How to sort a list of objects using an Angular 2 Pipe

Question

I am trying to create a pipe that will sort an array of a custom type.

// custom-object.ts
export class CustomObject {
    constructor(
        public someString: string,
        public someNumber: number
    ) {}
}

In my html is:

// custom-object-form.component.ts
<div class="form-group">
    <label for="cricos_course_code">Object: </label>
    <select class="form-control" required [(ngModel)]="model.someString">
        <option *ngFor="#object of (customObjects | sortArrayOfCustomObjects)" [value]="object.someString">{{object.someString}}</option>
    </select>
</div>

Then the code for the pipe:

// sort-custom-object.pipe.ts
import {Pipe, PipeTransform} from 'angular2/core';
import {CustomObject} from '../custom-object';

@Pipe({name: 'sortArrayOfCustomObjects'})
export class SortArrayOfCustomObjectsPipe implements PipeTransform {
  transform(arr:Array<CustomObject>): any {
    return arr.sort((a, b) => {
      if (a.someString > b.someString) {
        return 1;
      }
      if (a.someString < b.someString) {
        return -1;
      }
      // a must be equal to b
      return 0;
    });
  }
}

Now I am getting an error in the console:

EXCEPTION: TypeError: Cannot read property 'sort' of undefined in [(customObjects | sortArrayOfCustomObjects) in FormComponent@7:24]

Answer 1

Based off of your limited information I believe you want the following.

@Pipe({name: 'sortArrayOfCustomObjects'})
export class SortArrayOfCustomObjectsPipe implements PipeTransform {

  transform(arr: CustomObject[], args: any): CustomObject[]{
    return arr.sort((a, b) => {
      if (a.p1 > b.p1 || a.p2 < b.p2) {
        return 1;
      }
      if (a.p1 < b.p1 || a.p2 > b.p2) {
        return -1;
      }
      return 0;
    });
  }
}

This will sort the objects first by p1, and then by p2. For instance.

{p1: 'AA', p2: 3},
{p1: 'CC', p2: 2},
{p1: 'BB', p2: 5},
{p1: 'BB', p2: 1},
{p1: 'AA', p2: 2},
{p1: 'DD', p2: 4}

Would be sorted to

{p1: 'AA', p2: 3},
{p1: 'AA', p2: 2},
{p1: 'BB', p2: 5},
{p1: 'BB', p2: 1},
{p1: 'CC', p2: 2},
{p1: 'DD', p2: 4}

This is based on your comment:

I have an array of objects that contain properties p1: string, p2: number etc. In my form I want to have 2 select fields that show a list of p1 in alphabetical order and p2 in descending order.

Usage would be

*ngFor="#item of (items | sortArrayOfCustomObjects)"

UPDATE

For your error that you are now receiving be sure to guard against undefined values in arrays by using a guard.

transform(arr: CustomObject[], args: any): CustomObject[]{
  if(!arr){ return; }
  ...
}