function* permutatins(s) {
if (s.length > 1)
for (let i = 0; i < s.length; i++)
for (let t of permutatins(s.slice(0, i) + s.slice(i + 1)))
yield s.charAt(i) + t;
else yield s;
}
for (let s of permutatins('941'))
console.log(s);