Copy a variable s value into another

Question

I have a variable which has a JSON object as its value  I directly assign this variable to some other variable so that they share the same value  This is how it works   var a       some hidden var   val        b   a    This works and both have the same value  I use a mousemove event handler to update b through out my app  On a button click  I want to revert b to the original value  meaning the value stored in a       revert   on  click   function e       b   a        After this if I use the same mousemove event handler  it updates both a and b  when earlier it was updating only b as expected   I m stumped over this issue  What is wrong here

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I solved it myself for the time being  The original value has only 2 sub-properties  I reformed a new object with the properties from a and then assigned it to b  Now my event handler updates only b  and my original a stays as it is   var a     key1   value1   key2   value2         b   a       revert   on  click   function e         FAIL      b   a         WIN     b     key1  a key1  key2  a key2          This works fine  I have not changed a single line anywhere in my code except for the above  and it works just how I wanted it to  So  trust me  nothing else was updating a

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The reason for this is simple  JavaScript uses refereces  so when you assign b   a you are assigning a reference to b thus when updating a you are also updating b  I found this on stackoverflow and will help prevent things like this in the future by just calling this method if you want to do a deep copy of an object   function clone obj           Handle the 3 simple types  and null or undefined     if  null    obj     object     typeof obj  return obj          Handle Date     if  obj instanceof Date            var copy   new Date            copy setTime obj getTime             return copy                Handle Array     if  obj instanceof Array            var copy               for  var i   0  len   obj length  i  lt  len  i                  copy i    clone obj i                      return copy                Handle Object     if  obj instanceof Object            var copy               for  var attr in obj                if  obj hasOwnProperty attr   copy attr    clone obj attr                      return copy             throw new Error  Unable to copy obj  Its type isn t supported

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newVariable   originalVariable valueOf     for objects you can use   b   Object assign    a

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I found using JSON works but watch our for circular references  var newInstance   JSON parse JSON stringify firstInstance

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It s important to understand what the   operator in JavaScript does and does not do   The   operator does not make a copy of the data   The   operator creates a new reference to the same data   After you run your original code   var a       some hidden var   val        b   a    a and b are now two different names for the same object   Any change you make to the contents of this object will be seen identically whether you reference it through the a variable or the b variable  They are the same object   So  when you later try to  revert  b to the original a object with this code   b   a    The code actually does nothing at all  because a and b are the exact same thing  The code is the same as if you d written   b   b    which obviously won t do anything   Why does your new code work   b     key1  a key1  key2  a key2      Here you are creating a brand new object with the       object literal  This new object is not the same as your old object  So you are now setting b as a reference to this new object  which does what you want   To handle any arbitrary object  you can use an object cloning function such as the one listed in Armand s answer  or since you re using jQuery just use the   extend   function  This function will make either a shallow copy or a deep copy of an object   Don t confuse this with the     clone   method which is for copying DOM elements  not objects    For a shallow copy   b     extend      a      Or a deep copy   b     extend  true      a      What s the difference between a shallow copy and a deep copy  A shallow copy is similar to your code that creates a new object with an object literal  It creates a new top-level object containing references to the same properties as the original object   If your object contains only primitive types like numbers and strings  a deep copy and shallow copy will do exactly the same thing  But if your object contains other objects or arrays nested inside it  then a shallow copy doesn t copy those nested objects  it merely creates references to them  So you could have the same problem with nested objects that you had with your top-level object  For example  given this object   var obj         w  123      x            y  456          z  789            If you do a shallow copy of that object  then the x property of your new object is the same x object from the original   var copy     extend      obj    copy w   321  copy x y   654    Now your objects will look like this      copy looks as expected var copy         w  321      x            y  654          z  789              But changing copy x y also changed obj x y  var obj         w  123      changing copy w didn t affect obj w     x            y  654      changing copy x y also changed obj x y         z  789            You can avoid this with a deep copy  The deep copy recurses into every nested object and array  and Date in Armand s code  to make copies of those objects in the same way it made a copy of the top-level object  So changing copy x y wouldn t affect obj x y   Short answer  If in doubt  you probably want a deep copy

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I do not understand why the answers are so complex   In Javascript  primitives  strings  numbers  etc  are passed by value  and copied   Objects  including arrays  are passed by reference   In any case  assignment of a new value or object reference to  a  will not change  b    But changing the contents of  a  will change the contents of  b    var a    a   var b   a  a    c      b      a   var a    a  a    var b   a  a    c  c       b      a  a   and a    c  c    var a    a  a    var b   a  a a    c      b a      c  and a a      c    Paste any of the above lines  one at a time  into node or any browser javascript console   Then type any variable and the console will show it s value

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Most of the answers here are using built-in methods or using libraries frameworks  This simple method should work fine   function copy x        return JSON parse  JSON stringify x           Usage var a    some   var b   copy a   a     thing    console log b       some   var c     x  1    var d   copy c   c x   2   console log d        x  1

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the question is already solved since quite a long time  but for future reference a possible solution is  b   a slice 0     Be careful  this works correctly only if a is a non-nested array of numbers and strings

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For strings or input values you could simply use this   var a       some hidden var   val    b   a substr 0

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A solution for AngularJS    scope targetObject   angular copy  scope sourceObject

[javascript] Copy a variable's value into another

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