React Knowledge Boost

—

### 🧠 PHASE 1: Core React (Easy to Intermediate)
**1. JSX & Components**
– **JSX**: Syntactic sugar for `React.createElement`. Looks like HTML but it's JavaScript.
– **Functional Components**: Basic building blocks. Start simple:

function Hello() {
    return <h1>Hello, world!</h1>;
  }

**2. Props & State**
– **Props**: Read-only inputs to components.
– **State**: Internal data a component can change.
– Example:

function Counter({ initial }) {
    const [count, setCount] = useState(initial);
    return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
  }

**3. Events & Conditional Rendering**
– `onClick`, `onChange`, `onSubmit`, etc.
– `if`, `&&`, `ternary` operators inside JSX

**4. useEffect**
– Lifecycle in functional components.
– Example:

useEffect(() => {
    console.log('Component mounted');
    return () => console.log('Component unmounted');
  }, []);

—

### 🧠 PHASE 2: Intermediate Concepts

**5. Lifting State Up**
– Move shared state to the closest common parent.

**6. Controlled vs Uncontrolled Components**
– Controlled: input value comes from state.
– Uncontrolled: uses `ref`.

**7. useRef, useMemo, useCallback**
– `useRef`: DOM or persistent values.
– `useMemo`: Memoize expensive calculations.
– `useCallback`: Memoize functions to avoid unnecessary re-renders.

**8. Context API**
– Global state management (lightweight alternative to Redux).

const ThemeContext = React.createContext();

**9. Error Boundaries**
– Catch rendering errors in class components using `componentDidCatch`.

—

### 🧠 PHASE 3: Advanced Concepts

**10. Custom Hooks**
– Reusable hook logic.

function useWindowWidth() {
    const [width, setWidth] = useState(window.innerWidth);
    useEffect(() => {
      const handleResize = () => setWidth(window.innerWidth);
      window.addEventListener('resize', handleResize);
      return () => window.removeEventListener('resize', handleResize);
    }, []);
    return width;
  }

**11. React Router**
– Routing with “, “, “, etc.
– Dynamic routes, route guards, nested routes.

**12. Forms with Formik or React Hook Form**
– Manage complex forms with validation.

**13. Redux / Zustand**
– Global state management for large apps.

**14. Performance Optimization**
– Code splitting (`React.lazy` & `Suspense`)
– Avoid re-renders (`React.memo`, `useMemo`, `useCallback`)

**15. Testing React**
– Unit tests with **Jest**
– Component testing with **React Testing Library**

—

### 🔥 Interview-Focused Practice (Optional)
I'll give you:
– Real-life React interview questions
– Scenario-based coding tasks
– Mini-projects to reinforce each concept

Would you like to start with **hands-on questions** now, or should I go deeper into any specific topic first?

—

## ✅ PHASE 1: Core Concepts — "React Foundations"

### ✅ 1. **JSX & Functional Components**

🔸 **Concept**: JSX lets you write HTML-like syntax in JavaScript.
🔸 **Code Example**:

const Welcome = () => <h1>Welcome to React!</h1>;

🔹 **Interview Question**:
> ❓ What is JSX?
> ✅ JSX is a syntax extension for JavaScript used in React to describe UI structure, similar to HTML but with JS power.

—

### ✅ 2. **Props vs State**

🔸 **Concept**:
– Props = External data (read-only)
– State = Internal data (can change)

🔸 **Code**:

function Greet({ name }) {
  return <h2>Hello, {name}</h2>;
}

function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
}

🔹 **Interview Questions**:
> ❓ What's the difference between state and props?
> ✅ Props are passed from parent to child; state is local and managed inside the component.

—

### ✅ 3. **useEffect Hook**

🔸 **Concept**: Side effects (e.g., fetch, subscriptions)

useEffect(() => {
  console.log("Mounted");
  return () => console.log("Unmounted");
}, []);

🔹 **Interview Questions**:
> ❓ When does `useEffect` run?
> ✅ After render. It depends on the dependency array: `[]` runs once, `[value]` runs on value change.

