17 - Letter Combinations of a Phone Number

function letterCombinations(digits: string): string[] {
  if (digits.length === 0) return [];
  const keyboard:{[x:number]:string} = {"2": "abc", "3": "def", "4": "ghi",  "5": "jkl", "6": "mno", "7": "pqrs", "8": "tuv", "9": "wxyz" }
	let result:string[] = [];

	function recursive(currLetters:string, remainNums:string){
		if(remainNums.length===0){
			result.push(currLetters);
			return
		}
		let letters = keyboard[remainNums[0]];
		for(const singleLetter of letters){		
			recursive(currLetters+singleLetter,remainNums.slice(1))
		}
	}
	recursive("",digits)
  return result
};