React CheatSheet

// MyComponent.js
import React from 'react'; // importing React from react is optional in sub files in latest React , because it is done in app js and inherited but doing it is good practise and to be on the safer side.
import './MyComponent.css';  // we can directly import a css file in component

function MyComponent(props) {
  return (
    <div className="my-component"> // instead of `class` in html `className` is used.
      <h1>{props.title}</h1>       // All Props are passed as an Object
      <p>{props.description}</p>

      <div className="content">   
          {props.children}  // The Extra data/JSX passed can be accessed using props.children
      </div>
    </div>
  );
}

export default MyComponent;


// App.js
import React from 'react';
import MyComponent from './MyComponent';

function App() {
  return (
    <div>
      <MyComponent 
      title="My Title" 
      description="This is my description." > // Props/Attributes can be used to share data between components

      <p>This is some additional content.</p>  // We Can also simply pass extra data into components too
      </MyComponent>
      />  
    </div>
  );
}

export default App;

// Counter.js

import React, { useState } from 'react'; // useState is imported directly from react library.

const Counter = () => {
  // Declaring a state variable, initial value is used whenever the component is rendered for the first time.
  const [count, setCount] = useState(0);

  // useState returns 2 values that can be destructured.
  // first is the variable itself and second is a function that allows to update the variable.

  // Event handler for incrementing the count
  const handleIncrement = () => {
    setCount(count + 1); // Update the "count" state with a new value

    // set Action schedules the update , it takes a small time to update the specified value. 
    // therefore you cant instantly use the value right after setting it as the updation might be delayed.
    // in case of multiple set methods right after another they are batched together and executed together
  };

  // Event handler for decrementing the count
  const handleDecrement = () => {
    setCount(count - 1); // Update the "count" state with a new value
  };

  return (
    <div>
      <h1>Counter</h1>
      <p>Count: {count}</p>
      <button onClick={handleIncrement}>Increment</button>
      <button onClick={handleDecrement}>Decrement</button>
    </div>
  );
};

export default Counter;

import React, { useState } from "react";

const FormExample = () => {
  const [formData, setFormData] = useState({
    firstName: "",
    email: "",
  });

  // name of handler functions 
  const handleChange = (e) => { 
  // this function gets the data whenever it is called through the element
    const { name, value } = e.target;
    setFormData({
      ...formData,
      [name]: value,
    });
  };

  const handleSubmit = (e) => {
    e.preventDefault(); // we prevent the normal functionality of refreshing when form submits and add our logic.=
    alert(
      `Submitted Form Data:\nFirst Name: ${formData.firstName}\nEmail: ${formData.email}`
    );
    // Reset form fields after submitted
    setFormData({
      firstName: "",
      email: "",
    });
  };

  return (
    <form onSubmit={handleSubmit}> // All the form events have "on" prefixed to them in react
      <h1>Form Example</h1>
      <label htmlFor="firstName">First Name:</label> // for is changed to htmlFor in React
      <input
        type="text"
        id="firstName"
        name="firstName"
        value={formData.firstName}
        onChange={handleChange} // we pass the function to be called on change
      />
      <br />
      <br />
      <label htmlFor="email">Email:</label>
      <input
        type="email"
        id="email"
        name="email"
        value={formData.email}
        onChange={handleChange}
      />
      <br />
      <button type="submit" onClick={handleSubmit}> // onClick is used for Events on Buttons
        Submit
      </button>
    </form>
  );
};

export default FormExample;

import React, { useRef } from "react";

const MyForm = () => {
  const inputRef = useRef(); // Ref for input field
  const countRef = useRef(0); // Ref for mutable count value

  const handleButtonClick = () => {
    // Accessing DOM properties . here we are using it to just get values
    console.log("Input value:", inputRef.current.value);

    // Triggering DOM methods , triggering dom methods according to need
    inputRef.current.focus();
    inputRef.current.select();

    // Storing mutable value , normally useRef should not be used to update values as it does not rerender the component.
    // There it can be useful at place where only one value is needed to be change and rerender is not needed.
    countRef.current++;
    console.log("Current count:", countRef.current);
  };

  return (
    <div>
      <input ref={inputRef} type="text" />
      <button onClick={handleButtonClick}>Get Input Value, Focus Input, Increment Count</button>
    </div>
  );
};

export default MyForm;