—

## 🧠 Let's Practice a Mini Task:

🧪 **Task**: Create a `UserGreeting` component.
👉 If `isLoggedIn` is true, show "Welcome, Neeraj!", otherwise "Please log in".

Can you write this small functional component with conditional rendering using ternary?

(If you want, I’ll show you the answer afterward—but try it yourself first.)

Would you like:
– to try writing the code?
– or to proceed to the **next level: Lists, Events, and Forms** with challenges?

const UserGreeting = () =>{
const [isLoggedin , setIsLoggedIn] = useState(false)

return
{isLoggedin ? "welcome" : " Please Login" }
setIsLoggedIn(!isLoggedin)}>{isLoggedin ? "logout" : "login"}

}
export default UserGreeting ;

– `useState` usage ✅
– Conditional rendering with ternary ✅
– Dynamic button label ✅
– State toggle logic ✅

Here’s a quick **interview insight** you can use when showing or explaining this:

—

🧠 **Interviewer Might Ask**:
> ❓ How would you handle this if the login state came from a parent component or a global auth context?

✅ **Your Smart Answer**:
> "If login state is managed globally, I’d pass it as a prop or use Context API for scalability. This avoids prop drilling and centralizes state."

—

### 🎯 Next Challenge (Lists + Events):

Let’s level up:

### 👉 Task:
Create a `TodoList` component that:
– Uses `useState` to manage an array of todos
– Has an input to type a new todo
– Adds it to the list on button click
– Renders all todos using `.map()`

💡 (Try it on your own — you’re building real-world logic now. Let me know if you want hints or review.)

After this, we’ll move to `useEffect`, `useRef`, and `Custom Hooks` with interview questions. Ready?

const TodoList = () =>{
const [input , setInput] = useState("");
const[lists , setLists] = useState([])
const handleSubmit = (e)=>{
e.preventDefault();
if(input.trim()===""){
setLists([…lists , input])
setInput("")
}
}
return

setInput(e.target.value)}/>
Add

{lists.map((list , index)=>{
return {list}
})}

}
export default TodoList ;

—

### ❌ Problem:
Your condition is:

if (input.trim() === "") {
  setLists([...lists, input]);
  setInput("");
}

You're checking if the input **is empty**, and **only then** adding it. That’s reversed.

—

### ✅ Fix:
You should **only add** the item if the input is **not empty**:

if (input.trim() !== "") {
  setLists([...lists, input]);
  setInput("");
}

—

### ✅ Final Corrected Version:

import React, { useState } from 'react';

const TodoList = () => {
  const [input, setInput] = useState("");
  const [lists, setLists] = useState([]);

  const handleSubmit = (e) => {
    e.preventDefault();
    if (input.trim() !== "") {
      setLists([...lists, input]);
      setInput("");
    }
  };

  return (
    <>
      <form onSubmit={handleSubmit}>
        <input
          type="text"
          value={input}
          onChange={(e) => setInput(e.target.value)}
        />
        <button type="submit">Add</button>
      </form>
      <ul>
        {lists.map((list, index) => (
          <li key={index}>{list}</li>
        ))}
      </ul>
    </>
  );
};

export default TodoList;

—

### 🧠 Interview-Level Takeaways:
> ❓ What would you do to prevent duplicate todos?

✅ “I can check if the new input already exists in `lists` using `.includes(input)` before adding it.”

> ❓ Why use `index` as a key here?

✅ “For simple static lists, index is fine, but for dynamic content where items can be added/removed, using a unique ID (like UUID) is better to avoid rendering bugs.”

