Qs: React

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Getting Started

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What are the pros of using Component-Based Architecture?

If we split the UI into isolated and reusable pieces we can build it just like we do with Lego pieces.

That makes development a lot faster and more reliable - we can unit test our components and we eliminate undesired side effects.

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What are the key concepts of a component framework?

Component frameworks, aka component-based frameworks.

• OOP - Components as the main abstraction

• Declarative Programming for simpler/readable component structures (JSX)

• The State Machine pattern - to model and manage data in a deterministic way

• Virtual DOM to make UI reconciliation fast

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Why is React So Fast?

Virtual DOM, Diffing, etc

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What's Declarative Programming?

In react, it is done with JSX.

Simplifies UI construction by allowing developers to declare what the UI should look like, rather than manually manipulating the DOM.

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What is the result of transpiling JSX code to JavaScript?

JSX compiles to React.createElement() calls that build the Virtual Dom.

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What are Code Transpilers?

Code Transpilers are frontend tools that can make code transformations like JSX to JS and TS to JS.

They also add polyfills depending on the source code and what the target environments(users’ browsers) support.

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Explain the term stateless components ?

Pure functions that render DOM-based solely on the properties provided to them.

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Can You Name two types of React components?

• Function component

• Class component

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What are Pure Components in React?

TLDR: a special component type that avoids unnecesarry re-renders by checking if the props and state really changed.

Pure Components are legacy React components written with classes, with React.PureComponent and React.Component.

PureComponent does a shallow comparison on state change: when comparing scalar values it compares their values, but when comparing objects it compares only references.

Use React.PureComponent when you can satisfy any of the below conditions.

• State/Props should be an immutable object

• State/Props should not have a hierarchy

• You should call forceUpdate when data changes

https://legacy.reactjs.org/docs/react-api.html#reactpurecomponent

https://stackoverflow.com/questions/41340697/react-component-vs-react-purecomponent

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Can React components inherit from each other?

No, React favours composition over inheritance - components can assemble in different ways but do not inherit from each other.

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What's the main difference between Props and State?

The main difference is that the State is mutable and Props are immutable.

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Explain the use of 'key' in react list ?

Keys allow you to provide each list element with a stable identity.

Keys should be unique.

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Explain error boundaries?

Error boundaries help you to catch Javascript error anywhere in the child components.

They are most used to log the error and show a fallback UI.

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What are react portals?

Portal allows you to render children into a DOM node. CreatePortal method is used for it.

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When optimizing React code by decreasing "re-rendering time"", what should you focus on?

• Components at the bottom of the component tree - they will most likely re-render the most

• Components that contain Context - they trigger the re-render of all the components connected to that context

• Decrease re-rendering time with useCallback and useMemo

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Explain React Router ?

Routing library allowing you to add new screen flows to your application.

It also keeps URL in sync with what’s being shown on the page.

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What is Context?

React context helps you to pass data using the tree of react components.

It helps you to share data globally between various react components.

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What is a reduction?

In programming, especially in state management (like in React, Redux, or functional programming),

The process of taking a current state and an action, and returning a new state.

Reduction → The process of applying a reducer to state + action.

Reducer → The function that defines how the reduction happens.

Result → A new, updated state.

function reducer(state, action) {
  switch (action.type) {
    case "increment":
      return { count: state.count + 1 };
    case "decrement":
      return { count: state.count - 1 };
    default:
      return state;
  }
}

// Applying a reduction:
const currentState = { count: 0 };
const newState = reducer(currentState, { type: "increment" });
// newState = { count: 1 }

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Define the term Redux in React ?

Redux is a library used for front end development. It is a state container for JavaScript applications which should be used for the applications state management. You can test and run an application developed with Redux in different environments.

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What is the 'Store' feature in Redux?

Redux has a feature called 'Store' which allows you to save the application's entire State at one place. Therefore all it's component's State are stored in the Store so that you will get regular updates directly from the Store. The single state tree helps you to keep track of changes over time and debug or inspect the application.

