Extended version of built-in v8 promises

promiseExtensions.js

// TODO: Implement `forEach` and `reduce`

class P extends Promise {
	constructor(fn) {
		super(fn);
	}

	tap(fn) {
		return this.then(res => {
			return Promise
				.resolve(fn(res))
				.then(() => res);
		});
	}

	map(fn) {
		return this.then(res => Promise.all(res.map(fn)));
	}

	filter(fn) {
		return this.then(res => {
			return Promise
				.all(res.map(fn))
				.then(filters => res.filter((_, i) => filters[i]));
		});
	}
}

P.promisify = function promisify(fn, ctx) {
	return function promisifedMethod() {
		return new P((resolve, reject) => {
			const args = new Array(arguments.length + 1);

			args[arguments.length] = function promisifedMethodCallback(err) {
				if (err) {
					return reject(err);
				}

				if (arguments.length < 3) {
					return resolve(arguments[1]);
				}

				let res = new Array(arguments.length - 1);
				for (let i = 0; i < res.length; i++) {
					res[i] = arguments[i + 1];
				}

				return resolve(res);
			};

			for (let i = 0; i < arguments.length; i++) {
				args[i] = arguments[i];
			}

			fn.apply(ctx, args);
		});
	};
};

Built quick and untested for personal use. When I use node.js scripts as a replacement for bash scripts I prefer to not rely any dependencies. But, it's handy to have a few of the common Bluebird helpers.