it let obj2   arr     now  obj2 is another purple arrow that is indicating the value of arr obj let obj3     a    b    c    obj2 push 6      first pic below - making a new hand for the blue circle to point the 6   obj2    1  2  3  4  5  6    arr    1  2  3  4  5  6    we changed the blue circle object value  type1-value  and due to arr and obj2 are indicating that so both of them changed obj2   obj3    next pic below - changing the direction of obj2 array from blue circle to orange circle so obj2 is no more  1 2 3 4 5 6  and it s no more about changing anything in it but we completely changed its direction and now obj2 is pointing to obj3   obj2     a    b    c      obj3     a    b    c

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In JavaScript  the type of the value solely controls whether that value will be assigned by value-copy or by reference-copy    Primitive values are always assigned passed by value-copy     null undefined string number boolean symbol in ES6   Compound values are always assigned passed by reference-copy   objects  arrays function   For example  var a   2  var b   a      b  is always a copy of the value in  a  b    a     2 b     3  var c    1 2 3   var d   c      d  is a reference to the shared   1 2 3   value d push  4    c      1 2 3 4  d      1 2 3 4    In the above snippet  because 2 is a scalar primitive  a holds one initial copy of that value  and b is assigned another copy of the value  When changing b  you are in no way changing the value in a   But both c and d are separate references to the same shared value  1 2 3   which is a compound value  It s important to note that neither c nor d more  owns  the  1 2 3  value -- both are just equal peer references to the value  So  when using either reference to modify   push 4   the actual shared array value itself  it s affecting just the one shared value  and both references will reference the newly modified value  1 2 3 4    var a    1 2 3   var b   a  a      1 2 3  b      1 2 3      later b    4 5 6   a      1 2 3  b      4 5 6    When we make the assignment b    4 5 6   we are doing absolutely nothing to affect where a is still referencing   1 2 3    To do that  b would have to be a pointer to a rather than a reference to the array -- but no such capability exists in JS   function foo x        x push  4        x      1 2 3 4          later     x    4 5 6       x push  7        x      4 5 6 7     var a    1 2 3    foo  a     a      1 2 3 4   not   4 5 6 7    When we pass in the argument a  it assigns a copy of the a reference to x  x and a are separate references pointing at the same  1 2 3  value  Now  inside the function  we can use that reference to mutate the value itself  push 4    But when we make the assignment x    4 5 6   this is in no way affecting where the initial reference a is pointing -- still points at the  now modified   1 2 3 4  value   To effectively pass a compound value  like an array  by value-copy  you need to manually make a copy of it  so that the reference passed doesn t still point to the original  For example   foo  a slice        Compound value  object  array  etc  that can be passed by reference-copy  function foo wrapper        wrapper a   42     var obj         a  2     foo  obj     obj a     42   Here  obj acts as a wrapper for the scalar primitive property a  When passed to foo      a copy of the obj reference is passed in and set to the wrapperparameter  We now can use the wrapper reference to access the shared object  and update its property  After the function finishes  obj a will see the updated value 42   Source

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The variable doesn t  hold  the object  it holds a reference  You can assign that reference to another variable  and now both reference the same object  It s always pass by value  even when that value is a reference       There s no way to alter the value held by a variable passed as a parameter  which would be possible if JavaScript supported passing by reference

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The most succinct explanation I found was in the AirBNB style guide    Primitives  When you access a primitive type you work directly on its value   string number boolean null undefined    E g    var foo   1      bar   foo   bar   9   console log foo  bar        gt  1  9    Complex  When you access a complex type you work on a reference to its value   object array function    E g    var foo    1  2       bar   foo   bar 0    9   console log foo 0   bar 0         gt  9  9   I e  effectively primitive types are passed by value  and complex types are passed by reference

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An object outside a function is passed into a function by giving a reference to the outside object   When you use that reference to manipulate its object  the object outside is thus affected  However  if inside the function you decided to point the reference to something else  you did not affect the object outside at all  because all you did was re-direct the reference to something else

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The MDN docs explain it clearly  without being too verbose        The parameters of a function call are the function s arguments    Arguments are passed to functions by value  If the function changes   the value of an argument  this change is not reflected globally or in   the calling function  However  object references are values  too  and   they are special  if the function changes the referred object s   properties  that change is visible outside the function          Source  https   developer mozilla org en-US docs Web JavaScript Reference Functions Description

