Tips and suggestions always appreciated

```
const letterCombinations = (digits, current = "", res = []) => {
const numbers = {
2: "abc",
3: "def",
4: "ghi",
5: "jkl",
6: "mno",
7: "pqrs",
8: "tuv",
9: "wxyz"
}
if (digits.length === 0 && current === "") {
return res;
}
if (digits.length < 1) {
return(res.push(current));
}
let currentNumber,
currentLetter;
currentNumber = numbers[digits[0]];
for (let i = 0; i < currentNumber.length; i++) {
currentLetter = numbers[digits[0]][i];
letterCombinations(digits.slice(1), current + currentLetter, res);
}
return res;
}
```