Exercise: the longest word

Write a function function longestWord(words) that returns the longest word in the array words.

• longestWord(["sun", "planet", "moon"]) → "planet"
• longestWord(["cat", "dog"]) → "cat"
• longestWord([]) → ""

Constraints: words holds 0 to 10 non-empty strings; if several words are the longest, return the one that comes first; for an empty array return the empty string "".

Do not add extra imports unless the task says so explicitly.