filter() is a very important method of an array.
This example iterates the array a and builds a new array with elements of a that returned true when running the function f() on each a element
const b = a.filter(f)
A good example of using filter() is when you want to remove an item from the array:
const items = ['a', 'b', 'c', 'd', 'e', 'f']
const valueToRemove = 'c'
const filteredItems = items.filter(item => item !== valueToRemove)
// ["a", "b", "d", "e", "f"]
Here is how you could remove multiple items at the same time:
const items = ['a', 'b', 'c', 'd', 'e', 'f']
const valuesToRemove = ['c', 'd']
const filteredItems = items.filter(item => !valuesToRemove.includes(item))
// ["a", "b", "e", "f"]