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What is an action in Redux?

It is a function which returns an action object. The action-type and the action data are always stored in the action object. Actions can send data between the Store and the software application. All information retrieved by the Store is produced by the actions.

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Explain synthetic event in React ?

Synthetic event is a kind of object which acts as a cross-browser wrapper around the browser’s native event. It also helps us to combine the behaviors of various browser into signal API.

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Inline Styling, External Styling

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Pros and Cons of Inline vs External Styling in React?

Inline-Styling in React are written as JavaScript objects and applied directly to elements using the style prop.

Inline-Styling Pros:

• Scoped by default – no risk of style leaks.

• Dynamic styling – can use JS variables and state easily.

• Quick & simple for small components or one-off styles.

Inline-Styling Cons:

• Partial CSS Support - No pseudo-classes (:hover, :focus) or media queries.

• Partial CSS Support - Can’t leverage CSS features like keyframe animations or complex selectors.

• Verbose – not ideal for large components.

• Harder to maintain and re-use styles.

External styling refers to using CSS files, CSS Modules, or CSS-in-JS libraries.

External-Styling Pros:

• Full CSS support (pseudo-classes, media queries, animations).

• Cleaner & reusable – easier to manage across large apps.

• Separation of concerns – keeps JSX less cluttered.

• Works well for team projects.

External-StylingCons:

• Might need build setup (CSS Modules, CSS-in-JS).

• Slightly more overhead than inline styles.

• Scoped CSS (with CSS Modules) requires different syntax.

TLDR:

• Inline styles → Quick fixes, dynamic per-element styling, or when styles depend heavily on props/state.

• External styles (CSS/CSS-Modules/CSS-in-JS) → Larger projects, reusable styles, complex UI with pseudo-classes and media queries.

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What are CSS Modules ?

Regular CSS files, but class names are locally scoped by default using unique hashes.

Prevents naming conflicts without extra setup.

Pros:

• Full CSS support.

• Scoped CSS by default → no global pollution.

• Familiar CSS syntax.

Cons:

• Styling is separate from component logic (JSX) → potential context change inefficiency.

• Dynamic styles (based on props) can be tricky → usually require multiple classes or inline styles.

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What's CSS-in-JS ?

CSS-in-JS = Styled Components.

Uses JavaScript to create styled React components with template literals.

Styles are scoped automatically.

Pros:

• Full CSS support.

• Scoped CSS by default → no global pollution.

• Familiar CSS syntax.

• Styles live with the component → easy to manage per-component styles.

• Dynamic styling is straightforward using props.

• Encourages component-driven styling → aligns with React philosophy.

Cons:

• Slight runtime overhead (injecting styles in JS).

• CSS syntax inside JS → may feel less familiar.

• Can increase bundle size if heavily used.

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You are working on a big front-end code base. In the last releases, you had several bugs due conflicts between CSS classes with the same name. Some CSS classes were used and extended in different parts of the code leading to unpredictable element styles. Which of the following would solve the problem?

Adding JSS (CSS in JS) which eliminates conflicts by generating unique class names(using the component/file name).

JSS (CSS in JS) automatically prefixes the CSS classes with a unique hash generated at build time.

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Components

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What are Default Props in React Components ?

Default props allow you to specify fallback values for props if none are provided.

 function Button({ label = "Click me", color = "blue" }) {
  return <button style={{ backgroundColor: color }}>{label}</button>;
 }

 <Button /> // renders "Click me" with blue background
 <Button label="Submit" /> // renders "Submit" with blue background

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What's PropTypes in React ?

prop-types is a package for runtime type checking of props.

It helps catch bugs by warning if props are the wrong type.

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What are Controlled vs Uncontrolled components ?

Refers to how form inputs are managed and where their state lives.

1. Controlled Components

React fully controls the form input’s value through state.

The input’s value is accessed and set via useState. The source of truth is React state.

