Immutable Data Structures

What's the difference?

Normal object in JavaScript

var person = {
    name: 'Byomkesh Bakshi',
    occupation: 'being awesome'
};

person.occupation = 'being not so awesome';
console.log(person); 
// Object {name: "Byomkesh Bakshi", occupation: "being not so awesome"}

With immutable-js (a library from facebook)

var map1 = Immutable.Map({a:1, b:2, c:3});
var map2 = map1.set('b', 50);
map1.get('b'); // 2
map2.get('b'); // 50

Why use immutable ds?

Easy undo/redo/time-travel (actually trivial). Very hard to do using mutable data structures
Easier concurrency primitives (no explicit locks/synchronized)
- Thread safe
- Because they never change, no worrying about if they change mid operation in a thread
Easy value comparision between data (== returns instantly). No deep comparisons required.
Immutable data can be used as map keys or set values
Easy caching (they never change, so just cache the reference and not the whole thing)

Why use immutable ds?

No defensive copying
Easy to find bugs
Most of the bugs are due to state change and when using mostly immutable data structures you know exactly where to look for while finding bugs (hint: where you do state changes)

DEMO

http://localhost:3001

Code for undo/redo

History.prototype.undo = function() {
  if(currentIndex === 0) {
    return this.history[currentIndex];
  }

  currentIndex -= 1;
  return this.getCurrentItem();
}

History.prototype.redo = function() {
  if(currentIndex === (this.history.length - 1)) {
    return this.history[currentIndex];
  }

  currentIndex += 1;
  return this.getCurrentItem();
}

But aren't they slow?

Nope
Implemented using persistent data structures
- Uses hash mapped array tries and path copying

Immutable Data Structures

What's the difference?

Why use immutable ds?

Why use immutable ds?

DEMO

Code for undo/redo

But aren't they slow?

JavaScript has it?

Questons

Questions