const numericCharAlphabetTuples = [
  ['0','a'],
  ['1','b'],
  ['2','c'],
  ['3','d'],
  ['4','e'],
  ['5','f'],
  ['6','g'],
  ['7','h'],
  ['8','i'],
  ['9','j'],
];

function replaceNumbersInStringToAlphabet(string) {
  numericCharAlphabetTuples.forEach(tuple => {
    const numericChar = tuple[0];
    const alphabet = tuple[1];
    string = string.replace(numericChar, alphabet);
  });

  return string;
}