null.js

"use strict";

function ng(gf) {
	let gen = gf();
	let it = gen.next();
	while (!it.done) {
		if (it.value === null || it.value === undefined) {
			return it.value;
		}
		it = gen.next(it.value);
	}
	return it.value;
}

function div(x, y) {
	if (y === 0) {
		return undefined;
	}
	return x / y;
}

let res1 = ng(function * () {
	let x = 1, y = 2;
	let z = (yield div(x, y)) * 2;
	return z;
});

console.log(res1); // 1

let res2 = ng(function * () {
	let x = 1, y = 0;
	let z = (yield div(x, y)) * 2;
	return z;
});

console.log(res2); // undefined