  // We have 2 options , use 3 states to save different values or have just one state with object containing all 3 values


  // Single states for name, age, and email = Multi State
  const [name, setName] = useState('');
  const [age, setAge] = useState(0);
  const [email, setEmail] = useState('');

  // Object state for user = One State
  const [user, setUser] = useState({
    name: '',
    age: 0,
    email: ''
  });

  // Event handler for updating name
  const handleNameChange = (event) => {
    setName(event.target.value);
  };
  // That's the simple way

  // Event handler for updating user object
  const handleUserChange = (event) => {
   // updating the object replaces it as a whole so we have to use the old object using spread operator
    setUser({
      ...user,
      [event.target.name]: event.target.value
    });
   // The issue here is we are using the old state, in case it is being updated at the same time we might get the wrong value for oldState because of scheduling
   setUser((prevState) => {
      ...prevState,
      [event.target.name]: event.target.value
    });
    // We solve the above issue , by using the function variation of setState which itself provides the UPDATED old value and will never be wrong. Especially helpful when running Asynchronously
  };

import React, { useState } from 'react';

// Child Component
const ChildComponent = ({ childData, onChildDataChange }) => { // A child component can recieve data in props
  const handleInputChange = (event) => {
    const newChildData = event.target.value;
    onChildDataChange(newChildData); // data is saved in parent onChange
    // a child component can also recieve functions from parent component, this can be used to send data to parents
    // this concept is LIFTING STATE UP
  };

  return (
    <div>
      <h2>Child Component</h2>
      <input type="text" value={childData} onChange={handleInputChange} />
      <p>Child Data: {childData}</p>
    </div>
  );
};

// Parent Component
const ParentComponent = () => {
  const [parentData, setParentData] = useState('Hello from parent!');

  const handleChildDataChange = (newChildData) => {
    setParentData(newChildData);
  };

  return (
    <div>
      <h1>Parent Component</h1>
      <p>Parent Data: {parentData}</p>
      <ChildComponent childData={parentData} onChildDataChange={handleChildDataChange} />
      // Here parent is sending the data and a function to update data , to the child
      // Passing data to child , this concept is PASSING STATE DATA
    </div>
  );
};

export default ParentComponent;

// MyComponent.js - rendering lists
import React from 'react';
import DataItem from './DataItem'; // Import the child component

const MyComponent = ({ data }) => {
// here data provides us with an array of object of data that we want to render

  const renderDataItems = () => {
    // as jsx can automatically render element of arrays as html elements, we can directly return from map.
    return data.map(item => (
      // specifying a unique key for every component is very important as it helps react to identify items individually, and not update all the items when asked to. This prevents performance issues and unwanted bugs.
      <DataItem key={item.id} item={item} /> 
      // Render the DataItem child component with props , doing this helps to breakdown the logic and make it easily changeable and reusable.
      // We should never use index as the key as it becomes inconsitent when data is changed.
    ));
  };

  return (
    <div>
      <h1>My Data Items:</h1>
      {renderDataItems()} {/* we use a function to return the list of items we want to render */}
      {/* Call the renderDataItems function to render the data items and not directly map here for a clean code */}
    </div>
  );
};

export default MyComponent;

// DataItem.js - Rendering Content Conditionally
import React from 'react';

const DataItem = ({ item }) => {
  return (
    <div>
      <p>ID: {item.id}</p>
      <p>Name: {item.name}</p>
      {item.age && <p>Age: {item.age}</p>}
      {/* && operator allows to Conditionally render age only if it exists, else its false and doesnt render */}
      {item.gender ? <p>Gender: {item.gender}</p> : null}
      {/* We can also use Ternary operator, but in most cases && does the work*/}
    </div>
  );
};

export default DataItem;

import React, { useState } from 'react';

const FormValidationComponent = () => {
  const [formData, setFormData] = useState({
    email: '',
    password: '',
  });

  const [errors, setErrors] = useState({
    email: '',
    password: '',
  });

  const handleInputChange = (e) => {
    const { name, value } = e.target;
    setFormData({ ...formData, [name]: value });
  };

  const handleInputBlur = (e) => {
    const { name, value } = e.target;
    validateInput(name, value);
  };

