### Yes, You can now use [`ImmutableArray`][2] with .NET Core 1.0+

```cs
using System.Collections.Immutable;

var arr1 = new [] {1,2,3}.ToImmutableArray();
var arr2 = ImmutableArray.Create(new [] {1,2,3});
```

[Demo in DotNetFiddle][1]

**See Also**: [**Please welcome ImmutableArray**][3] by Immo Landwerth

[2]: https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable.immutablearray
[1]: https://dotnetfiddle.net/FRwOey
[3]: https://devblogs.microsoft.com/dotnet/please-welcome-immutablearrayt/

Is it possible to somehow mark a `System.Array` as immutable.  When put behind a public-get/private-set they can't be added to, since it requires re-allocation and re-assignment, but a consumer can still set any subscript they wish:

    public class Immy
    {
        public string[] { get; private set; }
    }

I thought the `readonly` keyword might do the trick, but no such luck.

Are immutable arrays possible in .NET?

If you really like the ergonomics of Array.Join, you can add an [Extension Method][1] like this:

```cs
public static class MyExtensions {
	public static string Join(this IEnumerable<string> arr, string seperator) =>
		string.Join(seperator, arr);
}
```

And then use like this:

```cs
var list = new string[] {"A", "B", "C"};
var output = list.Join(",");
// "A,B,C"
```

### [Demo in .NET Fiddle][2]

[1]: https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods
[2]: https://dotnetfiddle.net/4HMObc

Ok, this is a dumb thing that I'm sure I've done dozens of times but for some reason I can't find it.

I have an array... And want to get a string with the contents of that array separated by a delimited...

Where is the .Join() method that I can't find?

(This is .Net 2.0, I don't have any LINQ stuff)

Thank you!

Array.Join in .Net?

`-Filter` is more performant than `-Include`, so if you don't have a lot of different extensions, simply concatenating two filtered lists might be faster.



    $files  = Get-ChildItem -Path "H:\stash\" -Filter *.rdlc -Recurse 
    $files += Get-ChildItem -Path "H:\stash\" -Filter *.rdl  -Recurse 

I compared the output with a timer like this:



    $stopwatch = [System.Diagnostics.Stopwatch]::StartNew()
    # Do Stuff Here
    $stopwatch.Stop()
    Write-Host "$([Math]::Round($stopwatch.Elapsed.TotalSeconds)) seconds ellapsed"

I'm trying to write a PowerShell script to build a list of files, from several directories. After all directories have been added to the main list, I'd like to do the same processing on all files.

This is what I have:

    $items = New-Object Collections.Generic.List[IO.FileInfo]

    $loc1 = @(Get-ChildItem -Path "\\server\C$\Program Files (x86)\Data1\" -Recurse)
    $loc2 = @(Get-ChildItem -Path "\\server\C$\Web\DataStorage\" -Recurse)

    $items.Add($loc1) # This line fails (the next also fails)
    $items.Add($loc2)

    # Processing code is here

which fails with this error:

> Cannot convert argument "0", with
> value: "System.Object[]", for "Add" to
> type "System.IO.FileInfo": "Cannot
> convert the "System.Object[]" va lue
> of type "System.Object[]" to type
> "System.IO.FileInfo"."

I am mostly interested in what is the correct approach for this type of situation. I realize that my code is a very [C][1] way of doing it -- if there is a more PowerShell way to acomplish the same task, I'm all for it. The key, is that the number of `$loc#'s` may change over time, so adding and removing one or two should be easy in the resulting code.

  [1]: http://en.wikipedia.org/wiki/C_%28programming_language%29


Combine the results of two distinct Get-ChildItem calls into single variable to do the same processing on them

While [Krzysztof's suggestion][Krzysztof] was helpful, I couldn't quite get the binding to work with `ng-list` because the `ng-list` will always convert the array of objects into an  array of strings, thus overriding any objects I had on scope.

