[Immutable.js] Exploring Sequences and Range() in Immutable.js

Understanding Immutable.js's Map() and List() structures will likely take you as far as you want to go with immutable programming. They have only small semantic differences between each other and the remaining structures in the Immutable.js family. Sequence, however, has one major difference: it's lazy--which opens a new realm of functional possibilities. Let's write a simple sequence to start.

Seq is lazy — Seq does as little work as necessary to respond to any method call. Values are often created during iteration, including implicit iteration when reducing or converting to a concrete data structure such as a List or JavaScript Array.

let numbers = Immutable.Range(0, 100);

let seq = Immutable.Seq.of(...numbers).take(9); // Seq do nothing now

//Use toArray() to actually make it works
console.log(seq.toArray()); // [0, 1, 2, 3, 4, 5, 6, 7, 8]

Cache for Seq -- You are able to use .cacheResult() method to cache the Seq:

  it('should cache results of Seq()', () => {

    let objects = Immutable.Range(0, 1000).map(() => { return new Object(); });
    
    let take100 = objects.take(100).toArray();
    let take100Again = objects.take(100).toArray();
    
    take100.forEach((obj, index) => {
      expect(obj === take100Again[index]).to.be.false;
    })

    let cachedObjects = Immutable.Range(0, 1000).map(() => { return new Object(); }).cacheResult();

    expect(cachedObjects.size).to.equal(1000); 
    
    let take100Cached = cachedObjects.take(100).toArray();
    let take100CachedAgain = cachedObjects.take(100).toArray();
    
    take100Cached.forEach((obj, index) => {
      expect(obj === take100CachedAgain[index]).to.be.true;
    })
    
  });

Example shows each time Seq runs will create a new objects, so if you compare 'take100' and 'take100Again', they are different object, because everytime go thoguth the Seq, it will create a new object.

But when you apply cache, the 'take100Cached' and 'take100CachedAgain' they are the same.

var squares = Immutable.Seq.of(1,2,3).map(x => {console.log("here");return x * x});
var res = squares.join() + squares.join();

If see the console.log(); there are six times "here";

var squares = Immutable.Seq.of(1,2,3).map(x => {console.log("here");return x * x}).cacheResult();
var res = squares.join() + squares.join();

When cache applies, only console.log three times.