  const validateInput = (name, value) => {
    let error = '';

    switch (name) {
      case 'email':
        if (!value) {
          error = 'Email is required';
        } else if (!isValidEmail(value)) {
          error = 'Invalid email format';
        }
        break;
      case 'password':
        if (!value) {
          error = 'Password is required';
        } else if (value.length < 6) {
          error = 'Password must be at least 6 characters';
        }
        break;
      default:
        break;
    }

    setErrors({ ...errors, [name]: error });
  };

  const handleSubmit = (e) => {
    e.preventDefault();
    // Perform form submission and additional validation if needed
  };

  const isValidEmail = (email) => {
    // Validate email format, you can use a library or regex for more complex validation
    return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
  };

  return (
    <div>
      <h1>Form Validation Example</h1>
      <form onSubmit={handleSubmit}>
        <div>
          <label>Email:</label>
          <input
            type="email"
            name="email"
            value={formData.email}
            onChange={handleInputChange}
            onBlur={handleInputBlur}
          />
          {errors.email && <p>{errors.email}</p>}
        </div>
        <div>
          <label>Password:</label>
          <input
            type="password"
            name="password"
            value={formData.password}
            onChange={handleInputChange}
            onBlur={handleInputBlur}
          />
          {errors.password && <p>{errors.password}</p>}
        </div>
        <button type="submit">Submit</button>
      </form>
    </div>
  );
};

   import { useReducer } from "react";
   import NameContext from "./name-context";

   const defaultNameState = {
      arr: [],
      amount: 0,
   };

   const cartReducer = (state, action) => {
      if (action.type === "ADD") {
         //your logic 
         // can update the state directly here and action will contain the props sent with the dispatch
         const newArr = state.arr.concat(action.item);
         const updatedAmount = state.amount + 1;
         return {
            arr: newArr,
            amount: updatedAmount,
         };
      }

       // can add checks for other actions here

      return defaultCartState;
      // if the action is not specified default value will be returned.
   };

   const CartProvider = (props) => {
      const [nameState, dispatchNameAction] = useReducer(
         nameReducer,
         defaultNametate
      );

       // we could also use useState if its not complex data
       // we can also skip creating a state here and directly access context , but that brings the limitation to create local states

      const addItemToNameHandler = (item) => {
         dispatchNameAction({ type: "ADD", item: item });
      };

      const removeItemFromNameHandler = (id) => {
         dispatchNameAction({ type: "REMOVE", id: id });
      };

      const clearNameHandler = () => {
         dispatchNameAction({ type: "CLEAR" });
      };

      const cartContext = {
         arr: cartState.arr,
         amount: cartState.amount,
         addItem: addItemToNameHandler,
         removeItem: removeItemFromNameHandler,
         clearCart: clearNameHandler,
      };

      return (
         //this provider gives the access to all the children for the context
         <NameContext.Provider value={nameContext}>
            //the value prop gives access to all the children with the current updated value
            {props.children}
         </NameContext.Provider>
      );
   };

   export default NameProvider;

   <NameProvider>
   //other components that need to use this store
   // if we didnt create this middleware provider of ours , we would have had to add the NameContext.Provider directly where we want and manage a state at that place itself
   // creating a custom provider helps to totally isolate all the functionality at one place.
   </NameProvider> 

   import useContext from 'react';
   import NameContext from '../store/name-context';
   const compo = (props) => {
       const nameCtx = useContext(NameContext);
       // now this component will be automatically reevaluated whenever CartContext changes
       // another to access context is Context.Consumer which needs a different and lengthy setup    

       // to access context data 
       const data = nameCtx.arr;

       // we can update the state simply using the functions we created to dispatch data
       nameCtx.addItem(item);

