Lazy filtering

We've seen multiple ways to build sequences so far—now let's introduce a real need for them. When you filter sequences, each value from the collection is passed through the predicate function and into the side-effect, one at a time.

Basic lazy filtering

To help illustrate the concept of lazy filtering, let's implement a basic predicate function with some extra logging:

const predicate = (item) => {
  console.log('filtering', item);
  return item % 2;
};

This logging will make it easy for us to see when the actual filtering of a value happens relative to when the value is used in the side-effect. Let's try this on an indexed sequence:

const myList = List.of(1, 2, 3, 4, 5, 6);
myList
  .toSeq()
  .filter(predicate)
  .forEach(item => console.log('myList', item));
  // -> filtering 1
  // -> myList 1
  // -> filtering 2
  // -> filtering 3
  // -> myList 3
  // -> filtering 4
  // -> filtering 5
  // -> myList 5
  // -> filtering 6

If you pay close...