In this case, the simplest thing to do was to upgrade from ng-list to something with full object support like most tagging libraries:

* [ngTagsInput][ngTagsInput]
* [angular-tags][angular-tags]
* [bootstrap-tagsinput][bootstrap-tagsinput]

I Implemented ngTagsInput like this:

**HTML**:



    <tags-input ng-model="arrayObjects" display-property="text"></tags-input>

**JavaScript**:



    $scope.arrayObjects = [
      { "text": "one"   , "selected": false },
      { "text": "two"   , "selected": false },
      { "text": "three" , "selected": false }
    ];

This way each object is given it's own element where the text property is displayed and they are all formatted together to live inside of an input.  Changing one object just updates that single element

### [Demo in Pluner][plunk]



[Krzysztof]: https://stackoverflow.com/a/24966775/1366033

[angular-tags]: http://decipherinc.github.io/angular-tags/

[ngTagsInput]: http://mbenford.github.io/ngTagsInput/
[bootstrap-tagsinput]: http://timschlechter.github.io/bootstrap-tagsinput/examples/#angular

[plunk]: http://plnkr.co/edit/ruzBkQWeYaYtRhxDRyqg?p=preview

How can I convert between a delimited string and an array of **objects** with data binding? 

[`ng-list`][ng-list] will work with an array of strings.  But I have an array of objects, in which I would like to delimit the text property so the text in the array can be editable.

**Works with Array of Strings**:



    $scope.arrayStrings = ["one", "two", "three"];



    <textarea ng-model="arrayStrings" ng-list="&#10;" ></textarea>

**How can I get this to work with objects**:



    $scope.arrayObjects = [{
      text: "one",
      selected: true
    }, {
      text: "two",
      selected: true
    }, {
      text: "three",
      selected: true
    }];



    <textarea ng-model="arrayObjects" ng-list="&#10; | text" ></textarea>

### [Here's a demo in plunker][plunk 1]


---

One possible idea I had was to use ng-list on the array of strings and then map that to the array of objects.  The use a `$watch` to update the object array anytime the string array changed.  However, this still falls short because it will overwrite any other properties on the object every time the array updates. (Demo in previous plunker revision)



    $scope.$watch('arrayStrings', function() {
      $scope.arrayObjects = $scope.arrayStrings.map(function(s){
        return {
          text: s,
          selected: true
        }
      });
    })

---

### Update:

There still seem to be issues when using `ng-list` even when using [Krzysztof's suggestion][Krzysztof]:

    toString: function() { return s }

Overriding the `toString` method helps the set of objects when initially displayed, but as soon as you type anything, `ng-list` converts the set of objects back into an array of strings because at that point `toString` has not come into play. 

To clarify what I'm trying to do.  I really want a list of objects, with editable titles, that can also be selected.  I'd like to preserve selections even when the title has changed or items have been added.

### [Here's another plunker to demonstrate][plunk 2]

![screenshot][screenshot]

[ng-list]: https://docs.angularjs.org/api/ng/directive/ngList
[plunk 1]: http://plnkr.co/edit/ZudJcIak5sAWuiq3b1Hu?p=preview
[plunk 2]: http://plnkr.co/edit/0Q2jVYIU4dEc1GZ4EaPY?p=preview

[screenshot]: http://i.imgur.com/cWRGzzC.gif
[Krzysztof]: https://stackoverflow.com/a/24966775/1366033

Convert delimited string and an array of objects

Put simply, you can't make something into a function if it's not already a function, and you can't _really_ extend arrays.

