Five-Minute Promise Implementation

example.js

let a = new Prom((reject, resolve) => {
	setTimeout(() => resolve('Promise A'), 1000);
}).then(result => {
	console.log(result);
});

let b = new Prom((reject, resolve) => {
	resolve('Promise B');
}).then(result => {
	console.log(result);
});

prom.js

class Prom {
	constructor(fn) {
		this.resolved = false;
		this.rejected = false;

		this.result = null;

		this.thenHandlers = [];
		this.failHandlers = [];

		let resolve = result => {
			this.resolved = true;
			this.result = result;
			this.thenHandlers.forEach(fn => fn(result));
		};

		let reject = result => {
			this.rejected = true;
			this.result = result;
			this.failHandlers.forEach(fn => fn(result));
		};

		fn(reject, resolve);
	}
	then(fn) {
		if (this.resolved) {
			fn(this.result);
		} else {
			this.thenHandlers.push(fn);
		}
		return this;
	}
	fail(fn) {
		if (this.rejected) {
			fn(this.result);
		} else {
			this.failHandlers.push(fn);
		}
		return this;
	}
}