Merge two arrays

Write a function mergeArrays(a, b) that returns a new array with all elements of a and then all elements of b.

• a and b are arrays of numbers and strings (one array can have both). Each one can have any length, also 0.
• Keep the order: first the elements of a in their order, then the elements of b in their order.
• Keep repeated values. If a value is in both arrays, or twice in one array, it is in the result every time.
• Always return a new array, even when a or b is empty. Do not return a or b itself.
• Do not change (do not mutate) a or b.

The interviewer asked for exactly one for loop. The tests check only the result; keep to the rule yourself. One loop over the indexes from 0 to a.length + b.length - 1 is enough.