What you can do is create a wrapper function that wraps an array and provides the functionality you want, and also include the ability to get back the original array if you need it:






<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>




    var wrapper = (function() {
      function getColumnNumber(str) {
        return Array.prototype.reduce.call(str.toUpperCase(), function (t, c) {
            return 26 * t + c.charCodeAt(0) - 64;
        }, 0) - 1;
      }

      return function(arr) {
        return function(col, val) {
          if (arguments.length === 0) {
            return arr;
          }
          if (arguments.length > 1) {
            arr[getColumnNumber(col)] = val;
          }
          return arr[getColumnNumber(col)];
        };
      };
    })();

    var w = wrapper([10, 20, 30, 40, 50]);

    snippet.log(w('D')); // 40

    w('D', 33);          // set value

    snippet.log(w('D')); // 33

    w()[3] = 42;         // access underlying array directly
    w().push(60);

    snippet.log(w('D')); // 42
    snippet.log(w('F')); // 60

In google app scripts I have a one dimensional data array that I can get values from like this:

    data[0]

I'd love to be able to pass in the column name instead like this:

    data("A")

That way I don't have to convert letters into their array position.  So I'd like to extend the array object (extending isn't really risky since it's running in an isolated script environment).

I know I can add a function to the array prototype using this [letter to number function][1] and this [object extension question][2] like this:





    Array.prototype.byCol = function(colName) {
      return this[getColumnNumber(colName) - 1];
    }

    function getColumnNumber(str) {
      var out = 0, len = str.length;
      for (pos = 0; pos < len; pos++) {
        out += (str.charCodeAt(pos) - 64) * Math.pow(26, len - pos - 1);
      }
      return out;
    }

    var data = [1,2,3,4];

    document.write(data.byCol("B"));




But this is a slightly bulkier calling syntax than I wanted.  

Based on this question on [default functions][3], it looks like it's possible to assign a default function to an object, but they're doing so by just creating a function object like this:



    var test = new func(function() {
        // do something
    });

Can I get extend the array so that it will execute a default function when called as a method?

[1]: https://stackoverflow.com/a/29040784/1366033
[2]: https://stackoverflow.com/q/11337849/1366033
[3]: https://stackoverflow.com/q/10535447/1366033

Extend a JavaScript object with a default function

Alternatively, you can create a table from JSON using the the [Build Table Widget][3] 

Make sure you include the build table widget:



<pre><code>&lt;script src="<a href="https://cdnjs.cloudflare.com/ajax/libs/jquery.tablesorter/2.31.0/js/widgets/widget-build-table.min.js">https://cdnjs.cloudflare.com/ajax/libs/jquery.tablesorter/2.31.0/js/widgets/widget-build-table.min.js</a>";&gt;&lt;/script&gt;
</code></pre>


If you have an array of values, you can add the header to the first row like this:



<pre><code>var tableData = [
  <b>// header
  [ "Move", "Start Up"],</b>
  // rows
  [ "Phoenix Smasher", 15],
  [ "Bone Breaker",    16],
  [ "DeathFist",       60],
  [ "Thruster",        20],
  [ "S Jab",           10]
]
</code></pre>

Then pass the array and some params to the build source options in to the `widgetOptions`  like this:


    
    $("#jsonTable").tablesorter({
      theme: 'materialize',
      widgets: ['zebra'],
      widgetOptions: {
        build_type: 'array', 		  // can sometimes be detected if undefined
        build_source: tableData,
        build_headers: {
          rows    : 1,  			  // Number of header rows from the array
          widths  : [ '70%', '20%' ]  // set header cell widths
        },
        build_footers : {
          rows    : 0,  		      // Number of header rows from the array
        },
      }
    });

### Demo in [jsFiddle][5] & StackSnippets:





    var tableData = [
      // header
      [ "Move" , "Start Up"],
      // rows
      [ "Phoenix Smasher", 15],
      [ "Bone Breaker", 16],
      [ "DeathFist", 60],
      [ "Thruster", 20],
      [ "S Jab", 10]
    ]

    $("#jsonTable").tablesorter({
      theme: 'materialize',
      widgets: ['zebra'],
      widgetOptions: {
        build_type: 'array', 		 		// can sometimes be detected if undefined
        build_source: tableData,
        build_headers: {
          rows    : 1,  						 // Number of header rows from the array
          widths  : [ '70%', '20%' ] // set header cell widths
        },
        build_footers : {
          rows    : 0,  						 // Number of header rows from the array
        },
      }
    });



