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

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