<input type="text" value={myVar} onChange={(e) => setMyVar(e.target.value)} />

Pros:

• Predictable, React always knows the current value.

• Easy to validate, transform, or reset inputs.

• Best for complex forms.

Cons:

• More code, and re-renders happen on every keystroke.

2. Uncontrolled Components

An uncontrolled component lets the DOM itself handle the form state, instead of React.

The input’s value is accessed via a ref when needed. The source of truth is the DOM.

Pros:

• Less code, simpler for small forms.

• No re-renders on every keystroke.

Cons:

• Harder to enforce validation or sync UI with input state.

• React doesn’t “know” the current value until you read it manually.

3. Rule of thumb

Use controlled components when you need React to always “know” and manage the input value (validation, dynamic inputs, UI sync).

Use uncontrolled components for quick forms or when you don’t need real-time validation.

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What's a "pure function" in React ?

TLDR: deterministic + no side effect.

A function that, given the same inputs, always returns the same output and does not produce side effects.

In React, we use function components so this means:

• A component should return the same JSX for the same props and state.

• It should NEVER modify external variables, directly change props, or cause unintended side effects during rendering.

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Why React prefers purity

React’s rendering model relies on being able to re-render components predictably.

If components are pure:

• React can optimize rendering with tools like memoization (React.memo) and concurrent rendering.

• Debugging and testing are easier, since the UI is just a predictable function of state + props.

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Can you give examples of "impure functions" in React ?

TLDR: NON-deterministic + HAS side effect.

Some operations are inherently impure:

• data fetching

• logging

• subscriptions

• DOM manipulation

React’s approach is to isolate these side effects:

• Class components: use lifecycle methods (componentDidMount, componentWillUnmount, etc).

• Function components: use hooks (useEffect, useLayoutEffect).

This way, the render phase (component body) stays pure, while effects are handled after rendering.

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Impure component example:

 function MyComponent({ name }) {
  console.log("Rendering..."); // side effect inside render
  return <h1>Hello {name}</h1>;
 }

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Pure render + side effect in useEffect:

 function MyComponent({ name }) {
  useEffect(() => {
   console.log("Mounted or updated"); // side effect isolated
  }, [name]);

  return <h1>Hello {name}</h1>;
 }

So the best way to phrase it:

• React components should be pure functions of their props and state during rendering.

• Side effects should be moved into hooks (or lifecycle methods).

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Difference between functional and class components?

These are the two main ways to define components.

1. Definition Differs

 function Greeting(props) {    // Functional Component
  return <h1>Hello, {props.name}!</h1>;
 }

 class Greeting extends React.Component {   // Class Component
  render() {
    return <h1>Hello, {this.props.name}!</h1>;
  }
 }

1. State Management Differs

 function Counter() {    // Functional Components are originally stateless
  const [count, setCount] = React.useState(0);   // but with React Hooks (like useState, useEffect), they can manage state and lifecycle logic
  return <button onClick={() => setCount(count + 1)}>Clicked {count} times</button>;
 }

 class Counter extends React.Component {
  state = { count: 0 };
  render() {
    return (
      <button onClick={() => this.setState({ count: this.state.count + 1 })}>   // Class Components: State updated with this.setState()
        Clicked {this.state.count} times    // Class Components: State getter is this.state 
      </button>
    );
  }
 }

1. Lifecycle Methods

Functional Components : use hooks like useEffect to handle lifecycle behavior (mount, update, unmount).

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

Class Components : Use lifecycle methods such as componentDidMount, componentDidUpdate, componentWillUnmount.

 componentDidMount() { console.log("Mounted"); }
 componentWillUnmount() { console.log("Unmounted"); }

1. Simplicity

Functional Components are simpler:

• Easier to write and read.

• Encourages cleaner and more modular code.

• No need to worry about this binding.

Class Components:

• More verbose.

• Requires this keyword for state and props.

• Can get harder to manage in large projects.