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It s interesting in JavaScript  Consider this example   x000D   x000D  function changeStuff a  b  c      a   a   10    b item    changed     c    item   changed       var num   10  var obj1    item   unchanged    var obj2    item   unchanged     changeStuff num  obj1  obj2    console log num   console log obj1 item   console log obj2 item   x000D   x000D   x000D   This produces the output  10 changed unchanged   If obj1 was not a reference at all  then changing obj1 item would have no effect on the obj1 outside of the function  If the argument was a proper reference  then everything would have changed  num would be 100  and obj2 item would read  quot changed quot    Instead  the situation is that the item passed in is passed by value  But the item that is passed by value is itself a reference  Technically  this is called call-by-sharing  In practical terms  this means that if you change the parameter itself  as with num and obj2   that won t affect the item that was fed into the parameter  But if you change the internals of the parameter  that will propagate back up  as with obj1

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My simple way to understand this      When calling a function  you are passing the content  reference or value  of the argument variables  not the the variables themselves   var var1   13  var var2     prop  2       13 and var2 s content  reference  are being passed here foo var1  var2     Inside the function  parameter variables  inVar1 and inVar2  receive the contents being passed   function foo inVar1  inVar2         changing contents of inVar1 and inVar2 won t affect variables outside     inVar1   20      inVar2     prop  7       Since inVar2 received the reference of   prop  2    you can change the value of the object s property   function foo inVar1  inVar2       inVar2 prop   7

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JavaScript is always pass-by-value  everything is of value type    Objects are values  and member functions of objects are values themselves  remember that functions are first-class objects in JavaScript   Also  regarding the concept that everything in JavaScript is an object  this is wrong  Strings  symbols  numbers  booleans  nulls  and undefineds are primitives   On occasion they can leverage some member functions and properties inherited from their base prototypes  but this is only for convenience  It does not mean that they are objects themselves  Try the following for reference   x    test   alert x foo   x foo   12  alert x foo     In both alerts you will find the value to be undefined

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There s some discussion about the use of the term  pass by reference  in JavaScript here  but to answer your question      A object is automatically passed by reference  without the need to specifically state it    From the article mentioned above

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Consider the following    Variables are pointers to values in memory  Reassigning a variable merely points that pointer at a new value  Reassigning a variable will never affect other variables that were pointing at that same object   So  forget about  pass by reference value  don t get hung up on  pass by reference value  because    The terms are only used to describe the behavior of a language  not necessarily the actual underlying implementation  As a result of this abstraction  critical details that are essential for a decent explanation are lost  which inevitably leads to the current situation where a single term doesn t adequately describe the actual behavior and supplementary info has to be provided These concepts were not originally defined with the intent of describing javascript in particular and so I don t feel compelled to use them when they only add to the confusion    To answer your question  pointers are passed        code var obj         name   Fred       num  1        illustration                 Fred                                  obj  ----                                                  1        code obj name    George        illustration                   Fred     obj  ----    -----  George                                                1        code obj           illustration                   Fred     obj          -----  George                                                 1        code var obj         text   Hello world          function parameters get their own pointer to     the arguments that are passed in  just like any other variable    someFunc obj        illustration  caller scope          someFunc scope                                                                                                                                                                                                                             Hello world    Some final comments    It s tempting to think that primitives are enforced by special rules while objects are not  but primitives are simply the end of the pointer chain  As a final example  consider why a common attempt to clear an array doesn t work as expected      var a    1 2   var b   a   a       console log b       1 2     doesn t work because  b  is still pointing at the original array

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It s always pass by value  but for objects the value of the variable is a reference   Because of this  when you pass an object and change its members  those changes persist outside of the function   This makes it look like pass by reference   But if you actually change the value of the object variable you will see that the change does not persist  proving it s really pass by value   Example    x000D   x000D  function changeObject x    x000D    x     member   bar     x000D    console log  in changeObject      x member   x000D    x000D   x000D  function changeMember x    x000D    x member    bar   x000D    console log  in changeMember      x member   x000D    x000D   x000D  var x     member   foo     x000D   x000D  console log  before changeObject      x member   x000D  changeObject x   x000D  console log  after changeObject      x member      change did not persist    x000D   x000D  console log  before changeMember      x member   x000D  changeMember x   x000D  console log  after changeMember      x member      change persists    x000D   x000D   x000D    Output   before changeObject  foo in changeObject  bar after changeObject  foo  before changeMember  foo in changeMember  bar after changeMember  bar

[javascript] Is JavaScript a pass-by-reference or pass-by-value language?

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