addcommas.pipe.ts

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

/**
 * Turns the array into a sentence.
 * 
 * @example 
 * {{ ['Stephen', 'Trevor' ] | AddCommas:'Customer' }}
 */
@Pipe({ name: 'AddCommas' })
export class AddCommasPipe implements PipeTransform {
  transform(arr: null | string[], noun: string) {
    const UNKNOWN = `${noun} Unknown`;
    if (!arr) {
      return UNKNOWN;
    }

    switch (arr.length) {
      case 0:
        return UNKNOWN;
      case 1:
        return arr[0];
      case 2:
        return arr.join(' and ');
      default:
        const last = arr[arr.length - 1];
        const remaining = arr.slice(0, -1);
        return `${remaining.join(', ')}, and ${last}`;
    }
  }
}