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State Management

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The relationship between state and the UI is said to be deterministic. What does that mean?

1. for each version of the state, there is a unique version of the UI(User Interface)

2. the state is the source of truth and the UI is a function of that state

Modern JS frameworks circle around the idea of "the UI as a function of state".

This simplifies development and testing because is easy to predict how the application will look like, if we know the state.

The opposite is having arbitrary UI changes and heavily imperative code (e.g. jQuery) which result in unpredictable UIs and bloated spaghetti code.

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What's the Best Practice for Sate Management?

Focus on Essential State: Identify and manage only the essential state.

Optimal State Placement: Instead of lifting state too high, place state in the lowest common ancestor component.

Leveraging Native Framework Features: Before turning to third-party libraries, fully utilize the native state management features of your framework. For React: useState, useReducer, and Context API.

Multiple Context Providers: Use multiple context providers to manage different pieces of global state, i.e. one for language, one for auth, one for theme. This approach reduces unnecessary re-renders and keeps your application more performant.

Use Reducers for Complexity: For complex state logic, especially when dealing with nested state or multiple state transitions, employ reducer patterns (like useReducer in React) to simplify state management and enhance readability.

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What are some State Management Anti-Patterns?

Overstoring State: Storing both essential and derived state variables in your state management system, leading to redundancy, unnecessary complexity, and potential performance issues

Lifting State Too High

Premature Use of State Libraries

Overloading a Single Context Provider: Consolidating all global states into a single provider, causing unnecessary re-renders across multiple components.

Neglecting Reducer for Complex State: not using the reducer pattern for complex state transitions, leading to convoluted state management by abusing simpler state hooks like useState or useEffect.

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What's the difference between essential state vs derived state?

essential state = independent state

derived state = calculated from other state variables

Redundant state is a derived state that is treated as essential state.

Redundant state will result in convoluted components that re-render too many times due to unecesarry state updates.

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What does "Lifting State Up" mean?

When multiple components need access to the same state, the state is 'lifted up' to their common ancestor and passed down as props.

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What's Prop Drilling?

When state is passed down from a parent component through various levels of nested child components as props.

This can lead to scenarios where intermediate components, which do not directly need the state for their own purposes, still have to pass it down to their children.

It can result in bloated components making the code more coupled, less testable, and more brittle.

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What's the Best Practice when using Context API?

Use of Context and Providers for distributing global state (like authentication data or translation settings) across multiple components.

These can be placed at different points in the component tree, not just at the top level.

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Our application requires users to authenticate for them to see our premium content. We want an easy way to show/hide certain components depending on if the user is logged in or not. What do you think is the cleanest(least coupled) way to do that?

Add our auth state to React.Context (useContext())and subscribe our components to it.

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What's useReducer() ?

useReducer is a React Hook that provides an alternative way to manage component state, particularly useful for more complex state logic.

It allows you to manage state using a reducer function, similar to how Redux manages state. The useReducer hook returns the current state and a dispatch function that triggers state updates based on actions

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What are 3rd-Party State Management Libraries?

These libraries offer pre-built solutions for managing complex application state and/or complex state transitions.

They often implement the Observer Pattern.

• Known libraries: Redux, XState, Recoil, Zustand.

• Cons: adds another dependency.

TLDR: shiny thing you might not need.

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React Hooks

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Does order matter for React Hooks?

Yes.

React doesn’t track hooks by their variable names.

Instead, it keeps an internal array of hook states for each component in the order they’re called.

So if hooks are called in a different order between renders, React’s mapping breaks.

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What Are The Rules of Hooks?

1. Always call them in the same order for every render.

2. Call hooks only at the top level — never inside conditionals, loops, or nested functions.

Calling hooks inside conditionals could change the order, so point 2 refers to the same goal as point 1.

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Do "infinite loops in hooks" happen often ?

Yes. Infinite loops in React hooks are a common pitfall.

Especially with useEffect, useMemo, and useCallback.