    <link href="https://cdnjs.cloudflare.com/ajax/libs/jquery.tablesorter/2.31.0/css/theme.default.min.css" rel="stylesheet"/>
    <link href="https://cdnjs.cloudflare.com/ajax/libs/jquery.tablesorter/2.31.0/css/theme.materialize.min.css" rel="stylesheet"/>

    <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.2.4/jquery.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery.tablesorter/2.31.0/js/jquery.tablesorter.combined.min.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery.tablesorter/2.31.0/js/widgets/widget-build-table.min.js"></script>


    <div id="jsonTable"></div>



**Further Reading**:

* [Issue #31 - Tablesorter + json data support][1]
* [Issue #500 - Initializing tablesorter from a JavaScript array or JSON data][2]
* [SO - Creating HTML table from JSON with JQuery][4]

[2]: https://github.com/Mottie/tablesorter/issues/500
[1]: https://github.com/Mottie/tablesorter/issues/31
[3]: https://mottie.github.io/tablesorter/docs/example-widget-build-table.html
[4]: https://stackoverflow.com/q/34888340/1366033
[5]: http://jsfiddle.net/KyleMit/f18s5mwp/

I am trying to use the jQuery plugin tablesorter (tablesorter.com) on an html table that is generated by a javascript array when my page loads. The table gets styled, by the table won't sort when the column headers are clicked.

Here is what I have so far:

    <!DOCTYPE html>
    <html>
    <head>
	  <script type="text/javascript" src="/lib/jquery-ui/jquery-ui-1.11.4/external/jquery/jquery.js"></script>
	  <script type="text/javascript" src="/lib/jquery-ui/jquery-ui-1.11.4/jquery-ui.js"></script>
	  <link type="text/css" rel="stylesheet" href="/lib/jquery-ui/jquery-ui-1.11.4/jquery-ui.css">
	
	  <script type="text/javascript" src="/lib/jquery/tablesorter/jquery.tablesorter.js"></script>
	  <link type="text/css" rel="stylesheet" href="/lib/jquery/tablesorter/themes/blue/style.css">
	  <style>
	  </style>
	  <script>
		$( document ).ready( function() {
			$( "table" ).tablesorter();
		
			$( "p" ).click( function() {
				$( this ).hide();
			});
		});
		
		$( function() {
			$( "#datepicker" ).datepicker();
		});
		
	  </script>
	  <script>
	    var frameData = [
		  ["Phoenix Smasher", 15],
		  ["Bone Breaker", 16],
		  ["DeathFist", 60],
		  ["Thruster", 20],
		  ["S Jab", 10]
	    ];

	    function pageLoad() {
		  var t = "";
		  t += "<thead>";
		  t += "<tr>";
		  t += "<th>Move</th>";
		  t += "<th>Start Up</th>";
		  t += "</tr>";
		  t += "</thead>";
		  t += "<tbody>";
		  for (var i = 0; i < frameData.length; i++) {
			t += "<tr>";
			t += "<td>" + frameData[i][0] + "</td><td>" + frameData[i][1] + "</td>";
			t += "</tr>";
		  }
		  t += "</tbody>";
		  document.getElementById("frameTable").innerHTML = t;
	    }
	  </script>
    </head>
    <body onload="pageLoad()">
	  <p>Click the table headers to sort the array in descending order.</p>
	  <br />
	  <br />
	  <div id="demo"></div>
	  <table id="frameTable" class="tablesorter">
	  </table>

	  <p>jQuery test. This text will disappear on click.</p>
	  <input type="text" id="datepicker">
    </body>
    </html>

<b>What I've tried:</b> Oddly enough, when I get rid of the javascript array and place actual html table data in between the `<table>` and `</table>` tags, the tablesorter plugin works fine. Also, I've tried re-arranging the array and `pageLoad()` function with the jQuery code, with no luck at all.

Any idea how to get this to work?

