Why is immutability so important (or needed) in JavaScript?

Although the other answers are fine, to address your question about a practical use case (from the comments on the other answers) lets step outside your running code for a minute and look at the ubiquitous answer right under your nose: git. What would happen if every time you pushed a commit you overwrote the data in the repository?

Now we're in to one of the problems that immutable collections face: memory bloat. Git is smart enough to not simply make new copies of files every time you make a change, it simply keeps track of the diffs.

While I don't know much about the inner workings of git, I can only assume it uses a similar strategy to that of libraries you reference: structural sharing. Under the hood the libraries use tries or other trees to only track the nodes that are different.

This strategy is also reasonably performant for in-memory data structures as there are well-known tree-operation algorithms that operate in logarithmic time.

Another use case: say you want an undo button on your webapp. With immutable representations of your data, implementing such is relatively trivial. But if you rely on mutation, that means you have to worry about caching the state of the world and making atomic updates.

In short, there's a price to pay for immutability in runtime performance and the learning curve. But any experienced programmer will tell you that debugging time outweighs code-writing time by an order of magnitude. And the slight hit on runtime performance is likely outweighed by the state-related bugs your users don't have to endure.

I have recently been researching the same topic. I'll do my best to answer your question(s) and try to share what I have learned so far.

The question is, why is immutability so important? What is wrong in mutating objects? Doesn't it make things simple?

Basically it comes down to the fact that immutability increases predictability, performance (indirectly) and allows for mutation tracking.

Predictability

Mutation hides change, which create (unexpected) side effects, which can cause nasty bugs. When you enforce immutability you can keep your application architecture and mental model simple, which makes it easier to reason about your application.

Performance

Even though adding values to an immutable Object means that a new instance needs to be created where existing values need to be copied and new values need to be added to the new Object which cost memory, immutable Objects can make use of structural sharing to reduce memory overhead.

All updates return new values, but internally structures are shared to drastically reduce memory usage (and GC thrashing). This means that if you append to a vector with 1000 elements, it does not actually create a new vector 1001-elements long. Most likely, internally only a few small objects are allocated.

You can read more about this here.

Mutation Tracking

Besides reduced memory usage, immutability allows you to optimize your application by making use of reference- and value equality. This makes it really easy to see if anything has changed. For example a state change in a react component. You can use shouldComponentUpdate to check if the state is identical by comparing state Objects and prevent unnecessary rendering. You can read more about this here.

Additional resources:

• The Dao of Immutability
• Immutable Data Structures and JavaScript
• Immutability in JavaScript

If I set say an array of objects with a value initially. I can't manipulate it. That's what immutability principle says, right?(Correct me if I am wrong). But, what if I have a new News object that has to be updated? In usual case, I could have just added the object to the array. How do I achieve in this case? Delete the store & recreate it? Isn't adding an object to the array a less expensive operation?

Yes this is correct. If you're confused on how to implement this in your application I would recommend you to look at how redux does this to get familiar with the core concepts, it helped me a lot.

I like to use Redux as an example because it embraces immutability. It has a single immutable state tree (referred to as store) where all state changes are explicit by dispatching actions which are processed by a reducer that accepts the previous state together with said actions (one at a time) and returns the next state of your application. You can read more about it's core principles here.

There is an excellent redux course on egghead.io where Dan Abramov, the author of redux, explains these principles as follows (I modified the code a bit to better fit the scenario):

import React from 'react';
import ReactDOM from 'react-dom';

// Reducer.
const news = (state=[], action) => {
  switch(action.type) {
    case 'ADD_NEWS_ITEM': {
      return [ ...state, action.newsItem ];
    }
    default: {
        return state;
    }
  }
};

// Store.
const createStore = (reducer) => {
  let state;
  let listeners = [];

  const subscribe = (listener) => {
    listeners.push(listener);

    return () => {
      listeners = listeners.filter(cb => cb !== listener);
    };
  };

  const getState = () => state;

  const dispatch = (action) => {
    state = reducer(state, action);
    listeners.forEach( cb => cb() );
  };

  dispatch({});

  return { subscribe, getState, dispatch };
};

// Initialize store with reducer.
const store = createStore(news);

// Component.
const News = React.createClass({
  onAddNewsItem() {
    const { newsTitle } = this.refs;

    store.dispatch({
      type: 'ADD_NEWS_ITEM',
      newsItem: { title: newsTitle.value }
    });
  },

  render() {
    const { news } = this.props;

    return (
      <div>
        <input ref="newsTitle" />
        <button onClick={ this.onAddNewsItem }>add</button>
        <ul>
          { news.map( ({ title }) => <li>{ title }</li>) }
        </ul>
      </div>
    );
  }
});

// Handler that will execute when the store dispatches.
const render = () => {
  ReactDOM.render(
    <News news={ store.getState() } />,
    document.getElementById('news')
  );
};

// Entry point.
store.subscribe(render);
render();

Also, these videos demonstrate in further detail how to achieve immutability for:

• Arrays
• Objects

The question is, why is immutability so important? What is wrong in mutating objects? Doesn't it make things simple?

