```ts
export function commonHead(words: string[]): string {
  if (words.length === 0) return '';

  let firstWord = words[0];
  for (let i = 1; i < words.length; i++) {
    const word = words[i];
    if (!word.startsWith(firstWord)) {
      break;
    }
  }

  return firstWord!;
}
```