Exercise: word frequency in a Map

Write a function function wordFrequency(text) that returns a Map: its keys are the words of the string text, and its values are how many times each word occurs. The keys must follow the order in which the words first appear in the string.

• wordFrequency("the cat and the hat") → a Map with the pairs the → 2, cat → 1, and → 1, hat → 1
• wordFrequency("b a b c a b") → a Map with the pairs b → 3, a → 2, c → 1
• wordFrequency("") → an empty Map

Constraints: words consist of lowercase Latin letters and are separated by one space, with no spaces at the start or the end of the string; for the empty string the result is an empty Map whose size is 0.

The tests first check that the result is a Map: got instanceof Map is true only for a Map.

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