Question2: transformDigitsToLetters

question2.js

// # Given a string containing digits from 2-9 inclusive,
// # return all possible letter combinations that the number could represent.
// # Return the answer in any order.

// # A mapping of digits to letters (just like on the telephone buttons) is shown in file telephone.png.
// # Note that 1 does not map to any letters.

// # Example 1:

// # Input: digits = "23"
// # Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
// # Example 2:

// # Input: digits = ""
// # Output: []
// # Example 3:

// # Input: digits = "2"
// # Output: ["a","b","c"]

const multipleArrays = (array1, array2) => {
  const product = [];

  for (let i = 0; i < array1.length; i++) {
    for (let j = 0; j < array2.length; j++) {
      product.push(array1[i] + array2[j]);
    }
  }
  return product;
};

const transformDigitsToLetters = digits => {
  const digitToLetterDict = {
    2: 'abc',
    3: 'def',
    4: 'ghi',
    5: 'jkl',
    6: 'mno',
    7: 'pqrs',
    8: 'tuv',
    9: 'wxyz',
  };
  let result = [];

  if (!digits) {
    return [];
  }

  // assign result first letters from first digit
  result = digitToLetterDict[digits[0]].split('');

  // travel from second digit to end with slice(1)
  digits
    .split('')
    .slice(1)
    .forEach(digit => {
      result = multipleArrays(result, digitToLetterDict[digit].split(''));
    });

  return result;
};

const result = transformDigitsToLetters('293');
console.log(`transformDigitsToLetters ${result}`);