Third largest number

Write a function thirdLargest(numbers) that returns the third largest element of an array of numbers.

• numbers is an array of integers. They can be positive, negative or 0.
• All numbers in the array are different.
• Return the third largest number: in [10, 40, 20, 30] it is 20.
• If the array has fewer than 3 numbers (also for an empty array), return null.

The interviewer asked you not to use built-in functions such as sort or Math.max: walk through the array with a loop. The tests check only the result; keep to the rule yourself.