DEV Community: Deepansh Bhargava

Virtual DOM in React

Deepansh Bhargava — Sun, 16 May 2021 11:31:57 +0000

Today we gonna learn about the Virtual DOM present in React, which is responsible for its fast performance and declarative nature.

What is DOM?

DOM stands for Document Object Model, which represents the node tree of all the UI elements present on a page. If any element on the UI changes, the DOM updates itself with the new changes.

This process of updating the DOM frequently makes the web page slow, so that's why there was a need for Virtual DOM. As we know DOM represents Tree Structure so the changes and updates are done quickly but the rerendering part takes a long time to complete.

If you know the Critical Rendering Path, the last step of it i.e Paint takes the longest time to complete, that's why browsers are optimized to repaint in minimum time.

Source: https://guillermo.at/browser-critical-render-path

What is Virtual DOM?

Virtual DOM represents a copy of the actual DOM. React maintains a copy of the DOM and renders the UI according to this DOM.

React works on the concept of observable pattern. It listens to all the changes made in the actual DOM and updates the virtual DOM only with that changes. React then renders the UI only with the changes that happened in the virtual DOM. This process is called diffing.

The process of maintaining sync between the actual DOM and the virtual DOM is called reconciliation.

How Virtual DOM is faster?

As soon as any update is done in the actual DOM, the reconciliation algorithm marks the element as dirty, and goes on to process the next update. When all the updates are processed in the DOM, they are passed to the virtual DOM in batches, instead of one by one as we have in the actual DOM. This makes the virtual DOM a lot faster and thus makes React performant.

Keys - A solution of rerenders

The reference of a UI element in the actual DOM and in the Virtual DOM need not be the same. So what happens in this case, the react will rerender everything even though we changed a single line.

Suppose we have this piece of code. The DOM will render it fully as it's the first time.

<div>
  <p>Hello! I am a piece of text.</p>
  <p>Don't underestimate me!</p>
</div>

Now, if we add another paragraph the DOM will render again all the unchanged paragraphs too.

<div>
  <p>Hello! I am a piece of text.</p>
  <p>Don't underestimate me!</p>
  <p>You don't know my powers!</p>
</div>

Now you'll be like why React is not behaving as it's supposed to be.

So here as I said the reference need not be the same, the first two paragraphs in the actual DOM and in the virtual DOM are different for React and it's considering them as new additions to the DOM. This lead to rerendering of the DOM unnecessarily. This problem can be solved with the help of keys.

<div>
  <p key="1">Hello! I am a piece of text.</p>
  <p key="2">Don't underestimate me!</p>
</div>

While rendering lists, generally we give keys to the element and for many developers, it's a headache to decide what will be the key. So to solve this problem I stumbled upon this syntax of React.

<div>
  {React.Children.toArray(texts.map((text) => <p>{text}</p>))}
</div>

That's it for now folks. Thanks for reading this article. Let me know if anything is not clear.

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Happy Coding!

Async Await in Javascript

Deepansh Bhargava — Mon, 10 May 2021 08:04:05 +0000

So in the last post, we learned about how promises work in javascript.

Today, we gonna learn about how async-await works together for the seamless working of promises and makes the developer experience better.

The concept of async-await comes into use when we have more than 2 promises, the code becomes messy and sometimes unbearable.

If you remember the previous example of making a promise, that's great. If not here it is:

const isNumEven = (num) => new Promise((resolve, reject)=> {
  if(num % 2 === 0) {
    resolve(true)
  } else {
    reject(false)
  }
})

Now this promise can be used as:

async function isEven(num) {
  try {
    const result = await isNumEven(num);
    return result;
  } catch(err) {
    console.log('Something went wrong!')
  }
}

isEven(4); // true

As you can see, it makes our code less sloppy and easier to manage. Now if the isNumEven function returns another promise, then we can use await again and get the result.

Now some of you must be wondering why try catch is used in this example?

