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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript Interview Guide — Basic to Extreme</title>
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.level-intermediate{background:rgba(59,130,246,0.15);color:var(--inter);border:1px solid rgba(59,130,246,0.3)}
.level-advanced{background:rgba(139,92,246,0.15);color:var(--adv);border:1px solid rgba(139,92,246,0.3)}
.level-extreme{background:rgba(239,68,68,0.15);color:var(--extreme);border:1px solid rgba(239,68,68,0.3)}
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<div class="logo">
<button class="mob-menu-btn" onclick="toggleSidebar()">☰</button>
JS <span>Interview Master</span>
</div>
<a href="../index.html" class="back-hub">← Mastery Hub</a>
</div>
<div class="layout">
<div class="sidebar" id="sidebar">
<div class="sb-header">
<input type="text" class="search" placeholder="Search questions..." onkeyup="filterQ()">
</div>
<div class="q-list" id="qList"></div>
</div>
<div class="main" id="mainContent">
<div class="welcome" id="welcomeScreen">
<h1>JS: Basic to Extreme</h1>
<p>A comprehensive deep dive into JavaScript internals, patterns, and senior-level challenges.</p>
<div style="color:var(--accent); font-size: 2rem;">⚡</div>
</div>
<div class="ans-card" id="ansCard" style="display:none"></div>
</div>
</div>
<script>
const questions = [
// ── 1. JAVASCRIPT BASICS ──
{
id: 1,
title: "What is JavaScript?",
level: "basic",
category: "1. Basics",
def: "<strong>JavaScript</strong> is a high-level, interpreted (JIT), multi-paradigm language that serves as the engine of the modern web.",
expl: [
"Dynamic & Weakly Typed: Variables are not bound to a specific type.",
"Single-threaded: Executes one task at a time on the main thread.",
"Non-blocking: Uses the Event Loop to handle asynchronous operations.",
"Prototype-based: Objects inherit directly from other objects."
],
code: `const msg = "JS is everywhere";\nconsole.log(typeof msg);`,
output: "\"string\"",
tip: "Emphasize its 'Single-threaded but non-blocking' nature as a core strength."
},
{
id: 2,
title: "Difference between JavaScript and TypeScript?",
level: "basic",
category: "1. Basics",
def: "TypeScript is a strongly typed superset of JavaScript developed by Microsoft.",
expl: [
"TS provides static typing; JS is dynamic.",
"TS catches type-related bugs at compile time.",
"TS requires a transpilation step (TSC) to run in browsers.",
"TS supports features like Interfaces and Generics more robustly."
],
code: `// TS\nlet id: number = 10;\n// JS\nlet id = 10;`,
output: "// Compiled JS is identical",
tip: "Mention that TS is great for large-scale enterprise applications to reduce runtime errors."
},
{
id: 3,
title: "Difference between var, let, and const?",
level: "basic",
category: "1. Basics",
def: "Declaration keywords with different scoping and hoisting rules.",
expl: [
"var: Function-scoped, hoisted (as undefined), can be redeclared.",
"let: Block-scoped, hoisted to TDZ, cannot be redeclared in same scope.",
"const: Block-scoped, immutable binding (reference cannot change), must be initialized."
],
mermaid: "graph TD\n V[var] --> FS[Function Scope]\n L[let] --> BS[Block Scope]\n C[const] --> BS\n BS --> TDZ[Temporal Dead Zone]",
code: `{\n var x = 1;\n let y = 2;\n}\nconsole.log(x); // 1\nconsole.log(y); // Error`,
output: "1\nReferenceError: y is not defined",
tip: "Explain the 'Temporal Dead Zone' for let/const to show deeper knowledge."
},
{
id: 4,
title: "What are primitive data types?",
level: "basic",
category: "1. Basics",
def: "Basic data types that are not objects and have no methods.",
expl: [
"String, Number, BigInt, Boolean, Undefined, Null, Symbol.",
"Primitives are passed by value and are immutable.",
"All other types (Array, Object, Function) are reference types."
],
code: `let a = 10;\nlet b = a;\nb = 20;\nconsole.log(a); // 10`,
output: "10",
tip: "Remember 'typeof null' is 'object'—it's a known JS bug."
