Appends ordinal (st, nd ,rd ,th) to number or turns number into ordinal. Typescript Angular2 Pipe

ordinal.pipe.ts

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

const ordinals: string[] = ['th','st','nd','rd'];

/*
 * Append ordinal to number (e.g. "1st" position)
 * Usage:
 *   value | ordinal:keepNumber
 * Example:
 *   {{ 23 |  ordinal}}
 *   formats to: '23rd'
 * Example:
 *   {{ 23 |  ordinal:false}}
 *   formats to: 'rd'
*/
@Pipe({name: 'ordinal'})
export class OrdinalPipe implements PipeTransform {

    transform(n: number, keepNumber: boolean = true) {
        let v = n % 100;
        return (keepNumber?n:'') + (ordinals[(v-20)%10]||ordinals[v]||ordinals[0]);
    }
}

So why minus 20? What difference does it make?

So why minus 20? What difference does it make?

1st, 2nd, 3rd, 11th, 12th, 13th, 21st, 22nd, 23rd

Great little pipe, thanks very much!