The answer to this is error handling. If any of the statements fail in the try block, then the code directly goes into the catch block. So, if our promise fails, the error will be handled by the catch block.

Some important points related to async await:

Avoid using it inside for loops, and if there is a need to perform an operation on all entities use Promise.all rather than async await.
If you have used the async keyword before a function, it'll return a promise every time.
Async await is just syntactic sugar for promises, the code runs the same way as it runs for promises. The code looks synchronous and the program waits until the promise is resolved.
If a function is async, then only you can use await inside it.

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Happy Coding!

Hooks in React

Deepansh Bhargava — Sun, 09 May 2021 06:55:02 +0000

Today we'll learn about the widely used hooks in React. First, let me tell you the need for hooks. As we all know React has functional and class components. Previously, the functional components were only used to render the data but didn't consist of the business logic or any side effect in itself.

So hooks were introduced to implement lifecycle methods and states in functional components. They also bought the idea of code reusability along with them, that's why it was so quickly accepted within the React Community.

** Drum rolls 🥁 **

Let's move ahead with the main topic.

useState hook

This hook helps us to use and maintain a state within the functional component. It's very easy to use. The useState hook returns an array having two elements, the first one is the state variable and the second is the function that modifies the state variable.

import { useState } from "react";

export default function App() {
  const [number, setNumber] = useState(0);

  return (
    <div className="App">
      <button onClick={() => setNumber(number - 1)}>Subtract</button>
      <input
        type="number"
        value={number}
        onChange={(e) => setNumber(e.target.value)}
      />
      <button onClick={() => setNumber(number + 1)}>Add</button>
    </div>
  );
}

Here, we have number as the state variable and setNumber as the function which will modify the number.

useEffect hook

This hook works as a replacement for all the lifecycle methods which were used in class components. It contains a callback and a dependencies array. The hook will run every time when any of the values in the dependencies array changes.

import { useEffect, useState } from "react";

export default function App() {
  const [number, setNumber] = useState(0);
  const [isEven, toggleEven] = useState(true);

  useEffect(() => {
    if (number % 2 !== 0) {
      toggleEven(false);
    } else {
      toggleEven(true);
    }
  }, [number]);

  return (
    <div className="App">
      <button onClick={() => setNumber(number - 1)}>Subtract</button>
      <input
        type="number"
        value={number}
        onChange={(e) => setNumber(e.target.value)}
      />
      <button onClick={() => setNumber(number + 1)}>Add</button>
      <p>{isEven ? "Even" : "Odd"}</p>
    </div>
  );
}

Here as you can see, the dependencies array contains number. So every time the user presses any of the buttons or changes the input value, the useEffect hook will trigger and it'll check the number whether it's odd or even.

Within this hook, you can call side effects such as changing something in DOM, fetching data from an API, etc.

One more thing used in this hook is the cleanup function, which runs when we return a function in the callback. This function helps in cleaning of side effects i.e clearing a timer, closing a web socket, etc.

useContext hook

As in the class components, we had the Context API in the same way, we have useContext hook in functional components. This hook basically helps you to maintain a common global state across components. When the state changes it triggers re-renders across all the child components under the Context, even if the parent component uses shouldComponentUpdate or React.memo.

This hook can be used to implement a loading state within the application, passing data to children components, etc.

To create it, we use the createContext function and pass the initial value. In the context provider, we pass a prop called value which consists of the context variables and functions to modify the context.

import { createContext } from "react";

export const UserContext = createContext({
  name: "Initial Name"
});

Then we use this context in our Parent Component and wrap our app inside the Context Provider and pass a value. So now all the child components have the access to the value prop.

import { useContext, useState } from "react";
import Person from "./Person";
import { UserContext } from "./Context";

export default function App() {
  const user = useContext(UserContext);

  const [person, togglePerson] = useState(user);

  return (
    <UserContext.Provider value={[person, togglePerson]}>
      <div className="App">Hello {user.name}!</div>
      <Person />
    </UserContext.Provider>
  );
}

Now in the child component Person we can access the value prop and we can change the context value too.

import { useContext } from "react";
import { UserContext } from "./Context";

function Person() {
  const [person, togglePerson] = useContext(UserContext);

  return (
    <div>
      Person: {person.name}
      <button
        onClick={() => {
          togglePerson({ name: "Stuart Little" });
        }}
      >
        Change
      </button>
    </div>
  );
}

export default Person;

This way you can use the useContext hook. Make a context, wrap the parent component with the context's provider and you are good to go!

