react_native_prep.md

React Native + Advanced JavaScript Interview Questions

JavaScript Fundamentals

Question	Answer
What is the difference between == and ===?	== performs type coercion, === checks both type and value strictly.
Difference between null and undefined	null = assigned empty value, undefined = declared but not assigned.
What is Hoisting?	JS moves variable and function declarations to the top before execution.
What is the Temporal Dead Zone (TDZ)?	Period between block start and initialization of let/const where access throws error.
Can you redeclare let and const variables?	No — redeclaring them in the same scope throws a SyntaxError.
Does const make values immutable?	No — only prevents reassignment; internal mutation allowed.
What is Strict Mode?	"use strict" enforces safer, more predictable code execution.
What is Scope?	Defines where variables are accessible during runtime.
What is the void operator?	Evaluates an expression but always returns undefined.
Key features of ES6	let, const, arrow functions, modules, destructuring, classes, promises, etc.

Functions & Closures

Question	Answer
What is a Closure?	Function that retains access to its outer scope after it executes.
What is an Anonymous Function?	Function without a name, often used as callbacks.
What is a First-Class Function?	Functions are treated as values — can be passed or returned.
What is a Higher-Order Function?	Takes or returns other functions; used in functional programming.
What is Currying?	Converts multi-arg function into nested single-arg functions.
Example of Currying in JS	const add = a => b => c => a + b + c;
What are Arrow Functions?	Lexically bind this; cannot be constructors or generators.
What is this keyword?	Refers to current context of execution; call-site dependent.
Difference between call, apply, and bind	call() and apply() invoke immediately; bind() returns new bound function.

Objects & Operators

Question	Answer
What is a Proxy object?	Intercepts object operations (get/set/has) for custom behavior.
How to copy properties between objects?	Object.assign(target, source) merges properties.
How to get object keys?	Object.keys(obj) returns array of property names.
What is Object.freeze() used for?	Makes object immutable by preventing modifications.

Arrays & Iteration

Question	Answer
What is Array.slice()?	Returns shallow copy of portion of array (non-mutating).
What is Array.splice()?	Adds/removes elements and mutates the original array.
What is Rest Parameter?	Collects remaining args into an array using ...args.
What happens if rest parameter isn’t last?	Syntax error — it must be last parameter.
What is Spread Operator?	Expands iterable values into function arguments or array literals.
Purpose of compareFunction in sort()	Defines custom sort order; (a, b) => b - a for descending.
How does synchronous iteration work?	Uses Symbol.iterator and next() for controlled iteration.

Async JavaScript

Question	Answer
What is the Event Loop?	Continuously checks call stack and queues for pending tasks.
What is the Call Stack?	Tracks function execution order (LIFO).
What is the Event Queue / Macrotask Queue?	Stores async callbacks waiting for execution.
What is a Promise?	Represents future completion or failure of an async task.
Why do we need Promises?	Handles async flow cleanly; avoids callback hell.
What is Callback Hell?	Nested callbacks making code unreadable.
What is Promise Chaining?	Sequential .then() calls to execute async tasks in order.
What is Promise.all()?	Resolves when all promises succeed or rejects if one fails.
What is Promise.race()?	Resolves/rejects when first promise settles.
What is Memoization?	Caching function results to optimize performance.

Functional Concepts

Question	Answer
What is a Pure Function?	Always returns same output for same input, no side effects.
Difference between Debounce and Throttle	Debounce waits until idle; Throttle limits execution frequency.
What is Currying?	Breaks function with multiple args into chained single-arg calls.

React Native Architecture

Question	Answer
How does reconciliation differ from React DOM?	RN maps components to native views, not HTML DOM.
How does the bridge affect render performance?	Async JSON bridge adds latency; JSI removes overhead.
What’s new in RN architecture (Fabric, JSI, TurboModules)?	Direct JS-native access, concurrent UI, lazy native modules.
What is Yoga?	RN’s Flexbox layout engine written in C++.
What are TurboModules?	Next-gen native modules using JSI, type-safe and lazy-loaded.
What is Hermes Engine?	Optimized JS engine improving startup and memory.
What is Fabric?	Modern rendering system improving concurrency and layout.
What is Flipper?	Debugging tool for network, layout, and performance.
What is Watchman?	File watcher used by Metro bundler for rebuilds.

Performance, Debugging & Security

Question	Answer
How to detect memory leaks in RN?	Use Flipper Memory, Android Profiler, cleanup listeners/timers.
How to optimize startup time?	Lazy imports, Hermes, inline requires, smaller bundles.
How to optimize bundle size?	Tree-shaking, dynamic imports, compress assets.
App crashes only in Android release build — what to check?	Minification issues, Hermes mismatch, native errors.
What is SSL Pinning?	Embeds server cert to prevent MITM attacks.
How to secure OTA updates?	Code signing, encrypted storage, avoid JS secrets.
How to handle offline-first design?	Cache data, queue requests, sync when online.

React Native APIs & Features

Question	Answer
What is Deep Linking?	Opens app via custom URLs (e.g., myapp://profile/1).
How to implement background tasks?	Use Headless JS or native background-fetch modules.
How to implement accessibility?	Use accessible, accessibilityLabel, accessibilityRole.
What is SafeAreaView?	Keeps content away from notches/status bars.

Components, State & Hooks

Question	Answer
Stateful vs Stateless Components	Stateful have internal state; Stateless rely on props only.
PureComponent vs React.memo	Both skip re-renders via shallow comparisons.
Controlled vs Uncontrolled Components	Controlled managed by React; Uncontrolled by native UI.
useLayoutEffect vs useEffect	useLayoutEffect runs sync after layout, useEffect async after paint.
How to persist app state?	AsyncStorage, Redux Persist, Recoil Persist, MMKV.
What is forwardRef?	Passes ref to child component or native view.

Native Modules & Background Behavior

Question	Answer
Example of a Native Module	Use @ReactMethod in Java/Kotlin and access via NativeModules in JS.
What are TurboModules?	JSI-based modules — faster, lazy-loaded, type-safe.
How does setTimeout/setInterval behave in background?	Suspended or delayed; use native background services for reliability.