[Angular 2] Understanding Pure & Impure pipe

First, how to use a build in pipe:

            <div class="pipe-example">
                <label>Uppercase Pipe: {{ message | uppercase }}</label>
            </div>
            
            <div class="pipe-example">
                <label>Lowercase Pipe: {{ message | lowercase }}</label>
            </div>            

            <div class="pipe-example">
                <label>Slice Pipe: {{ message | slice:0:5 }}</label>
            </div>                       

            <div class="pipe-example">
                <label>Date Pipe: {{ date | date:'yyyy-MMM-dd' }}</label>
            </div>

            <div class="pipe-example">
                <label>Number Formatting: {{ pi | number:'5.1-2' }}</label>
            </div>            

            <div class="pipe-example">
                <label>Percent Pipe: {{ percentage | percent:'2.1-2' }}</label>
            </div>                  

            <div class="pipe-example">
                <label>Currency Pipe: {{ amount | currency:'USD':true:'2.1-2' }}</label>
            </div>                         

Then how to create a custom pipe:

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({
    name: 'sort'
})
export class SortPipe implements PipeTransform {
    transform(value: any[], args): any[] {

        let sorted = [
            ...value
        ].sort();

        if(args.length > 0 && args === "DESC"){
            sorted = sorted.reverse();
        }

        return sorted;
    }
}

Notice here, we use:

let sorted = [
            ...value
        ].sort();

The reason doing this is because sort() is mutate opreation, once you apply sort(), it will mutate the original data, which is not good. So here we create a copy of data and apply sort() fucntion to enhance immutatbility. 

Pure and Impure:

See example at: https://angular.io/resources/live-examples/pipes/ts/plnkr.html

Pure pipe it means:  

It won't go thought Angular Change Detection if the primitive type reference doesn't change.

    if (this.mutate) {
    // Pure pipe won't update display because heroes array reference is unchanged
    // Impure pipe will display
    this.heroes.push(hero);
    } else {
      // Pipe updates display because heroes array is a new object
      this.heroes = this.heroes.concat(hero);
    }

So if 'this.mutate' is ture, then we mutate this.heros array, so primitive type (array) reference is changed. So this will trigger change detection. So that, everytime you add a hero into the this.hero, piped result will update.

If this.mutate is false, then reference won't change, therefore,  if you add new hero, even set 'can fly' tag to ture, it won't update the piped result.

Impure pipe: 

Angular executes an impure pipe during every component change detection cycle. An impure pipe will be called a lot, as often as every keystroke or mouse-move.

To set pure or impure you just need to add a pure flag:

@Pipe({
    name: 'sort',
    pure: true //false
})

Default is ture.

So when build a custom pipe, you need to decide whether it should be pure or impure.