Actually, the opposite is true: mutability makes things more complicated, at least in the long run. Yes, it makes your initial coding easier because you can just change things wherever you want, but when your program goes bigger it becomes a problem – if a value changed, what changed it?

When you make everything immutable, it means data can't be changed by surprise any more. You know for certain that if you pass a value into a function, it can't be changed in that function.

Put simply: if you use immutable values, it makes it very easy to reason about your code: everyone gets a unique* copy of your data, so it can't futz with it and break other parts of your code. Imagine how much easier this makes working in a multi-threaded environment!

Note 1: There is a potential performance cost to immutability depending on what you're doing, but things like Immutable.js optimise as best they can.

Note 2: In the unlikely event you weren't sure, Immutable.js and ES6 const mean very different things.

In usual case, I could have just added the object to the array. How do I achieve in this case? Delete the store & recreate it? Isn't adding an object to the array a less expensive operation? PS: If the example is not the right way to explain immutability, please do let me know what's the right practical example.

Yes, your news example is perfectly good, and your reasoning is exactly right: you can't just amend your existing list, so you need to create a new one:

var originalItems = Immutable.List.of(1, 2, 3);
var newItems = originalItems.push(4, 5, 6);

A Contrarian View of Immutability

TL/DR: Immutability is more a fashion trend than a necessity in JavaScript. If you are using React it does provide a neat work-around to some confusing design choices in state management. However in most other situations it wont add enough value over the complexity it introduces, serving more to pad up a resume than to fulfill an actual client need.

Long answer: read below.

Why is immutability so important(or needed) in javascript?

Well, I'm glad you asked!

Some time ago a very talented guy called Dan Abramov wrote a javascript state management library called Redux which uses pure functions and immutability. He also made some really cool videos that made the idea really easy to understand (and sell).

The timing was perfect. The novelty of Angular was fading, and JavaScript world was ready to fixate on the latest thing that had the right degree of cool, and this library was not only innovative but slotted in perfectly with React which was being peddled by another Silicon Valley powerhouse.

Sad as it may be, fashions rule in the world of JavaScript. Now Abramov is being hailed as a demigod and all us mere mortals have to subject ourselves to the Dao of Immutability... Wether it makes sense or not.

What is wrong in mutating objects?

Nothing!

In fact programmers have been mutating objects for er... as long as there has been objects to mutate. 50+ years of application development in other words.

And why complicate things? When you have object cat and it dies, do you really need a second cat to track the change? Most people would just say cat.isDead = true and be done with it.

Doesn't (mutating objects) make things simple?

YES! .. Of course it does!

Specially in JavaScript, which in practice is most useful used for rendering a view of some state that is maintained elsewhere (like in a database).

What if I have a new News object that has to be updated? ... How do I achieve in this case? Delete the store & recreate it? Isn't adding an object to the array a less expensive operation?

Well, you can go the traditional approach and update the News object, so your in-memory representation of that object changes (and the view displayed to the user, or so one would hope)...

Or alternatively...

You can try the sexy FP/Immutability approach and add your changes to the News object to an array tracking every historical change so you can then iterate through the array and figure out what the correct state representation should be (phew!).

I am trying to learn what's right here. Please do enlighten me :)

Fashions come and go buddy. There are many ways to skin a cat.

I am sorry that you have to bear the confusion of a constantly changing set of programming paradigms. But hey, WELCOME TO THE CLUB!!

Now a couple of important points to remember with regards to Immutability, and you'll get these thrown at you with the feverish intensity that only naivety can muster.

1) Immutability is awesome for avoiding race conditions in multi-threaded environments.

Multi-threaded environments (like C++, Java and C#) are guilty of the practice of locking objects when more than one thread wants to change them. This is bad for performance, but better than the alternative of data corruption. And yet not as good as making everything immutable (Lord praise Haskell!).

