Asp Forum - duplicator an array - comp.lang.javascript

jrough

4/14/2016 1:27:00 AM

I have one question for you.
The problem is to create a duplicator function like below. To make this one work:
[1,2,3,4,5].duplicator();
------------------------
Okay, first thing i did was just get it to work in a regular function. I think this works because I saw the duplcation of res in Firebug.
var arr =[1,2,3,4,5];
function duplicator(arr){
return arr.concat(arr.slice(0));
}
var res = duplicator(arr);
console.log(res.toString());
The only problem with this one is it doesn't output the string in the console.log.
Do you know why?

---------------------
Okay, supposing the above works then I thought I would make an anonymous function in the duplicator method. This would make it work the way the problem was first outlined above. The only problem is how do I capture the value of the two arrays?

I guess you would do something like Array.prototype.duplicator = (function (){
return this.concat(arr.slice(0));
})()

9 Answers

Aleksandro

4/14/2016 4:00:00 AM

On 13/04/16 22:27, JRough wrote:
>
> I have one question for you.
> The problem is to create a duplicator function like below. To make this one work:
> [1,2,3,4,5].duplicator();
> ------------------------
> Okay, first thing i did was just get it to work in a regular function. I think this works because I saw the duplcation of res in Firebug.
> var arr =[1,2,3,4,5];
> function duplicator(arr){
> return arr.concat(arr.slice(0));
> }
> var res = duplicator(arr);
> console.log(res.toString());
> The only problem with this one is it doesn't output the string in the console.log.
> Do you know why?

It works perfectly for me.

> Okay, supposing the above works then I thought I would make an anonymous function in the duplicator method. This would make it work the way the problem was first outlined above. The only problem is how do I capture the value of the two arrays?
>
>
> I guess you would do something like Array.prototype.duplicator = (function (){
> return this.concat(arr.slice(0));
> })()

Try:

Array.prototype.duplicate = function ()
{
this.push.apply(this, this.slice(0))
}

jrough

4/14/2016 4:53:00 PM

On Wednesday, April 13, 2016 at 9:00:18 PM UTC-7, Aleksandro wrote:
> On 13/04/16 22:27, JRough wrote:
> >
> > I have one question for you.
> > The problem is to create a duplicator function like below. To make this one work:
> > [1,2,3,4,5].duplicator();
> > ------------------------
> > Okay, first thing i did was just get it to work in a regular function. I think this works because I saw the duplcation of res in Firebug.
> > var arr =[1,2,3,4,5];
> > function duplicator(arr){
> > return arr.concat(arr.slice(0));
> > }
> > var res = duplicator(arr);
> > console.log(res.toString());
> > The only problem with this one is it doesn't output the string in the console.log.
> > Do you know why?
>
> It works perfectly for me.

I can't see it in firebug scratchpad console.log I'll try it in chrome tnx
>
> > Okay, supposing the above works then I thought I would make an anonymous function in the duplicator method. This would make it work the way the problem was first outlined above. The only problem is how do I capture the value of the two arrays?
> >
> >
> > I guess you would do something like Array.prototype.duplicator = (function (){
> > return this.concat(arr.slice(0));
> > })()
>
> Try:
>
> Array.prototype.duplicate = function ()
> {
> this.push.apply(this, this.slice(0))
> }

tnx again

Christoph M. Becker

4/14/2016 5:42:00 PM

On 14.04.2016 at 06:00, Aleksandro wrote:

> On 13/04/16 22:27, JRough wrote:
>
>> Okay, supposing the above works then I thought I would make an anonymous function in the duplicator method. This would make it work the way the problem was first outlined above. The only problem is how do I capture the value of the two arrays?
>>
>>
>> I guess you would do something like Array.prototype.duplicator = (function (){
>> return this.concat(arr.slice(0));
>> })()
>
> Try:
>
> Array.prototype.duplicate = function ()
> {
> this.push.apply(this, this.slice(0))
> }

The usual caveats wrt. augmenting built-in objects apply, though, see
e.g.
<http://peter.michaux.ca/articles/javascript-the-good-parts-built-in-object-augmentation-and-names....

--
Christoph M. Becker

Thomas 'PointedEars' Lahn

4/14/2016 8:10:00 PM

Christoph M. Becker wrote:

> On 14.04.2016 at 06:00, Aleksandro wrote:
>> On 13/04/16 22:27, JRough wrote:
>>> Okay, supposing the above works then I thought I would make an anonymous
>>> function in the duplicator method. This would make it work the way the
>>> problem was first outlined above. The only problem is how do I capture
>>> the value of the two arrays?
>>>
>>>
>>> I guess you would do something like Array.prototype.duplicator =
>>> (function (){
>>> return this.concat(arr.slice(0));
>>> })()
>>
>> Try:
>>
>> Array.prototype.duplicate = function ()
>> {
>> this.push.apply(this, this.slice(0))
>> }
>
>
> The usual caveats wrt. augmenting built-in objects apply, though, see
> e.g.
> <http://peter.michaux.ca/articles/javascript-the-good-parts-built-in-object-augmentation-and-names....

It should be noted that the referred article is *old*; it is dated
2008-06-18.

