Re: duplicator an array

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From	Aleksandro <aleksandro@gmx.com>
Newsgroups	comp.lang.javascript
Subject	Re: duplicator an array
Date	Sat, 16 Apr 2016 14:13:31 -0300
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On 16/04/16 09:27, Stefan Weiss wrote:
> Aleksandro wrote:
>> On 13/04/16 22:27, JRough wrote:
>>> var arr =[1,2,3,4,5];
>>> function duplicator(arr){
>>>     return  arr.concat(arr.slice(0));
>>> }
>>> var res = duplicator(arr);
> 
> ...
> 
>> Try:
>>
>> Array.prototype.duplicate = function ()
>> {
>> 	this.push.apply(this, this.slice(0))
>> }
> 
> Note that this is not equivalent to OP's function.
> 
> 1) push() modifies the original array, concat() does not.
> 
> 2) It handles sparse arrays badly:
> 
>     "use strict";
> 
>     let a = [0,,,,4],
>         b = a.slice(0);
> 
>     dir(a, "a");
>     dir(b, "b");
>     dir(a.concat(b), "concat()");
>     a.push.apply(a, b);
>     dir(a, "push() applied");
> 
>     function dir (arr, label)
>     {
>         console.log(label + ":");
>         for (let i in arr) console.log(`  [${i}] = ${arr[i]}`);
>     }
> 
> Output:
> 
>     a:
>       [0] = 0
>       [4] = 4
>     b:
>       [0] = 0
>       [4] = 4
>     concat():
>       [0] = 0
>       [4] = 4
>       [5] = 0
>       [9] = 4
>     push() applied:
>       [0] = 0
>       [4] = 4
>       [5] = 0
>       [6] = undefined
>       [7] = undefined
>       [8] = undefined
>       [9] = 4
> 
> This is because apply() fills in `undefined` for missing parameters.
> 
> 
> By the way, ES2015 introduced the spread operator `...`.
> With this operator, you can also write
> 
>     this.push(...this.slice(0))
> 
> instead of
> 
>     this.push.apply(this, this.slice(0))
> 
> 
> -- stefan

It could have been what the OP intended though.

Thread

duplicator an array JRough <janis.rough@gmail.com> - 2016-04-13 18:27 -0700
  Re: duplicator an array Aleksandro <aleksandro@gmx.com> - 2016-04-14 01:00 -0300
    Re: duplicator an array JRough <janis.rough@gmail.com> - 2016-04-14 09:53 -0700
    Re: duplicator an array "Christoph M. Becker" <cmbecker69@arcor.de> - 2016-04-14 19:41 +0200
      Re: duplicator an array Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-04-14 22:09 +0200
        Re: duplicator an array Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-04-14 22:23 +0200
    Re: duplicator an array Stefan Weiss <krewecherl@gmail.com> - 2016-04-16 14:27 +0200
      Re: duplicator an array Joao Rodrigues <jr@none.com> - 2016-04-16 12:01 -0300
        Re: duplicator an array Joao Rodrigues <jr@none.com> - 2016-04-16 13:43 -0300
      Re: duplicator an array Aleksandro <aleksandro@gmx.com> - 2016-04-16 14:13 -0300
  Re: duplicator an array bharat.bandgar009@gmail.com - 2017-05-03 01:33 -0700

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