   }

  import { forwardRef, useImperativeHandle } from 'react';

  const ChildComponent = forwardRef((props, ref) => {
    const [state, setState] = useState(initialValue);

    useImperativeHandle(ref, () => ({
      reset: () => setState(initialValue),
      increment: () => setState(state + 1),
    }));

    return <div>{state}</div>;
  });

  const ParentComponent = () => {
    const childRef = useRef(null);

    return (
      <div>
        <ChildComponent ref={childRef} />
        <button onClick={() => childRef.current.reset()}>Reset</button>
        <button onClick={() => childRef.current.increment()}>Increment</button>
      </div>
    );
  };

  const functionName = useCallback(() => {
//state updates or other code.
  }, []);

   const result = useMemo(() => {
       return array.sort((a,b) => a-b);
   }, [array]);

  class Product extends Component {
      render() {
      return <h2>Product</h2>
      }
  }

  constructor() {
      super()
      this.state = {
          name : "",
          bool : true,
          obj : {...}
      }
  }

    componentDidUpdate(prevProps, prevState){
        if(prevState.value !=== currentValue){
        //other code only executed if this value changed.
        }
    }
    ```



- componentWillUnmount() = Right before component is unmounted = cleanup in useEffect 

### Context in Class based

- Class-based can use Context in almost the same way as functions, only change is how we use it , without useContext() and we can connect only one Context to a single classBased component.

### Error Boundaries - Best use case for class-based

- generally whenever we throw an Error at any place , the app crashes with error , or we use try catch

- For this we can create a new class based component , example named ErrorBoundary and give it a lifecycle method `componentDidCatch()` and this does not have any hook for now.

- `componentDidCatch(){}`  this method is executed whenever any child throws an error.

- Example -



```javascript
  class ErrorBoundary extends Component {
      constructor() {
          super();
          this.state = {hasError: false}
      }
      componentDidCatch(error){
          this.setState({hasError: true})
      }
      render() {
      if(this.state.hasError){
          return <p>Something Went Wrong</p>
      }
          return this.props.children;
      }
  }

    fetch('url').then(response => {
        return response.json(); //this is also delayed and sends a response
    }).then(data => {
        console.log(data.results);   
    });
    ```



- Async/Await 

  - It is a replacement of .then and .catch block , a javascript feature that we can simply use with react.

- We can simply handle loading states and errors to show loading data  or error while requests are being made.

- Handling Errors 

  - 2xx codes = accepts

  - 4xx / 5xx responsed might have to be handled

  - We can catch errors using `.catch`  for .then or try catch for async await

  - **Fetch** dosent give an error for error status codes whereas axios can catch it.

  - To check for errors in fetch , we have to check response.ok or response.status and throw error manually

- Sending POST request

  - fetch takes a second argument with object where we can send post request and the data to be sent



  ```javascript
    fetch('url', {
        method: 'POST',
        body: JSON.stringify(data), //this sends data as json 
        headers:  {
        'Content-type' : 'application/json' //tells that data is json
        }
    })
    ```



## Custom React Hooks

- Custom hooks are simply functions containing stateful logic , that can use other hooks and react state.

- We can use custom hooks when we have shared functionality between multiple components that are using hooks too 

- Usage

  - We should create a separate file for the customhook in a folder named 'hooks' in src, and a must rule is for the function to start with 'use' this tells react that it is going to use other hooks.



  ```javascript
    const useCustom = () => {
        //code with states and hooks that you want to use at multiple places
    }
    ```



  - Now we can simply use our hook based code in our custom hook and use that at the place that we want by simple calling it like a function.

  - When we call a custom hook in a component the state and hooks of it are tied to that specific component, and not common between all components.

  - as the custom hook is tied to the function it is used in , it will re-evaluate on every change of the custom hook too. that is why we can use useCallback & useMemo to save it from re-rendering if needed.

  - we can return any state/number/variable from a custom component the the component that we want to use it in.

  - we can simply pass arguments/parameters to the custom hook for different use cases.

  - dont forget to update the inside hooks of custom hook , with dependencies of props

## Redux - State Management , App wide

- Local State - data in single component = useState and useReducer

- Cross-Component State - affects multiple components = uses props to transfer state [useState, useReducer]

- App-Wide State - state that effects entire app = prop chains , context , redux

- Why Redux in place of Context ?

  - context has some disadvantages that redux can fulfill [therefore we can use both also too]

  - context can become very complex because of multiple providers

  - context is not that performant , not good for high frequency changes

- How does Redux Work

  - Redux always has all the data at one central store [state]

  - Components make a subscription to get the data directly from store

  - To update the store data , we have Reducer Functions which mutate /change the data

  - Components dispatch/trigger actions with descriptions which forward info to reducer and update the store.

### Basic Redux without React

Redux is based on observer pattern , which is the reason why it is able to do state updates as its able to observe all the changes in the state.

1. install - `npm install redux`

2. Create a reducer , with initial state and actions to perform



   ```javascript
     const nameReducer = (state = {counter : 0}, action)=> {
         if(action.type === "..."){
              return {
                 counter: state.counter + 1,
             }
         }
         //and so on state updates
     }
     ```



   - automatically called by redux, always recieves old/existing state and  an action. 

