Mastering Immutable.js

Multiple filter levels

Let's see how lazy evaluation works with more than one sequence operation. To do so, let's first implement a function to compose filter predicates, as follows:

const equals = first => (second) => {  console.log(`${first} === ${second}`);  return first === second;};

Now we have an easy way to build a simple predicate that logs what we're comparing and then returns the result of the actual comparison. Let's put equals() to use by chaining together several filters:

myList  .toSeq()  .filterNot(equals(1))  .filterNot(equals(3))  .filterNot(equals(5))  .forEach(item => console.log('myList', item));  // -> 1 === 1  // -> 1 === 2  // -> 3 === 2  // -> 5 === 2  // -> myList 2  // -> 1 === 3  // -> 3 === 3  // -> 1 === 4  // -> 3 === 4 // -> ...