map elements of an array then make an action

1let numbers = [1, 2, 3, 4]
2let filteredNumbers = numbers.map(function(num, index) {
3  if (index < 3) {
4     return num
5  }
6})
7// index goes from 0, so the filterNumbers are 1,2,3 and undefined.
8// filteredNumbers is [1, 2, 3, undefined]
9// numbers is still [1, 2, 3, 4]
10
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