map() is key method of an array when it comes to thinking in functional programming terms.
This example iterates a and builds a new array with the result of executing f() on each a element:
const b = a.map(f)
Given an array, we can use map() to create a new array from the initial one, and then filtering the result using filter(). This short example creates a new array to get the first letter of each item in the list array, and filters the one that matches A:
const list = ['Apple', 'Orange', 'Egg']
list.map(item => item[0]).filter(item => item === 'A') //'A'