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

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

  return prefix;
}
```