Pointers in JavaScript

Question

Can we pass a reference of a variable that is immutable as argument in a function   Example   var x   0  function a x        x      a x   alert x     Here I want to have 1 instead of 0

User · Answer

In your example you actually have 2 variables with the same name. The (global) variable x and the function scoped variable x. Interesting to see that javascript, when given a choice of what to do with 2 variables of the same name, goes with the function scoped name and ignores the out-of-scope variable.

It's probably not safe to presume javascript will always behave this way...

Cheers!

User · Answer

Javascript should just put pointers into the mix coz it solves a lot of problems  It means code can refer to an unknown variable name or variables that were created dynamically  It also makes modular coding and injection easy   This is what i see as the closest you can come to c pointers in practice   in js   var a   78           creates a var with integer value of 78  var pointer    a     note it is a string representation of the var name eval  pointer       12       equivalent to  eval   a   12    but changes value of a to 12   in c   int a   78           creates a var with integer value of 78  int pointer    amp a     makes pointer  to refer to the same address mem as a  pointer   12       changes the value of a to 12

User · Answer

Since JavaScript does not support passing parameters by reference  you ll need to make the variable an object instead  x000D   x000D  var x    Value  0   x000D   x000D  function a obj  x000D    x000D      obj Value    x000D    x000D   x000D  a x   x000D  document write x Value     Here i want to have 1 instead of 0 x000D   x000D   x000D    In this case  x is a reference to an object   When x is passed to the function a  that reference is copied over to obj   Thus  obj and x refer to the same thing in memory   Changing the Value property of obj affects the Value property of x   Javascript will always pass function parameters by value  That s simply a specification of the language  You could create x in a scope local to both functions  and not pass the variable at all

User · Answer

This question may help  How to pass variable by reference in javascript  Read data from ActiveX function which returns more than one value  To summarise  Javascript primitive types are always passed by value  whereas the values inside objects are passed by reference  thanks to commenters for pointing out my oversight   So to get round this  you have to put your integer inside an object     x000D   x000D  var myobj    x 0   x000D   x000D  function a obj  x000D    x000D      obj x    x000D    x000D   x000D  a myobj   x000D  alert myobj x      returns 1 x000D   x000D     x000D   x000D   x000D

User · Answer

Depending on what you would like to do  you could simply save the variable name  and then access it later on like so   function toAccessMyVariable variableName     alert window variableName       var myFavoriteNumber   6    toAccessMyVariable  myFavoriteNumber      To apply to your specific example  you could do something like this    var x   0  var pointerToX    x    function a variableName        window variableName       a pointerToX   alert x     Here I want to have 1 instead of 0

User · Answer

I have found a slightly different way implement pointers that is perhaps more general and easier to understand from a C perspective  and thus fits more into the format of the users example     In JavaScript  like in C  array variables are actually just pointers to the array  so you can use an array as exactly the same as declaring a pointer  This way  all pointers in your code can be used the same way  despite what you named the variable in the original object   It also allows one to use two different notations referring to the address of the pointer and what is at the address of the pointer   Here is an example  I use the underscore to denote a pointer    var  x     10     function foo  a        a 0     10     foo  x    console log  x 0      Yields  output  20

User · Answer

It just doesn t support pointers  end of story  -    I would like to add that   while I m brand new and coming from a C background  from what I m reading about the event loop and stack  you should keep whichever object as close to its caller as possible   As I m brand new   I could have this wrong    A lot of answers here suggest to make it global  but again if I m reading correctly   that could possibly increase the stack hops and in a way  possibly fragment your event loop needlessly  depending on what the called object is currently doing     Also   any example given that mimics  pointers   is just 1 in many ways to do the same thing   which will not be a  pointer

User · Answer

I m thinking that in contrary to C or any language with pointers        Javascript     var o    x 10 y 20   var o2    z 50 w 200      obviously in javascript you cannot access to objects o and o2 addresses in memory but you can t compare their address neither    no trivial possibility to sort them  and then access by dichotomy       o    o2    obviously false   not the same address in memory o  lt   o2    true   o  gt   o2    also true      that s a huge problem      it means that you can list manage every objects created  allocated  by your application   from that huge list of objects  compute some information about these  how they are linked together for example  but when you want to retrieve the information you created about a specific object  you cannot find it in your huge list by dichotomy   there is no unique identifier per object that could be used as a replacement of the real memory address   this finally means that this is a huge problem  if you want to write in javascript     a javascript debugger   IDE  or a specially optimized garbage collector a data structure  drawer   analyzer

User · Answer

JavaScript doesn t support passing primitive types by reference  There is a workaround  though   Put all the variables you need to pass in an object  In this case  there s only one variable  x  Instead of passing the variable  pass the variable name as a string  which is  x     x000D   x000D  var variables    x 0   x000D  function a x  x000D    x000D      variables x     x000D    x000D  a  x    x000D  alert variables x   x000D   x000D   x000D

User · Answer

It might be impossible since JavaScript doesn t have Perl s     operator to get a primitive by reference  but there is a way to create an  effectively a pointer  object for a primitive using this pattern   This solution makes the most sense for when you already have the primitive so you can t put it into an object anymore without needing to modify other code   but still need to pass a pointer to it for other parts of your code to tinker with its state  so you can still tinker with its state using the seamless proxy which behaves like a pointer   var proxyContainer             attaches a pointer-lookalike getter setter pair      to the container object  var connect   function container             primitive  can t create a reference to it anymore     var cant touch this   1337            we know where to bind our accessor mutator          so we can bind the pair to effectively behave          like a pointer in most common use cases      Object defineProperty container   cant touch this              get   function                 return cant touch this                                      set   function val                cant touch this   val                             not quite direct  but still  touchable  var f   function x        x cant touch this    1      connect proxyContainer         looks like we re just getting cant touch this      but we re actually calling the accessor  console log proxyContainer cant touch this         looks like we re touching cant touch this      but we re actually calling the mutator  proxyContainer cant touch this   90        looks like we touched cant touch this      but we actually called a mutator which touched it for us  console log proxyContainer cant touch this    f proxyContainer         we kinda did it     console log proxyContainer cant touch this

User · Answer

In JavaScript  you cannot pass variables by reference to a function  However you can pass an object by reference

User · Answer

Late answer but I have come across a way of passing primitive values by reference by means of closures  It is rather complicated to create a pointer  but it works   function ptr get  set        return   get  get  set  set       function helloWorld namePtr        console log namePtr get         namePtr set  jack        console log namePtr get       var myName    joe   var myNamePtr   ptr      function      return myName         function  value    myName   value        helloWorld myNamePtr      joe  jack console log myName      jack   In ES6  the code can be shortened to use lambda expressions   var myName    joe   var myNamePtr   ptr   gt  myName  v   gt  myName   v    helloWorld myNamePtr      joe  jack console log myName      jack

User · Answer

In JavaScript that would be a global   However  your function would look more like this   function a       x         Since x is in the global context  you don t need to pass it into the function

User · Answer

You refer to  x  from window object  var x   0   function a key  ref        ref   ref    window      object reference - default window     ref key        a  x                         string alert x

[javascript] Pointers in JavaScript?

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