The page is on my server: http://sketchcarellz.com/multiArray.html

Using jQuery tablesorter with javascript table array

You could also do it by using [`Object.fromEntries()`][1] like this:

```js
function flattenArrayToObject(arr) {
  let entries = arr.map(el => [el.id, el.name])
  let obj = Object.fromEntries(entries)
  return obj;
}
```

Or in a one liner if you really needed to:

```js
let flattenArray = arr => Object.fromEntries(arr.map(el => [el.id, el.name]))
```

### Demo in Stack Snippets





    let friends = [
        {id: "158d5", name: "Gloria"},
        {id: "3e426", name: "Shawn"}
    ]

    function flattenArrayToObject(arr) {
      let entries = arr.map(el => [el.id, el.name]) // [["158d5","Gloria"], ["3e426","Shawn"]]
      let obj = Object.fromEntries(entries)
      return obj;
    }

    console.log(flattenArrayToObject(friends))




[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/fromEntries

I Have a JSON input format, here's an exemple :


    {
      "friends": [
        {
          "id": "5a8d4euyiuyiuyhiuyc022c7158d5",
          "name": "Gloria Coffey"
        },
        {
          "id": "5a8d4e2rytuyiyuyiytiy3e426",
          "name": "Shawn Ellison"
        }
      ]
    }

I would transform it to an array key: value arrays, something like this :

    [[id : "5a8d4euyiuyiuyhiuyc022c7158d5", name:"Gloria Coffey"],[id : "5a8d4e2rytuyiyuyiytiy3e426", name:"Shawn Ellison"]]

What I have done : 

    search(event) {
	    this.searchRmpmService.getResults(event.query).then(data => {
        this.results = data.friends;
        console.log(this.results);
        let output = [];
        output= Object.entries(this.results);
        console.log(output);
    });

the first console.log of `this.results` prints me an array of objects

then my `output` array prints: 

    0:Array(2)
    0:"0" <-- ??
    1:{
       id:"5a8d4e2ffead0c022c7158d5",
       name:"Gloria Coffey"
    }length:2__proto__:Array(0)

what I would is 

    id : 5a8d4e2ffead0c022c7158d5
    name : Gloria Coffey

Converting Array of objects to arrays of key, value array

The array can store the actual functions themselves, instead of the function names. 



    function italy()  { console.log('italy') }
    function czech()  { console.log('czech') }
    function russia() { console.log('russia') }

    var arrAnimations = [ italy, czech, russia ]

Then locate the item in the array, and call it:



    var activeScene = 0;
    arrAnimations[activeScene]()


### Demo in Stack Snippets





    function italy()  { console.log('italy') }
    function czech()  { console.log('czech') }
    function russia() { console.log('russia') }

    var arrAnimations = [ italy, czech, russia ]

    var index = 0;

    function callNextFunction() {
        index = index >= arrAnimations.length - 1 ? 0 : index + 1
        
        arrAnimations[index]()
    }

    var btn = document.getElementById('animate')
    btn.addEventListener("click", callNextFunction)



    <button id="animate">Animate</button>

I’m trying to create a simple loop of animation functions. I've stored their names in an array and am able to log out each object as a string with a click event. However I can't seem to call the corresponding functions of that event

I've tried to do this but I get an error nt[rt] is not a function

    arrAnimations[activeScene]()

I've tried many approaches from stack overflow from similar questions, such as creating a helper function like this

    myFunction = function(){};   
    var arrAnimations = {italy: myFunction};

    arrAnimations['activeScene']();//executes the function

and this

    var tmp = arrAnimations[activeScene]
    window[tmp]

