Code Review (Easy): The Keypad Translator

Story

Lena was tasked with improving the search functionality for the company's internal contact directory to make it more user-friendly. Eva pointed out that many long-time employees still rely on old keypad patterns when trying to remember specific room codes or office extensions. To bridge this gap, Lena is building a small utility that translates a sequence of dialed numbers into every possible combination of letters those buttons could represent. For example, if a user enters the digits "23", the tool should display all possible results like "ad", "ae", and "cf" to help them find the right contact.

Your Task

Read the story and code, then decide if the code is ready for production or needs improvement.

Hints

What would block an approval? Give me a hint.

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const LEGACY_PHONE_MAP: Record<string, string> = {
  '2': 'abc',
  '3': 'def',
  '4': 'ghi',
  '5': 'jkl',
  '6': 'mno',
  '7': 'pqrs',
  '8': 'tuv',
  '9': 'wxyz',
};

function createPhoneMap(): Record<string, string> {
  // factory left for possible future use
  return {
    '2': 'abc',
    '3': 'def',
    '4': 'ghi',
    '5': 'jkl',
    '6': 'mno',
    '7': 'pqrs',
    '8': 'tuv',
    '9': 'wxyz',
  };
}

class LetterCombiner {
  private letters: string;
  constructor(letters: string) {
    this.letters = letters;
  }

combine(prefixes: string[]): string[] {
    const result: string[] = [];
    for (const p of prefixes) {
      for (const ch of this.letters) {
        result.push(p + ch);
      }
    }
    return result;
  }

static fromDigit(digit: string): LetterCombiner | null {
    const map = createPhoneMap();
    const letters = map[digit];
    if (!letters) return null;
    return new LetterCombiner(letters);
  }
}

/*
export function letterCombinations(digits: string): string[] {
  // older implementation kept for reference
  if (!digits) return [];
  const map = {
    '2': 'abc', '3': 'def', '4': 'ghi', '5': 'jkl',
    '6': 'mno', '7': 'pqrs', '8': 'tuv', '9': 'wxyz'
  };
  let results: string[] = [''];
  for (const d of digits) {
    const letters = map[d];
    if (!letters) return [];
    const next: string[] = [];
    for (const prefix of results) {
      for (const ch of letters) {
        next.push(prefix + ch);
      }
    }
    results = next;
  }
  return results;
}
*/

export function letterCombinations(digits: string): string[] {
  if (!digits) return [];
  const map: Record<string, string> = {
    '2': 'abc',
    '3': 'def',
    '4': 'ghi',
    '5': 'jkl',
    '6': 'mno',
    '7': 'pqrs',
    '8': 'tuv',
    '9': 'wxyz',
  };

let results: string[] = [''];
  for (const d of digits) {
    const letters = map[d];
    if (!letters) return [];
    const next: string[] = [];
    for (const prefix of results) {
      for (const ch of letters) {
        next.push(prefix + ch);
      }
    }
    results = next;
  }
  return results;
}