—

Would you like the next level challenge?
We can do:
– `useEffect` with API fetch
– Or start `useRef`, `Context`, `Custom Hooks`
– Or jump into **React Router** or **Form validation**

👉 Let me know your interest:
**API**, **Hooks**, **Context**, **Router**, or **Forms**?

const TodoList = () =>{
const [input , setInput] = useState("");
const[lists , setLists] = useState([])
const handleSubmit = (e)=>{
e.preventDefault();
if(input.trim() !==""){
setLists([…lists , input])
setInput("")
}
}
return

setInput(e.target.value)}/>
Add

{lists.map((list , index)=>{
return {list}
})}

}
export default TodoList ;

—

### ✅ What You Did Perfectly:
– `useState` for both input and list ✔️
– `onChange` to capture typing ✔️
– `onSubmit` to handle form submission ✔️
– `.trim()` to avoid empty inputs ✔️
– Array update using spread `[…lists, input]` ✔️
– Rendering list with `.map()` and unique `key` ✔️

—

### 🧠 React Interview Deep Dives:

#### ❓ Q: Why do we use `e.preventDefault()` in forms?
✅ Prevents the default form submission which reloads the page.

—

#### ❓ Q: Why is using `index` as a key in `.map()` considered risky?
✅ If the list gets reordered or items are deleted, index-based keys may cause incorrect DOM updates. Prefer unique IDs when possible.

—

### 🚀 Ready for Level-Up?

Next topic: **`useEffect` in action with API fetching**

Would you like to:
– 🔍 See a real example (`fetch()` + `useEffect`)
– 🎯 Solve an API-related challenge (e.g. fetch users and display them with loading/error states)
– 🛠 Or jump into `useRef` + `custom hooks`

Let me know and we’ll build step-by-step. You're progressing like a pro.

—

### ✅ Interview-Ready Answer:

> 🧠 **Q: Can I use `try…catch` inside `useEffect`?**
> ✅ Yes! But since `useEffect` doesn't support `async` directly, I define an inner async function and call it. I wrap `await fetch()` in a `try…catch` to handle network or API errors cleanly.

—

### 💡 Example (Interview-Ready Code):

“`jsx
import React, { useState, useEffect } from 'react';

const UsersList = () => {
const [users, setUsers] = useState([]);
const [loading, setLoading] = useState(true);
const [error, setError] = useState(null);

useEffect(() => {
const fetchUsers = async () => {
try {
const

—

### ✅ Interview Answer (No code, just logic):
> "Yes, when fetching API data in React (typically in `useEffect`), I wrap the async call in a `try…catch` block. This helps me catch and handle network errors, log issues, or show fallback UI like error messages."

—

### 🎯 Example API to Mention:
You can use a public API like:

– `https://jsonplaceholder.typicode.com/users` (dummy user data)
– `https://api.github.com/users` (GitHub profiles)
– `https://fakestoreapi.com/products` (ecommerce-style data)
– `https://api.coindesk.com/v1/bpi/currentprice.json` (Bitcoin price data)

So if an interviewer asks:

> ❓ How would you safely fetch data in a component?

You can respond:

> "I’d use `useEffect` with an async function inside it. I'd fetch data from something like `https://jsonplaceholder.typicode.com/users` and use `try…catch` to gracefully handle any errors, like network failure or a non-200 status."

—

Let me know when you're ready to build a `useEffect`-based API fetch yourself — I’ll review it, no spoilers! Or we can go deeper into loading states, error UIs, or pagination.

—

### ✅ React Component: Fetch users with error + loading handling

“`jsx
import React, { useEffect, useState } from 'react';

const FetchUsers = () => {
const [users, setUsers] = useState([]);
const [loading, setLoading] = useState(true);
const [error, setError] = useState(null);

useEffect(() => {
const fetchUsers = async () => {
try {
const res = await fetch('https://jsonplaceholder.typicode.com/users');
if (!res.ok) {
throw new Error(`HTTP error! Status: ${res.status}`);
}
const data = await res.json();
setUsers(data);
} catch (err) {
setError(err.message);
} finally {
setLoading(false);
}
};