Here is the code:





    var arrAnimations = [
      'italy',
      'czech',
      'russia'
    ]

    var activeScene = 0;

    document.getElementById('animate').addEventListener("click",
      function incNumber() {
          if (activeScene < arrAnimations.length - 1) {
              activeScene++;
          } else if (activeScene = arrAnimations.length - 1) {
              activeScene = 0;
          }
          // console.log(arrAnimations[activeScene])
      }
    )

    function italy() { console.log('italy') }

    function czech() { console.log('czech') }

    function russia() { console.log('russia') }




    <div id="animate">Animate</div>

Javascript pass variable to function name

Maybe just a simple `for... i` loop that checks each array for an item in position `i`





    var input = [["one","two","three"],["uno","dos"],["1","2","3","4"],["1st","2nd","3rd"]]

    var output = []
    var maxLen = Math.max(...input.map(arr => arr.length));

    for (i=0; i < maxLen; i++) {
      input.forEach(arr => { if (arr[i] !== undefined) output.push(arr[i]) })
    }

    console.log(output)



Simple, but predictable and readable

-----

### Avoiding For Each Loop

If you need to avoid `forEach`, here's a similar approach where you could: get the [max child array length][0], [build a range of integers][1] that would've been created by the for loop (`[1,2,3,4]`), map each value to pivot the arrays, [flatten the multi-dimensional array][2], and then [filter][3] out the empty cells.

First in discrete steps, and then as a one liner:



    var input = [["one","two","three"],["uno","dos"],["1","2","3","4"],["1st","2nd","3rd"]];

Multiple Steps:



    var maxLen = Math.max(...input.map(arr => arr.length));
    var indexes = Array(maxLen).fill().map((_,i) => i);
    var pivoted = indexes.map(i => input.map(arr => arr[i] ));
    var flattened = pivoted.flat().filter(el => el !== undefined);


One Liner:



    var output = Array(Math.max(...input.map(arr => arr.length))).fill().map((_,i) => i)
                   .map(i => input.map(arr => arr[i] ))
                   .flat().filter(el => el !== undefined)



[0]: https://stackoverflow.com/q/4020796/1366033
[1]: https://stackoverflow.com/a/33457557/1366033
[2]: https://stackoverflow.com/q/10865025/1366033
[3]: https://stackoverflow.com/a/42194793/1366033

I'm trying to use `reduce()` combine a set of arrays in a "collated" order so items with similar indexes are together. For example:

    input = [["one","two","three"],["uno","dos"],["1","2","3","4"],["first","second","third"]]
    
    output = [ 'first','1','uno','one','second','2','dos','two','third','3','three','4' ]

It doesn't matter what order the items with similar index go as long as they are together, so a result of `'one','uno','1'...` is a good as what's above. I would like to do it just using immutable variables if possible.

I have a way that works: 

		const output = input.reduce((accumulator, currentArray, arrayIndex)=>{
            currentArray.forEach((item,itemIndex)=>{
                const newIndex = itemIndex*(arrayIndex+1);
                accumulator.splice(newIndex<accumulator.length?newIndex:accumulator.length,0,item);
            })
            return accumulator;
        })

But it's not very pretty and I don't like it, especially because of the way it mutates the accumulator in the forEach method. I feel there must be a more elegant method.

I can't believe no one has asked this before but I've tried a bunch of different queries and can't find it, so kindly tell me if it's there and I missed it. Is there a better way? 

To clarify per question in comments, I would like to be able to do this without mutating any variables or arrays as I'm doing with the `accumulator.splice` and to only use functional methods such as `.map`, or `.reduce` not a mutating loop like a `.forEach`.




reduce array of arrays into on array in collated order

### Update 2021

You can use the [`Array.at()`][1] method, which was moved to [Stage 4  in Aug, 2021][9]

```js
['a','b','c'].at(-1) // 'c'
```

### Further Reading

* **Docs**
  * [MDN - Array.prototype.at()][1]
  * [Proposal - Relative Indexing Method][5]
  * [tc39 spec - Array.at][6]
  * [CanIUse - JavaScript built-in: Array: at][7]

* **Stack Overflow**
  * [Get the last item in an array][2]
  * [Selecting last element in JavaScript array][3]
  * [Destructuring to get the last element of an array in es6][4]


