Promise.delay using Declaration Merging in TypeScript

Promise.delay.ts

/* On TypeScript Playground: https://goo.gl/EQVVUk */

// Interface merging: https://www.typescriptlang.org/docs/handbook/declaration-merging.html
// declare global {
    interface Promise<T> {
        delay(duration?: number): Promise<T>;
    }
// }
        
// Allow some sugar syntax
Promise.prototype.delay = function(duration: number) {
    return this.then(function(value) {
        return new Promise(function(resolve) {
            setTimeout(function() { resolve(value); }, duration);
        })
    }, function(reason) {
        return new Promise(function(resolve, reject) {
            setTimeout(function() { reject(reason); }, duration);
        })
    })
}

// Use this for throttling
const musicians = ['Clapton', 'U2', 'Muse', 'Coldplay'];

musicians.forEach((musician: string, index: number) => {
    Promise.resolve(true).delay(2000 * index)
        .then(() => {
            const span = document.createElement('span');
            span.innerHTML = new Date() + ': <em>' + musician + '</em><br>';
            document.body.appendChild(span);
        });
});