BUT ALAS! In JavaScript you always operate on a single thread. Even web workers (each runs inside a separate context). So since you can't have a thread related race condition inside your execution context (all those lovely global variables and closures), the main point in favour of Immutability goes out the window.

(Having said that, there is an advantage to using pure functions in web workers, which is that you'll have no expectations about fiddling with objects on the main thread.)

2) Immutability can (somehow) avoid race conditions in the state of your app.

And here is the real crux of the matter, most (React) developers will tell you that Immutability and FP can somehow work this magic that allows the state of your application to become predictable.

Of course this doesn’t mean that you can avoid race conditions in the database, to pull that one off you’d have to coordinate all users in all browsers, and for that you’d need a back-end push technology like WebSockets (more on this below) that will broadcast changes to everyone running the app.

Nor does it mean that there is some inherent problem in JavaScript where your application state needs immutability in order to become predictable, any developer that has been coding front-end applications before React would tell you this.

This rather confusing claim simply means that if you use React your application is prone to race conditions, but that immutability allows you to take that pain away. Why? Because React is special.. its been designed first and foremost as a highly optimised rendering library with state management subverted to that aim, and thus component state is managed via an asynchronous chain of events (aka "one-way data binding") that optimize rendering but you have no control over and rely on you remembering not to mutate state directly...

Given this context, its easy to see how the need for immutability has little to do with JavaScript and a lot to do with React: if have a bunch of inter-dependent changes in your spanking new application and no easy way to figure out what your state is currently at, you are going to get confused, and thus it makes perfect sense to use immutability to track every historical change.

3) Race conditions are categorically bad.

Well, they might be if you are using React. But they are rare if you pick up a different framework.

Besides, you normally have far bigger problems to deal with… Problems like dependency hell. Like a bloated code-base. Like your CSS not getting loaded. Like a slow build process or being stuck to a monolithic back-end that makes iterating almost impossible. Like inexperienced devs not understanding whats going on and making a mess of things.

You know. Reality. But hey, who cares about that?

4) Immutability makes use of Reference Types to reduce the performance impact of tracking every state change.

Because seriously, if you are going to copy stuff every time your state changes, you better make sure you are smart about it.

5) Immutability allows you to UNDO stuff.

Because er.. this is the number one feature your project manager is going to ask for, right?

6) Immutable state has lots of cool potential in combination with WebSockets

Last but not least, the accumulation of state deltas makes a pretty compelling case in combination with WebSockets, which allows for an easy consumption of state as a flow of immutable events...

Once the penny drops on this concept (state being a flow of events -- rather than a crude set of records representing the latest view), the immutable world becomes a magical place to inhabit. A land of event-sourced wonder and possibility that transcends time itself. And when done right this can definitely make real-time apps easier to accomplish, you just broadcast the flow of events to everyone interested so they can build their own representation of the present and write back their own changes into the communal flow.

But at some point you wake up and realise that all that wonder and magic do not come for free. Unlike your eager colleagues, your stakeholders (yea, the people who pay you) care little about philosophy or fashion and a lot about the money they pay to build a product they can sell. And the bottom line is that its harder to code for immutability and easier to break it, plus there is little point having an immutable front-end if you don't have a back-end to support it. When (and if!) you finally convince your stakeholders that you should publish and consume events via a push techology like WebSockets, you find out what a pain it is to scale in production.

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Now for some advice, should you choose to accept it.

A choice to write JavaScript using FP/Immutability is also a choice to make your application code-base larger, more complex and harder to manage. I would strongly argue for limiting this approach to your Redux reducers, unless you know what you are doing... And IF you are going to go ahead and use immutability regardless, then apply immutable state to your whole application stack, and not just the client-side, as you're missing the real value of it otherwise.

Now, if you are fortunate enough to be able to make choices in your work, then try and use your wisdom (or not) and do what's right by the person who is paying you. You can base this on your experience, on your gut, or whats going on around you (admittedly if everyone is using React/Redux then there a valid argument that it will be easier to find a resource to continue your work).. Alternatively, you can try either Resume Driven Development or Hype Driven Development approaches. They might be more your sort of thing.

In short, the thing to be said for immutability is that it will make you fashionable with your peers, at least until the next craze comes around, by which point you'll be glad to move on.

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Now after this session of self-therapy I'd like to point out that I've added this as an article in my blog => Immutability in JavaScript: A Contrarian View. Feel free to reply in there if you have strong feelings you'd like to get off your chest too ;).