   - it should always return a new state object 

   - it should be a pure function , without any calls / hooks

3. Create a store pointing at the reducer



   ```javascript
     import {createStore} from 'redux'
     const store = createStore(nameReducer);
     ```



4. Create a subcriber which gets triggered on every action and can get state



   ```javascript
     const nameSubscriber = () => {
       const latestState = store.getState();
     }
     ```



   - it is a function does not get any parameter , but can get the state in this because it gets triggered everytime a state update occurs.

5. subscribe your subscriber to the store



   ```javascript
     store.subscribe(nameSubscriber);
     ```



   - this function redux calls everytime state update occurs

6. now simply dispatch your actions to the store to perform actions on state



   ```javascript
     store.dispatch({type: 'increment'});
     ```



   - this calls reducer to run again and that calls the subscriber.

### Using Redux With React

- `npm install redux react-redux` to install redux in the project

- Create a folder "store" in your src for redux

- Create a file to create your store. ex-index.js



  ```javascript
  import {createStore} from 'redux'

  // Create a reducer , with initial state and actions to perform
  const nameReducer = (state = {counter : 0}, action)=> {
       if(action.type === "..."){
           return {
               counter: state.counter + 1,
           }
       }
       //and so on state updates

      return state;
   }

  //automatically called by redux, always recieves old/existing state and an action.
  // it should always return a new state object
  // it should be a pure function , without any calls / hooks

  const store = createStore(nameReducer);

  //this helps us access the specific state.
  export default store;

  //in app.js
  //other imports ...
  import {Provider} from 'react-redux'
  import store from "./store/index";

   ReactDOM.render(
        <Provider store={store}>
            <App />
        </Provider>,
        document.getElementById('root')
   )

  import {useSelector} from 'react-redux';

    const compo = () => {

        const counter = useSelector(state => state.counter);
        //like this we can get specific state data we want too
        //useSelector manages creation of subscription for state updates automatically
        // it creates a subscription when component is mounted and updates the data on its own
        // it also unsubscribes when the component is removed from DOM 

        //... other code
        // can simply use the counter data normally we will get updated on its own

    }

    const dispatch = useDispatch(); 
    //this returns a dipatch function with which we can call the redux store
    const increment = () => {
        dispatch({type: 'increment'});
    }

  import {connect} from 'react-redux';
  class Counter extends Component {
      //we can use the data sent in props props
      incrementHandler() {
          this.props.increment(); //accessing the dispatch function
      } 

      value = this.props.counter; //accessing the value

      //other code...
  }

  const mapStateToProps = state => {
      return {
          counter: state.counter 
          // accessing the redux state , keys are passed as props
      }
  }
  // function names can be anything , but this is conventional
  const mapDispatchToProps = dispatch => {
      // this function will recieve the dispatch
      return {
          increment: () => dispatch({type: 'increment'}),
          decrement: () => dispatch({type: 'decrement'})
      }
  }

  export default connect(mapStateToProps, mapDispatchToProps)(Counter);

  // in the reducer simply access the value using action
  const nameReducer = (state = {counter : 0}, action)=> {
     if(action.type === "ADD"){
         return {
             counter: state.counter + action.amount,
         }
     }
  }

  // now add the data while dispatching from the component
  dispatch({type: 'ADD', amount: 5});
  // this amount will be accessible in the reducer 

  // in the file with your reducer 

  // creating an overall initial state
  const initialState = {
      counter: 0,
      showCounter: true
  }

  const counterReducer = (state = initialState, action) => {

      if(action.type === 'ADD') {
          // never update the existing state directly because it is a reference , it can work but it can cause unforeseen issues
          return {
