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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Cloning an object
Date: Fri, 06 Jul 2018 00:24:09 +0200
Organization: PointedEars Software (PES)
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Xref: csiph.com comp.lang.javascript:35356

Cezary Tomczyk wrote:

> var o = {
>    a: 1,
>    b: [
>      {
>        test: 'test'
>      }
>    ]
> };
> 
> var cloned = clone(o);
> 
> o.b[0].test = 'something';
> 
> expect(cloned.b[0].test).toBe('test');
> 
> This test doesn't pass. Basically, modifications on original object
> should not modify cloned object.

The object referred to by “o.b[0]” is NOT part of the original object, 
therefore if you do _not_ make a “deep copy”, then only the *reference 
value* stored in the “b” property of the original object is cloned (copied).

Despite the syntax that suggests otherwise, ECMAScript objects are NOT like 
that:

  ,---------------------------------.
  :          o : Object             :
  :---------------------------------:
  : +a : number = 1                 :
  : ,-----------------------------. :
  : :         +b : Array          : :
  : : ,-------------------------. : :
  : : :       +0 : Object       : : :
  : : :-------------------------: : :
  : : : +test : string = "test" : : :
  : : `-------------------------' : :
  : `-----------------------------' :
  `---------------------------------'
  
But like this:

        ,-----------------.
  o --> :    : Object     :
        :--------------:--:
        : +a : number = 1 :   ,--------------.
        : +b : Array -------> :   : Array    :
        `-----------------'   :--------------:   ,-------------------------.
                              : +0 : Object ---> :        : Object         :
                              `--------------'   :-------------------------:
                                                 : +test : string = "test" :
                                                 `-------------------------'

When you clone an object without making a “deep copy”, i.e. a *clone* of the 
*objects* that the first object’s properties *refer to*, then the clone will 
access the same objects:

        ,-----------------.
  o --> :    : Object     :
        :--------------:--:
        : +a : number = 1 :   ,------------.-.
        : +b : Array -------> :   : Array    :
        `-----------------'   :--------------:   ,-------------------------.
                              : +0 : Object ---> :        : Object         :
                              `--------------'   :-------------------------:
                                     ^           : +test : string = "test" :
                                     :           `-------------------------'
                                     :
             ,-----------------.     :
  cloned --> :    : Object     :     :
             :--------------:--:     :
             : +a : number = 1 :     :
             : +b : Array -----------'
             `-----------------'

vs. (“deep copy”)

        ,-----------------.
  o --> :    : Object     :
        :--------------:--:
        : +a : number = 1 :   ,------------.-.
        : +b : Array -------> :   : Array    :
        `-----------------'   :--------------:   ,-------------------------.
                              : +0 : Object ---> :        : Object         :
                              `--------------'   :-------------------------:
                                                 : +test : string = "test" :
                                                 `-------------------------'

             ,-----------------.
  cloned --> :    : Object     :
             :--------------:--:
             : +a : number = 1 :   ,------------.-.
             : +b : Array -------> :   : Array    :
             `-----------------'   :--------------:   
                                   : +0 : Object -----.
                                   `--------------'   :
                                                      :
                         ,-------------------------.  :
                         :        : Object         :<-'
                         :-------------------------:
                         : +test : string = "test" :
                         `-------------------------'

This important difference is why I insist on saying “the object referred to 
by” instead of just “object”, and only using shorthand terminology with 
previous definition.

-- 
PointedEars
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