const powerset = arr =>
  arr.reduce((a, v) => a.concat(a.map(r => r.concat(v))), [[]]);
  
powerset([1, 2]); // [[], [1], [2], [1, 2]]

const getTimezone = () => Intl.DateTimeFormat().resolvedOptions().timeZone;

// Example
getTimezone();

if (process.env.NODE_ENV === "production") {
	app.use(express.static("client/build"));

	app.get("*", (req, res) => {
	res.sendFile(path.resolve(__dirname, "client", "build", "index.html"));
	});
}

arr = [] // empty array
const isEmpty = arr => !Array.isArray(arr) || arr.length === 0;

// Examples
isEmpty([]); // true
isEmpty([1, 2, 3]); // false

const primes = num => {
  let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
    sqroot = Math.floor(Math.sqrt(num)),
    numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
  numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
  return arr;
};

primes(10); // [2, 3, 5, 7]

const head = arr => (arr && arr.length ? arr[0] : undefined);

head([1, 2, 3]); // 1
head([]); // undefined
head(null); // undefined
head(undefined); // undefined