              counter: state.counter + 1,
              showCounter: state.showCounter
              // we are having to specify the full state because react dosent merge it instead it just updates the overall state with what we return    
              // always return a brand new object with all the properties
          }
      }

      if(action.type === 'toggle'){
          return {
              showCounter: !state.showCounter,
              counter: state.counter
          }
      }

      return state;

  }

  const store = createStore(counterReducer);
  export default store

    import {createSlice, configureStore} from '@reduxjs/toolkit';

    // 1. Create slice for the state you want
    // createSlice wants an object as argument
    const counterSlice = createSlice({
        name: 'counter', //any custom name that you wish,
        initialState : {counter: 0, showCounter: true}, // this gives it an initial value to start with
        reducers: { // this takes all the reducers we need to perform state updates
            increment(state){ // we dont need if checks here , cause we can directly access these 
               state.counter++; // toolkit uses immer which automtically mutates and does the update , so we can directly work with the state without having to copy it
            },
            increase(state,action){
               state.counter += action.payload; // toolkit automatically attaches any data in a property names payload which cannot be changed
            }
        }

    })

    const initialAuthState = {
        isAuthenticated: false,
    }

    const authSlice = createSlice({
        name: 'authentication',
        initialState : initialAuthState, 
        reducers: {
            login(state) {
                state.isAuthenticated = true;
            },
            logout(state) {
                state.isAuthenticated = false;
            }  
        }
    })

    // 2. To let the store know about the slice and reducer to use
    const store = createStore(counterSlice.reducer);
    // this is good if we have just one slice but when we are working with multiple slices

    // 2. We can use configureStore in toolkit, alternative to combineReducers of redux to send multiple reducers in state.
    const store = configureStore({
        // reducer : counterSlice.reducer   // this is in case of one main reducer
        reducer : { 
            counter : counterSlice.reducer,
            auth: authSlice.reducer
        }
        // this can be used for multiple slices, these slices will be merged together and used in store automatically
    });

    export default store;

  counterSlice.actions.increment();
  // toolkit creates action objects when we call these reducers , 
  // which are unique by default, that is why these are also called action creators
  // these returned objects will already have uniques properties automatically being handled behind the scenes.
  // that is why we dont have to worry about creating unique actions and adding if else

  export const counterActions = counterSlice.actions;
  export const authAction = authSlice.actions;

  import {counterActions} from '.../store/index';
  import {useSelector , useDispatch} from 'react-redux'

  const Counter = () => {
      // const counter = useSelector(state => state.counter);
      // if single slice is in store, we can also simply access our redux state in the same old way
      const counter = useSelector(state => state.counter.counter);
      // we have to specify the key of that slice when multiple are set
      const authenticated = useSelector(state => state.auth.isAuthenticated);

      const dispatch = useDispatch();

      const incrementHandle = () => {
          dispatch(counterActions.increment());
      }

      const increaseHandle = () => {
          dispatch(counterActions.increase(10)); 
          // example dipatches {type: UNIQUE_IDENTIFIER, payload: 10}
          // this value is automatically stored in payload and can be access only as payload
          // we can also send any data , array object anything in place of 10 and access as payload
      }    
  }

   import {BrowserRouter} from 'react-router-dom'

   //...


   root.render(

   <BrowserRouter>
       <App />
     </BrowserRouter>
   );

import {Route} from 'react-router-dom';


//import Home component
//import other components
// as above components will be loaded as pages , we can store them
// in a different folder named pages or screens.