We now have String.prototype.trim(), and we have .toJSON() as a hook for
JSON.stringify() in the language standard (ES 2015, Â§ 24.3.2, step 12), and
it is implemented. So it would seem that either Douglas Crockford was
prophetic or, more likely, that his were good examples that later made it
into the standard. JFTR, String.prototype.trim() has been in the FAQ for
quite a while (currently in Â§ 8.1), and I have written toJSON() methods.

Also, one important reason for not augmenting the object referred to by
Array.prototype, and by extension any prototype, has disappeared: we have
standard property descriptors since ECMAScript Edition 5/5.1
(2009-12/2011-06) so that user-defined properties need no longer be
enumerable and need not mess up for-in iteration (useful for sparse arrays
if order is irrelevant):

try
{
Object.defineProperty(Array.prototype, "duplicate", {
value: function () {
return this.concat(this.slice());
},

/* optional; the default for each attribute is â??falseâ? */
configurable: true,
enumerable: false,
writable: true
});
}
catch
{
console.warn("Array.prototype.duplicate cannot be (re)configured.");
}

In this case, however, there is little benefit of calling the method instead
of inlining it with a.concat(a.slice()) [the 0 argument is optional as that
is the implicit default (ES 2015, Â§ 22.1.3.22, step 5)].

Also note that a.duplicate() as defined above does nothing of consequence;
in order for it to duplicate the array items in the desired way it would
have to define array items explicitly or its return value would need to be
assigned to something.

In any case, it is possible to safely augment built-in prototype objects if
you limit the scope of the change:

(function () {
var arrayDuplicate = Array.prototype.duplicate;

Array.prototype.duplicate = function () {
return this.concat(this.slice());
};

/* do something */

Array.prototype.duplicate = arrayDuplicate;
}());

This is safe only if there are no concurrent threads in the same global
context as all share the same â??Arrayâ? object and one of them might rely on a
different implementation. DNS-based namespaces, as suggested in the
article, might be a solution to that; but all parties involved have to agree
to that for it to work reliably.

Otherwise the only safe approach is to augment the instance instead of its
prototype:

(function () {
var a = [1, 2, 3];

a.duplicate = function () {
return this.concat(this.slice());
};

// â?¦

/* [1, 2, 3, 1, 2, 3] */
a = a.duplicate();

// â?¦
}());

--
PointedEars
FAQ: <http://PointedEars.... | SVN: <http://PointedEars.de...
Twitter: @PointedEars2 | ES Matrix: <http://PointedEars.de/es-...
Please do not cc me. / Bitte keine Kopien per E-Mail.

Thomas 'PointedEars' Lahn

4/14/2016 8:23:00 PM

Thomas 'Ingrid' Lahn wrote:

> try
> {
> Object.defineProperty(Array.prototype, "duplicate", {
> value: function () {
> return this.concat(this.slice());
> },
>
> /* optional; the default for each attribute is â??falseâ? */
> configurable: true,
> enumerable: false,
> writable: true
> });
> }
> catch
.--------^
:
(e)

> {
> console.warn("Array.prototype.duplicate cannot be (re)configured.");
> }
>
> [â?¦]
> Otherwise the only safe approach is to augment the instance instead of its
> prototype:
>
> (function () {
> var a = [1, 2, 3];
>
> a.duplicate = function () {
> return this.concat(this.slice());
> };

Or like above.

--
PointedEars
FAQ: <http://PointedEars.... | SVN: <http://PointedEars.de...
Twitter: @PointedEars2 | ES Matrix: <http://PointedEars.de/es-...
Please do not cc me. / Bitte keine Kopien per E-Mail.

Stefan Weiss

4/16/2016 12:27:00 PM

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

Joao Rodrigues

4/16/2016 3:02:00 PM

On 16/04/2016 09:27, Stefan Weiss wrote:

>
> 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))
>

Yeah, the spread operator makes life a lot easier: [1]:

let arr1 = ['a', 'b'];
let arr2 = ['c'];
let arr3 = ['d', 'e'];

console.log([...arr1, ...arr2, ...arr3]);
// [ 'a', 'b', 'c', 'd', 'e' ]

[1] <http://www.2ality.com/2015/08/getting-started-es...

Cheers,
Joao Rodrigues

Joao Rodrigues

4/16/2016 4:44:00 PM

On 16/04/2016 12:01, Joao Rodrigues wrote:
> On 16/04/2016 09:27, Stefan Weiss wrote:
>
>>
>> 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))
>>
>
> Yeah, the spread operator makes life a lot easier: [1]:
>
> let arr1 = ['a', 'b'];
> let arr2 = ['c'];
> let arr3 = ['d', 'e'];
>
> console.log([...arr1, ...arr2, ...arr3]);
> // [ 'a', 'b', 'c', 'd', 'e' ]
>
> [1] <http://www.2ality.com/2015/08/getting-started-es...
>

As an exercise, the OP's code might be rewritten in ES6 to:

Array.prototype.duplicator = function () {
return [...this, ...this];
};
[1,2,3,4,5].duplicator();
// [1, 2, 3, 4, 5, 1, 2, 3, 4, 5]

Tested with latest versions of MS Edge, Chrome and FF.

--
Joao Rodrigues

Aleksandro

4/16/2016 5:14:00 PM

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.

comp.lang.javascript

duplicator an array

jrough

Aleksandro

jrough

Christoph M. Becker

Thomas 'PointedEars' Lahn

Thomas 'PointedEars' Lahn

Stefan Weiss

Joao Rodrigues

Joao Rodrigues

Aleksandro

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