fetchUsers();
}, []);

if (loading) return Loading users…;
if (error) return Error: {error};

return (

User List

{users.map(user => (

const ApiFetch = () =>{
const [users , setUsers] = useState([]);
const [loading , setLoading] = useState(true);
const [error , setError] = useState(null);

useEffect(()=>{
const fetchApi = async ()=>{
try{
const response = await fetch('https://jsonplaceholder.typicode.com/users');
if (!response.ok){
throw new Error ("issue in data fetching")
}
const data = await response.json();
setUsers(data)
}
catch(err){
setError(err.message)
}
finally{
setLoading(false)
}
}
fetchApi()
} , [])
if(loading) return loading….
if (error) return Error : {error}
return
Users Data Fetch
{users.map((user , index)=>{
return {user.name}
})}

}
export default ApiFetch ;

—

### ✅ What You Did Perfectly:
– `useEffect` to fire the fetch on mount ✅
– `async/await` with `try…catch` for safe error handling ✅
– `.ok` check on the response before parsing ✅
– Clean use of `finally` to toggle the loading state ✅
– Conditionally rendering based on `loading` and `error` ✅
– Semantic, easy-to-read UI ✅

—

### 🧠 Interview Insight (What You Can Say):

If you're asked:

> ❓ How do you handle async data fetching in React?

Your killer answer:

> “I use `useEffect` to fire an async function when the component mounts. I wrap the fetch call inside `try…catch` to handle both network failures and bad HTTP statuses. I also use `finally` to cleanly update the loading state. If the API is successful, I update the state — otherwise, I show a relevant error message.”

You can also say:

> “I usually fetch from public APIs like `https://jsonplaceholder.typicode.com/users` for mock data in dev.”

—

### 🔄 Optional Next Step Challenge:
Would you like to:
1. Show **user emails with toggle details** (intro to conditional UI)
2. Add a **search box** to filter users by name (real app scenario)
3. Move to `useRef`, `memo`, or `custom hooks`

You’re moving like a seasoned dev now — tell me where you'd like to go deeper!

Add a search box to filter users by name (real app scenario)

yes
let me know without code spoiler

—

## 🧩 1. **Toggle Email on User Click**

### ✅ Goal:
– Show a list of users (which you already have)
– When you click on a user’s name, it **toggles their email visibility**

### 🧠 How to Think:
– You’ll need a **piece of state** to track which user's email is visible
– When you click a user's name:
– If it’s currently visible → hide it
– If not visible → show it

💡 Hint: You could store the ID of the currently "expanded" user — or use a list of IDs if you want multiple users expanded at once.

—

## 🔍 2. **Search Box to Filter Users by Name**

### ✅ Goal:
– Add a search input above the list
– As you type, the list **filters in real time** to only show names that match

### 🧠 How to Think:
– Use `useState` for `searchTerm`
– As the user types, update this value
– Use `.filter()` on the `users` array during render — only show names that include the search string (case-insensitive)

💡 Bonus UX Tip: Add a `.toLowerCase()` comparison so it works regardless of casing.

—

### 🗣️ What to Say in Interviews:
> “I’ve implemented toggle-based detail UIs using conditional rendering and local state. I’ve also built live search functionality by filtering stateful data arrays based on an input value, which updates in real time using controlled components.”

—

🚀 Try building both features in your `ApiFetch` component. When you're done, paste it here — I’ll give you pro-level feedback and optimization tips.

Ready to go?