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Why do infinite loops happen in hooks?

React components re-render when state or props change.

If a hook (like useEffect) updates state without proper dependency control, it keeps triggering re-renders → infinite loop.

 function BadComponent() {          // Infinite Loops Example
  const [count, setCount] = useState(0);

  useEffect(() => {
    setCount(count + 1);   // Infinite loop: setCount triggers re-render every time
  }, [count]);

  return <p>{count}</p>;
 }

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How to avoid infinite loops in hooks?

1. Use Correct Dependencies

   • Always list only what’s necessary in the dependency array.

   • If you leave it out, it may run every render.

   • If you put too much, it may loop unnecessarily.

2. Use Functional State Updates

   • sets state based on the previous state, setCount(prev => prev + 1);

3. Don’t Derive State From State Inside Effects

   • use useMemo instead

4. Be Careful With Asynchronous Calls

   • If an effect triggers an async call that updates state, you may loop.

   • Solution: use guards (conditions) or cancelation

   • Why: prevent state updates or side effects from running after a component unmounts (removed from the DOM)

 useEffect(() => {
  let isMounted = true;    // guard flag

  async function loadData() {
    const data = await fetchSomething();
    if (isMounted) setValue(data);     // condition, only update if component is "still" mounted
  }

  loadData();
  return () => { isMounted = false };
 }, []); // condition, only runs once

1. Memoize Functions Passed as Dependencies

   • If a function is defined inside component (inline), it'll be re-created on every render

   • useEffect sees a "new" dependency → runs again → re-renders (if state updates inside) → infinite loop.

   • Fix with useCallback.

 function MyComponent() {
  // const fetchData = async () => {
  //  const res = await fetch("/api/data");
  //  return res.json();
  // };
  // below is the correct way - above is the former version that creats the infinite loop
  const fetchData = useCallback(async () => {
    const res = await fetch("/api/data");
    return res.json();
  }, []);

  useEffect(() => {
    fetchData();
  }, [fetchData]); // 🚨 redefined each render
 }

• Rule of Thumb

  • If fetchData is external (defined outside component) and stable → [fetchData] or [] is fine.

  • If fetchData is inside component → wrap in useCallback, then use it in deps.

  • If fetchData has no dependencies → prefer [].

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Performance Opimization

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Top 10 React Optimizations Techniques

1. Use React.memo and Pure Component

2. Use useCallback and useMemo Hooks

3. Code Splitting (Dynamic Import)

4. Avoid Inline Functions and Objects in JSX

5. Optimize Lists with key and React.memo

6. Lazy Load Images and Components

7. Avoid Reconciliation with Proper State Management

8. Use Production Build

9. Debounce or Throttle Expensive Operations

10. Use React Profiler & DevTools

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What's the difference between useMemo() vs useCallback() ?

Both are hooks for caching, for performance optimization, but they serve slightly different purposes.

• useMemo() → caches the result of a function

• useCallback() → caches the function itself (not the result).

HOF / wrapper ???

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useMemo() Use Case and Basic Example?

Memoizes/Caches the result of a function.

Use case: When you have an expensive calculation and don’t want to recompute it unless dependencies change.

 const expensiveValue = useMemo(() => {
  console.log("Calculating...");
  return count * 2;     // imagine a heavy computation here
 }, [count]);        // calculation only runs when count changes

HOF / wrapper ???

for the result of a function to be memoized with useMemo(), it should be:

• deterministic: the same inputs always produce the same outputs ???

• pure: it should not have side effects ???

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useCallback() Use Case and Basic Example?

Memoizes/Caches the function itself (not the result).

Use case: When you pass callbacks to child components.

<CoolDisplay onClick={increment} />

CoolDisplay component will be re-rendered whenever increment is re-created, typically every time its parent component re-renders.

We can avoid that by caching increment since it's not a changing value.

HOF / wrapper ???

for a function to be memoized with useCallback(), it should be BOTH:

• deterministic: the same inputs always produce the same outputs ???

• pure: it should not have side effects ???

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What's the difference between React.memo() vs useCallback() ?

Both are hooks for caching functions, for performance optimization, but they serve slightly different purposes.

• useCallback() → caches a regular function

• React.memo() → caches a react function component

HOF vs HOC / wrappers ???

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What's React.memo() ?

Memoizes/Caches the rendered output of a react function component, meaning:

• React skips re-rendering the component if its props are the same as the previous render.

• If props change, it will re-render normally.

Performance optimization.

Prevents unnecessary re-renders of functional components.

React.memo() is a HOC: it's a wrapper / intersector.

for a react function component to be memoized with useCallback(), it should be BOTH:

• deterministic: the same inputs always produce the same outputs ???

• pure: it should not have side effects ???

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How: Code example for React.memo() ?

 import React from 'react';

 const MyComponent = React.memo(function MyComponent({ value }) {
  console.log('Rendered!');
  return <div>{value}</div>;
 });

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Why: Real-Life Use Cases For React.memo() ?

Components re-renders can hurt performance, especially for:

• Large component trees

• Components with expensive rendering logic (e.g., heavy calculations, complex UI)

• Lists of items that rarely change

These are common examples where React.memo() is especially useful.

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How does React.memo() compare the props?

By default, React.memo() does a shallow comparison of props.

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React.memo() comparing props: Is it possible to change the default behavior?

Yes, via a custom comparison function.

 const MyComponent = React.memo(
  ({ user }) => {     // function component
    console.log('Rendered!');
    return <div>{user.name}</div>;
  },
  (prevProps, nextProps) => {    // custom comparison function
    return prevProps.user.id === nextProps.user.id;    // only re-renders if user.id changes, even if other properties in user change
  }
 );

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Testing

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In a react app, what are some easiest components to Unit Test? What does it imply?

Components located at the bottom of the component tree are generally easier to Unit Test.

They tend to be stateless, simply rendering props, which makes them straightforward for testing.

This implies we test these first and then gradually move up the tree to test more components.

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What happens when as you move up the component tree when Unit Testing?

Testing complexity increases due to dependencies on child components.

In such cases, Unit Tests may become cumbersome, and it's better to switch to End-To-End Testing.

---

To make components easier to test we should:

1. Keep component dependencies minimal

2. Avoid using global state (unless necessary)

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Common Patterns

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How do you handle file uploads in React forms ?

TLDR: React just helps you access the file object. Uploading is handled by the browser via FormData and a request to the server.

React doesn’t manage file contents directly — the browser handles that.

React gives you access to the selected file(s) through event.target.files.

 function FileUploadForm() {
  const [file, setFile] = React.useState(null);

  const handleFileChange = (e) => {
    setFile(e.target.files[0]);      // get the first file
  };

  const handleSubmit = (e) => {
    e.preventDefault();
    if (!file) return;

    const formData = new FormData();   // FormData is a built-in browser API
    formData.append("myFile", file);

    // Send to backend
    fetch("/upload", {
      method: "POST",
      body: formData,
    })
      .then((res) => res.json())
      .then((data) => console.log("Success:", data))
      .catch((err) => console.error("Error:", err));
  };

  return (
    <form onSubmit={handleSubmit}>
      <input type="file" onChange={handleFileChange} />
      <button type="submit">Upload</button>
    </form>
  );
 }

Unlike text inputs, <input type="file"> is uncontrolled in React:

• You CANNOT set its value via React state (security reasons).

• You handle changes via onChange and keep references to files instead.

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How do you debounce/throttle input in React ?

TODO

What ?

Limit how often an expensive operation (like an API call, input handler, or scroll listener) is executed in response to rapidly firing events.

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TODO

redux store vs context api?

can u use hooks in class components?

Handling form submission with preventDefault

useRef for DOM access vs persistent values

Custom hooks – when and why to create them

useReducer vs useState

useLayoutEffect vs useEffect