function App() {
    return (
        <div>
            <Route path="/">  //yourDomain.com/
                <Home/>
            </Route>
            <Route path="/other"> //yourDomain.com/other
                <Other/>
            </Route>
        </div>
    )
}

export default App;

  import {Link} from 'react-router-dom';


  const Header = () => {

      return (
          <header>
              //nav ul li ...
                  <Link to='/home'>Home</Link>
                  <Link to='/products'>Products</Link>
          </header>
      )

  }

import {NavLink} from 'react-router-dom';

// ... imports , component data ... in the component


<NavLink activeClassName={classes.active} to='/home'>
Home
</NavLink>

  <Route path='/item-details/:itemId'> 
      <ItemDetails/>
  </Route>
  // this means we can load , domain/item-details/anything
  // whatever we write in anything , /item1 , /p2123w 
  // will redirect us to the item details page

  import {useParams} from 'react-router-dom'

  const ItemDetail = () => {
      const params = useParams();

      console.log(params.itemId); 
      //we can access multiple custom params using this key that we provided in the route
      // example /:itemId/:itemPrice
  }

  import { Route, Switch } from 'react-router-dom';
   // Switch makes it that only one of the matched routes are rendedered and not all
  // but because of router functionality , the first one in order will be rendered which is not always right
  // for that we can use exact which will only let the route render if its exactly the same.
  <Switch>
      <Route path='/welcome'>
         <Welcome />
      </Route>
      <Route path='/products' exact>
          <Products />
      </Route>
      <Route path='/products/:productId'>
           <ProductDetail />
      </Route>
  </Switch>

 <section>
    <h1>The Welcome Page</h1>
    <Route path="/welcome/new-user">
      <p>Welcome, new user!</p>
    </Route>
</section>

 // this will allow us to load the paragraph when , the new user route is added to welcome page.

  import {Redirect} from 'react-router-dom';
  // exact is needed or else route will always match and loop it.
  <Route path='/' exact> 
   <Redirect to='/home'>
  </Route>

  <Layout> //containing navbar and props.children to render these inside components
      <Switch>
          <Route path="/home"><Home/></Route>
          <Route path="/other"><Other/></Route>
      </Switch>
  </Layout>

  // add a route at the end of all your routes already created that
  // will catch all the remainging url and render not found page

  // this will work because react matches routes from top to bottom
  <Switch>
      <Route path="/home"><Home/></Route>
      // after all the routes at end add
      <Route path="*">
          <NotFoundPage />
      </Route> 
  </Switch>

  import { useHistory } from 'react-router-dom';

  const Component = () => {
      const history = useHistory();

      history.push('/other'); // sends to next page and allows the user to go back as it is pushed in history
      history.replace('/other'); // sends to the next page and dosent allow the user to go back to previous page
  }

  import { Prompt } from 'react-router-dom';

  <Prompt
      when={isFillingForm} //if this this true then the user will be prompted if he tries to leave the page
      message={(location) =>
        'Are you sure you want to leave? All your entered data will be lost!'
      }
  />

  const match = useRouteMatch();
  const location = useLocation();

history.push(`${location.pathname}?sort=${(Ascending ? 'asc' : 'desc')}`);

//can be written as

history.push({
   pathname: location.pathname,
   search: `?sort=${(Ascending ? 'asc' : 'desc')}`
});

  import {useLocation, useHistory} from 'react-router-dom';

  // in the component
  const history = usehistory();
  const location = useLocation();

  history.push('/page?sort=asc'); 
  // this will still rerender the current component even if we are in it

  const queryParams = new URLSearhParams(location.search);
  // this is built in constructor class of browser, which will return our query params as object

  const sortAsc = queryParams.get('sort') === 'asc';
  // now we can simply use the current param value

    // className can take a function in Navlinks now which will give the nav data to it.

    <NavLink className={(navData) => navData.isActive ? classes.active: ''} to='/home'>
      Home
    </NavLink>
    ```



- **Redirect** is replaced with Navigate