1. ✅ **Email toggle on user click**
2. ✅ **Search input to filter users by name**

—

import React, { useState, useEffect } from 'react';

const ApiFetch = () => {
  const [users, setUsers] = useState([]);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);
  const [visibleEmails, setVisibleEmails] = useState([]); // track email toggle
  const [searchTerm, setSearchTerm] = useState(""); // track search input

  useEffect(() => {
    const fetchApi = async () => {
      try {
        const response = await fetch('https://jsonplaceholder.typicode.com/users');
        if (!response.ok) {
          throw new Error("Issue in data fetching");
        }
        const data = await response.json();
        setUsers(data);
      } catch (err) {
        setError(err.message);
      } finally {
        setLoading(false);
      }
    };
    fetchApi();
  }, []);

  const toggleEmail = (id) => {
    setVisibleEmails((prev) =>
      prev.includes(id) ? prev.filter((uid) => uid !== id) : [...prev, id]
    );
  };

  const filteredUsers = users.filter((user) =>
    user.name.toLowerCase().includes(searchTerm.toLowerCase())
  );

  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error}</p>;

  return (
    <>
      <h2>Users Data Fetch</h2>
      <input
        type="text"
        placeholder="Search by name..."
        value={searchTerm}
        onChange={(e) => setSearchTerm(e.target.value)}
        style={{ padding: '8px', marginBottom: '20px', width: '100%' }}
      />
      <ul>
        {filteredUsers.map((user) => (
          <li key={user.id} style={{ marginBottom: '10px', cursor: 'pointer' }}>
            <strong onClick={() => toggleEmail(user.id)}>{user.name}</strong>
            {visibleEmails.includes(user.id) && <p>Email: {user.email}</p>}
          </li>
        ))}
      </ul>
    </>
  );
};

export default ApiFetch;

—

### 🔥 Highlights You Can Explain in Interviews:

– `visibleEmails` state lets you **track toggled items**
– `filter()` + `toLowerCase()` gives real-time **search filtering**
– Clean and minimal UX with smart interaction

Let me know if you want to enhance this further with:
– Avatar images
– Accordion-style toggle
– Debounced search input
– Pagination or Load More

You're building like a **React pro** now.

how yu did it?

—

## 🧩 PART 1: Email Toggle — `visibleEmails` state

### 💡 The Problem:
You want to **show/hide a user's email** when you click on their name.

### ✅ The Solution:
We use a new piece of state:

const [visibleEmails, setVisibleEmails] = useState([]);

– This array keeps track of the user IDs whose emails are currently **visible**.
– If a user ID is in this array → show their email
– Otherwise → don’t show it

—

### ⚙️ Logic on Click:
When you click a user's name:

onClick={() => toggleEmail(user.id)}

The function checks if that user's ID is in the `visibleEmails` list:

const toggleEmail = (id) => {
  setVisibleEmails((prev) =>
    prev.includes(id)
      ? prev.filter((uid) => uid !== id) // remove it
      : [...prev, id]                    // add it
  );
};

So:
– If the ID is already in the array → remove it (hide email)
– If it's not → add it (show email)

—

## 🧩 PART 2: Search Box — `searchTerm` + `.filter()`

### 💡 The Problem:
You want to filter users by name while typing in a search box.

### ✅ The Solution:

const [searchTerm, setSearchTerm] = useState("");

The search box input:

<input
  value={searchTerm}
  onChange={(e) => setSearchTerm(e.target.value)}
/>

Then, you filter the user list before displaying:

const filteredUsers = users.filter((user) =>
  user.name.toLowerCase().includes(searchTerm.toLowerCase())
);

This gives you only the users that **match the search input**.

—

### 🔄 Final Rendering:
You loop through the `filteredUsers`:

<ul>
  {filteredUsers.map((user) => (
    <li key={user.id} onClick={() => toggleEmail(user.id)}>
      {user.name}
      {visibleEmails.includes(user.id) && <p>{user.email}</p>}
    </li>
  ))}
</ul>

—

### 🧠 What You’ve Learned:
– `visibleEmails` is for **show/hide email** per user (by tracking ID)
– `searchTerm` lets you **filter user list** in real time
– `includes()` is used for both — searching inside a string and checking if an ID is in an array

—

Would you like me to show this visually as a flow or diagram?

Or would you like a **simplified version** of this concept to practice with first?