},
{
id: 5,
title: "Difference between null and undefined?",
level: "basic",
category: "1. Basics",
def: "Both represent absence of value, but with different semantic meanings.",
expl: [
"undefined: Variable declared but not assigned a value (automatic).",
"null: Intentional absence of any object value (manual).",
"typeof undefined is 'undefined'; typeof null is 'object'."
],
code: `let x;\nlet y = null;\nconsole.log(x == y); // true\nconsole.log(x === y); // false`,
output: "true\nfalse",
tip: "Use null to explicitly 'clear' a variable or indicate a missing result."
},
{
id: 6,
title: "Difference between == and ===?",
level: "basic",
category: "1. Basics",
def: "Comparison operators with and without type coercion.",
expl: [
"== (Abstract Equality): Compares values after converting to a common type.",
"=== (Strict Equality): Compares both value and type without conversion.",
"Standard best practice is to always use ===."
],
code: `console.log(5 == '5'); // true\nconsole.log(5 === '5'); // false`,
output: "true\nfalse",
tip: "Mention that == is useful only in very specific cases like checking for both null/undefined (val == null)."
},
{
id: 7,
title: "What is type coercion?",
level: "basic",
category: "1. Basics",
def: "The automatic or implicit conversion of values from one data type to another.",
expl: [
"Implicit: Happens automatically (e.g., '5' + 5 = '55').",
"Explicit: Done manually using functions like Number() or String().",
"Tricky cases: [] + [] = '', [] + {} = '[object Object]'."
],
code: `console.log('5' - 2); // 3 (String becomes Number)\nconsole.log('5' + 2); // '52' (Number becomes String)`,
output: "3\n\"52\"",
tip: "Know the difference between '+' (prefers string) and other operators (prefer numbers)."
},
{
id: 8,
title: "Truthy and Falsy values",
level: "basic",
category: "1. Basics",
def: "Values that evaluate to true or false in a boolean context.",
expl: [
"Falsy: false, 0, -0, 0n, '', null, undefined, NaN.",
"Truthy: Everything else (including [], {}, 'false').",
"Common mistake: Empty arrays and objects are truthy."
],
code: `if ([]) console.log("Truthy!");\nif ("") console.log("Won't run");`,
output: "\"Truthy!\"",
tip: "Be careful with 0 and empty strings in conditional checks; use explicit comparison if needed."
},
{
id: 9,
title: "What is Hoisting?",
level: "basic",
category: "1. Basics",
def: "The behavior where declarations are moved to the top of their scope during compilation.",
expl: [
"Function declarations are fully hoisted.",
"Var is hoisted and initialized as undefined.",
"Let/Const are hoisted but remain in the Temporal Dead Zone (TDZ).",
"Initializations (assignments) are NOT hoisted."
],
code: `console.log(a); // undefined\nvar a = 5;\n\nhello(); // "Hi"\nfunction hello() { console.log("Hi"); }`,
output: "undefined\n\"Hi\"",
tip: "Always declare variables at the top of their scope to avoid hoisting confusion."
},
{
id: 10,
title: "Explain scope in JavaScript",
level: "basic",
category: "1. Basics",
def: "The accessibility of variables and functions in different parts of code.",
expl: [
"Global Scope: Accessible everywhere.",
"Function Scope: Accessible only within the function (var).",
"Block Scope: Accessible only within the {} block (let/const).",
"Lexical Scope: Outer variables accessible to inner functions."
],
mermaid: "graph TD\n GS[Global Scope] --> FS[Function Scope]\n FS --> BS[Block Scope]",
code: `const global = "I am global";\nfunction test() {\n const local = "I am local";\n console.log(global);\n}\ntest();`,
output: "\"I am global\"",
tip: "Minimize global variables to prevent naming collisions and memory bloat."
},
{
id: 11,
title: "What is block scope?",
level: "basic",
category: "1. Basics",
def: "Scope restricted to the nearest curly braces {}.",
expl: [
"Introduced with ES6 (let/const).",
"Applies to if-statements, for-loops, and naked blocks.",
"Helps prevent variable leakage from loops and conditions."
],
code: `if (true) {\n let x = 10;\n}\nconsole.log(x); // ReferenceError`,
output: "Uncaught ReferenceError: x is not defined",
tip: "Block scope makes code more predictable and debugging easier."
},
{
id: 12,
title: "Function Declaration vs Expression?",
level: "basic",
category: "1. