Angular 2 ngFor - reverse order of output using the index

Question

Trying to learn something about filtering and ordering in Angular 2. I can't seem to find any decent resources and I'm stuck at how to order an ngFor output in reverse order using the index. I have written the the following pipe put it keeps giving me errors that array slice in not a function.

@Pipe({
    name: 'reverse'
})
export class ReversePipe implements PipeTransform {
    transform(arr) {
        var copy = arr.slice();
        return copy.reverse();
    }
}

and my ngfor looks like this.

<div class="table-row generic" *ngFor="let advert of activeAdverts | reverse let i = index;" [attr.data-index]="i" (click)="viewAd(advert.title)">      
    <div class="table-cell white-text">{{ i+1 }}</div>                    
    <div class="table-cell white-text">{{advert.title}}</div>
    <div class="table-cell green-text">{{advert.advert}}</div>
</div>

I guess you need to add `if(!ar) return;` to not get errors when `null` is passed. — Günter Zöchbauer, Nov 15 '16 at 19:55
theres definitely items in the array. I get output when I remove the pipe from the ngfor.. the kind of errors I am seeing are slice is not a function or reverse is not a function. — Hamburgersn Heroin, Nov 15 '16 at 20:01
also more precisely this is the error inline template:13:47 caused by: arr.slice is not a function — Hamburgersn Heroin, Nov 15 '16 at 20:06

score 13 · Answer 1 · edited Aug 25 '17 at 19:36

13

I would add .slice() as well

*ngFor="let advert of activeAdverts.slice().reverse() let i = index;"

edited Aug 25 '17 at 19:36

msanford

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answered Aug 25 '17 at 19:06

Chris Skura

161
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1

Why to use slice() here? And if we will not do slice() what will happen? – Satish Patro Feb 11 '19 at 05:22
2

Reverse() works in place. It means, it will change the original array. Slice without parameters returns a copy of activeAdverts, so original array, in this case, stays unchanged. – Prokhor Sednev Apr 26 '19 at 12:39

score 11 · Answer 2 · answered Dec 24 '16 at 17:46

11

For such common use cases, I would suggest you to use reverse pipe from ng-pipes module: https://github.com/danrevah/ng-pipes#reverse-1

From the DOCS:

in the controller:

this.items = [1, 2, 3];

in the view:

<li *ngFor="let item of items | reverse"> <!-- Array: [3, 2, 1] -->

answered Dec 24 '16 at 17:46

D_R

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1

Wow! it is working, even with the firebase Observable lists! – trojan Aug 28 '17 at 14:40

score 6 · Answer 3 · answered May 24 '18 at 21:42

6

I think @BhaskerYadav gave the best answer. Changing the original array is a bad idea and his/her idea has no side-effects, performance or otherwise. IMHO it just needs a slight improvement as all that index dereferencing is unreadable at scale.

<tr *ngFor="let _ of activeAdverts; let i = index">
  <ng-container *ngIf="activeAdverts[activeAdverts.length-i-1] as advert">
    <td>{{advert.p}}</td>
    <td>{{advert.q}}</td>
    ...
  </ng-container>
</tr>

answered May 24 '18 at 21:42

Mark Florence

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8

This worked for me. But, didn't understand what you mean by "index dereferencing is unreadable at scale". Could you please help explain. Thanks. – User3250 Aug 07 '18 at 16:07
Sure thing. All the examples show just a couple of table cells. But that isn't realistic. What if there were 10, 20? If you had to write `activeAdverts[activeAdverts.length-i-1]` in each, your template would rapidly become unreadable. The `as` keyword of `*ngIf` is underused but great for creating a local variable -- and assigning it with the `*ngIf` expression. So in my example, you only need write the lengthy index dereference once. – Mark Florence Aug 08 '18 at 18:46
Ahh! Got you. Thanks a ton! :) – User3250 Aug 09 '18 at 01:38
@MarkFlorence what if then I dynamically push activeAdverts? – Chirag Jan 16 '19 at 06:43

score 4 · Answer 4 · answered Jul 08 '17 at 09:47

4

Using the following method u can reverse the output, but orignal array won't be modified,

 <tr *ngFor="let advert of activeAdverts; let i = index" >
           <td>{{activeAdverts[activeAdverts.length-i-1]}}</td>
  </tr>

answered Jul 08 '17 at 09:47

BhaskerYadav

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score 2 · Answer 5 · answered Aug 23 '17 at 05:10

The most simple way I was able to make it work is to just use activeAdverts.reverse(). In your case:

<div class="table-row generic" *ngFor="let advert of activeAdverts.reverse() let i = index;" [attr.data-index]="i" (click)="viewAd(advert.title)">      
    <div class="table-cell white-text">{{ i+1 }}</div>                    
    <div class="table-cell white-text">{{advert.title}}</div>
    <div class="table-cell green-text">{{advert.advert}}</div>
</div>

I found this answer to do the trick

score 1 · Answer 6 · edited Apr 01 '20 at 16:19

1

Use this

*ngFor="let item of items.slice().reverse()"

edited Apr 01 '20 at 16:19

matthias_h

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answered Apr 01 '20 at 14:51

Trilok Singh

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score 0 · Answer 7 · answered May 24 '19 at 13:23

0

Lodash reverse worked for me.

import { reverse} from "lodash";

this.cloudMessages = reverse(this.cloudMessages)

answered May 24 '19 at 13:23

Sampath

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Angular 2 ngFor - reverse order of output using the index

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