eventually correct implementation of IO Monad in JS

IO.js

class IO extends Monad {
	constructor(fn) {
		super();
		this.__value = fn;
	}

	static of(fn) {
		const io = new IO(() => fn);
		Object.freeze(io);
		return io;
	}

	map(f) {
		return new IO(compose(this.__value, f));
	}

	mapP(res, rej) {
		return this.map(then(res, rej));
	}
	
	flatMap(f) {
		/*
		return new IO(() => {
			const r = this.map(f).performIO().performIO();
			return r;
		});
		*/
		
		// same result as above. So what would be better?
		return this.map(f).performIO();
	}

	join() {
		return this.__value;
	}

	performIO() {
		return this.__value();
	}
}