[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at
[2]: https://stackoverflow.com/q/3216013/1366033
[3]: https://stackoverflow.com/q/9050345/1366033
[4]: https://stackoverflow.com/q/33064377/1366033
[5]: https://github.com/tc39/proposal-relative-indexing-method
[6]: https://tc39.es/ecma262/#sec-array.prototype.at
[7]: https://caniuse.com/mdn-javascript_builtins_array_at
[8]: https://tc39.es/proposal-relative-indexing-method/#sec-array-prototype-additions
[9]: https://github.com/tc39/proposals/blob/main/finished-proposals.md

I have a list of array items like this:

```js
const items = [
  { a: 1 },
  { b: 2 },
  { c: 3 },
]
```

How can I return / log the last element: `{ c: 3 }`


Here's what I've tried so far:

```js
let newarray = items.map((item) => {
    console.log(item);
})

console.log(newarray);
```




How the get the last element in an array items using JavaScript

Refactored [Nina Scholz's answer][1] with longer variable names, comments, and slightly different control of flow:





    const combineAdjacentElements = array => array.reduce((acc, val, i) => {
        // if cur is different from prev value, add raw number
        if (val !== array[i - 1]) {
          acc.push(val)
          return acc
        }

        // if first repetition, replace val with grouped array
        if (typeof acc.at(-1) === 'number') {
            acc[acc.length - 1] = [val, 2];
            return acc
        }

        // if multiple repetition, increment counter in grouped array
        acc.at(-1)[1]++;
        return acc;
    }, []);


    const output = combineAdjacentElements([3, 2, 2, 2, 5, 1, 1, 7, 1])

    console.log(JSON.stringify(output, null, 0))
    // [3,[2,3],5,[1,2],7,1]




### Further Reading

* [Check whether variable is number][2]
* [How the get the last element in an array][3]
* [How to replace the last index value in array javascript][4]

[1]: https://stackoverflow.com/a/61756370/1366033
[2]: https://stackoverflow.com/q/1303646/1366033
[3]: https://stackoverflow.com/a/69563549/1366033
[4]: https://stackoverflow.com/q/48744911/1366033

I'd like to combine identical elements in an array, into a single term with how many times the value appears



```js
function combineArrayElements(arr) {
  return arr.map((e, i, ar) => {
    if (e === ar[i + 1] || (e[0] && e[0] === ar[i + 1])) {
      return [e, e[1] + 1]
    }
    return e;
  })
}
```

Some example input and output:

```js
// input  [3, 2, 2, 5, 1, 1, 7, 1]
// output [3,[2,2],5,[1,2],7,1]

// input  [1, 1, 1, 2, 1]
// output [[1,3], 2, 1]
```


How to reduce JS array length if next elements equal previous

As logically concluded by the linq example

> `None` is the same as `!Any`, so you could define your own extension method as follows:

```js
let none = (arr, callback) => !arr.some(callback)
```

And then call like this:

```js
let arr = ["a","b","c"]
let noBs = none(arr, el => el === "b")
```

---

*Or* if you want to [*extend `Array.proto`*][1], you can add the following method:


```js
Object.defineProperty(Array.prototype, 'none', {
    value: function (callback) { return !this.some(callback) }
});
```

Then call like this:

```js
let arr = ["a","b","c"]
let noBs = arr.none(el => el === "b")
```

[1]: https://stackoverflow.com/a/35518127/1366033

There's a similar question for LINQ: [Is there an equivalent of `None()` in LINQ?][5]

There are some boolean methods on collections / arrays:

* [`Array.some`][1] (similar to [`linq.Any`][3])
* [`Array.every`][2] (similar to [`linq.All`][4])

Can I check if no elements in array match a given function callback

A possible workaround is to `.filter` and then check `.length` and make sure it's zero:

```js
let arr = ["a","b","c"]
// make sure that no item in array = "b"
let noBs = arr.filter(el => el === "b").length === 0
```


[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some
[2]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every
[3]: https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.any?view=netcore-3.1
[4]: https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.all?view=netcore-3.1
[5]: https://stackoverflow.com/q/19338122/1366033