You can experiment with the above code here.

useReducer hook

If you know Redux then you already know what this hook does, if not then simply it changes the state according to the action passed. It helps in maintaining a local state easily whereas in Redux we can do that globally.

To use the reducer globally, we can make a context and pass it across the components. Geddit!

Here we have an initial state and a dispatch to modify the state. Within the dispatch, we pass actions along with some info called payload. The action then goes to the reducer which in turn changes the state.

import { useReducer } from "react";
import "./styles.css";

const initialState = [
  {
    name: "Mad Angles",
    quantity: 5
  },
  {
    name: "Pringles",
    quantity: 3
  },
  {
    name: "Lays",
    quantity: 2
  },
  {
    name: "Kurkure",
    quantity: 1
  },
  {
    name: "Uncle Chips",
    quantity: 9
  }
];

function reducer(state, action) {
  switch (action.type) {
    case "add":
      return [...state, action.payload];
    case "delete":
      return state.filter((_, index) => index !== action.payload);
    default:
      return state;
  }
}

export default function App() {
  const [state, dispatch] = useReducer(reducer, initialState);

  return (
    <div className="App">
      <h1>Hello CodeSandbox</h1>
      <ul>
        {state.map(({ name, quantity }, index) => (
          <div className="list-item">
            <li key={name}>
              <b>{name}</b> - {quantity}
            </li>
            <button
              onClick={() => {
                dispatch({ type: "delete", payload: index });
              }}
            >
              delete
            </button>
          </div>
        ))}
        <button
          onClick={() => {
            dispatch({
              type: "add",
              payload: {
                name: "Doritoes",
                quantity: 5
              }
            });
          }}
        >
          add
        </button>
      </ul>
    </div>
  );
}

You can find the above code here.

That's it for now. There are other hooks too like useRef, useCallback but they are rarely used. Thanks for reading this article. Let me know if anything is not clear.

Happy Coding!

Promises in Javascript

Deepansh Bhargava — Sun, 09 May 2021 06:28:32 +0000

A Promise represents the state of an asynchronous operation. This asynchronous operation can be anything i.e fetching data from an API, reading contents from a file, etc.

Before promises we had callbacks, but they had fewer capabilities and lead to a sloppy code. When dealing with multiple callbacks, our code will become a callback hell which becomes difficult to manage.

A Javascript Promise consists of four states:

Fulfilled: The operation was completed successfully.
Pending: The operation isn't completed, it's still in process.
Rejected: The operation is failed.
Settled: The operation is either fulfilled or rejected.

A promise can be created by using
Promise constructor.

const isNumEven = (num) => new Promise((resolve, reject)=> {
  if(num % 2 === 0) {
    resolve(true)
  } else {
    reject(false)
  }
})

Now the above-created promise can be used as

isNumEven(5)
  .then(res => console.log(res))
  .catch(err => console.log(err))

The then handler is invoked when the promise is fulfilled while the catch handler is invoked when the promise is rejected.

Here in this case the catch handler is invoked as the number we have passed to the function is not even.

Chaining Promises

When the result of a promise is another promise, we have to chain them to get the results.

isNumPrime(5)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.log(err))

This method of handling asynchronous operations looks fine when we have 2 to 3 promises but it gets unmanageable when there are more than that. Then async await comes into the picture. They help you to handle multiple promises very easily.

That's it for now. Thanks for making it to the last. Let me know any feedback or suggestions you have for this article. Also, let me know if you want me to write on async await.