  ```jsx
    <Route path='/' element={<Navigate to='/home'/>} />
    // this will push the next page into history

    <Route path='/' element={<Navigate replace to='/home'/>} />
    // this will work exactly like the old redirect and replace the history
    ```



- Now nested **Route** also need to be wrapped in **Routes** 

- Because of new logic of choosing routes , to access nested routes

  1. we have to write `/home/*` in the main route to catch all the nested routes inside.

  2. in the inner component containg the nested route , we no more have to write `/home/nest` instead normally `nest` in the path and react will understand it because now it is relative.

  3. This also effects the **Links** inside components , their path is also relative now.

- Another option to make nested routes , is by adding Route children in the main app.js



  ```jsx
    // in app.js

    <Route path='/home/*' element={<Home/>}>
        <Route path='other' element={<Other />} />
    </Route>


    // to define where the nested content has to be rendered , in home.js
    // A new component OUTLET is available and will be simply replaced with the nested content

    <section>
        <Outlet/>
        <h1>Hi</h1>
    </section>
    ```



- **useHistory** is no longer there in v6 and is replaced with **useNavigate**



  ```js
    const navigate = useNavigate();
    navigate('/home'); // will push into history
    navigate('/home', {replace: true}); // will redirect/replace in history

    // forward and backward navigation is also possible by
    navigate(-1) // backward
    navigate(1) // forward
    ```



- **Prompt** component also no more exists in v6

- For even more latest and better changes of v6.4 = [React Router 6.4 - Getting Started](https://www.youtube.com/watch?v=L2kzUg6IzxM)

## Deployment

- Before deployment steps 

  1. Test Code

  2. Optimize Code - and add Lazy Loading

  3. Build App for production

  4. Upload Production Code to Server

  5. Configure Server

### Adding Lazy Loading

- Most of the bundles are download on first render of the website.

- Lazy loading is divinding the code into chunks and only downloading them when speicific route/functionality needs it.

- To use it



```js
  import React from 'react';

  const NewPost = React.lazy(() => import('./pages/NewPost'));
  // this will make the app to not import this component right when the app is loaded
  // instead the component will only be downloaded when it is being rendered

  <Route path='/new-post'>
    <NewPost />  // the component is downloaded only when this URL is hit.
  </Route>

  import {Suspense} from 'react';

  <section>
      <Suspense fallback={<p>Loading...</p>}>  // can define any jsx in here
      // other elements/components inside
      </Suspense>
  </section>

  //Greeting.js
  import React from 'react';

  function Greeting({ name }) {
    return (
      <div>
        <h1>Hello, {name}!</h1>
      </div>
    );
  }

  export default Greeting;

  //Greeting.test.js
  import React from 'react';
  import { render,screen } from '@testing-library/react';
  import Greeting from './Greeting';

  test('renders a greeting message with the provided name', () => { 
  //test funtion is offered by jest , and here we write the test name
    const { getByText } = render(<Greeting name="John" />); 
  // we render the component on screen
    const greetingMessage = screen.getByText(/Hello, John!/i);
  // now we select the component
   expect(greetingMessage).toBeInTheDocument(); 
  //expect helps us asser the resulta and check if the check has passed
  });

  describe('Button component', () => {
    test('displays the button label', () => {
      const { getByText } = render(<Button label="Click me" />);
      const buttonElement = getByText(/Click me/i);
      expect(buttonElement).toBeInTheDocument();
    });
  });();
  });

  import React from 'react';
  import { render, screen } from '@testing-library/react';
  import userEvent from '@testing-library/user-event';
  import LoginForm from './LoginForm';

  describe('LoginForm component', () => {
    test('disables the submit button until both fields are filled in', () => {
      render(<LoginForm />);
      const usernameInput = screen.getByLabelText(/username/i);
      const passwordInput = screen.getByLabelText(/password/i);
      const submitButton = screen.getByRole('button', { name: /submit/i });

      // Verify that the submit button is disabled initially
      expect(submitButton).toBeDisabled();

      // Simulate user input in the username field
      userEvent.type(usernameInput, 'johndoe');

      // Verify that the submit button is still disabled
      expect(submitButton).toBeDisabled();

      // Simulate user input in the password field
      userEvent.type(passwordInput, 'password123');

      // Verify that the submit button is now enabled
      expect(submitButton).not.toBeDisabled();
    });
  });

  import React from 'react';
  import { render, screen } from '@testing-library/react';
  import UserList from './UserList';

  describe('UserList component', () => {
    test('fetches and displays a list of users', async () => {
      const mockUsers = [
        { id: 1, name: 'John Doe' },
        { id: 2, name: 'Jane Smith' },
      ];
      //this  replaces the fetch in the window , with a mock function
      // proviced by jest, and then create our own response to work with
      global.fetch = jest.fn(() =>
        Promise.resolve({
          json: () => Promise.resolve(mockUsers),
        })
      );

      render(<UserList />);

      expect(screen.getByText(/user list/i)).toBeInTheDocument();

      const userListItems = await screen.findAllByRole('listitem');
      expect(userListItems).toHaveLength(2);

      // Verify that fetch was called with the correct URL
      expect(fetch).toHaveBeenCalledWith(
        'https://jsonplaceholder.typicode.com/users'
      );

    });

  });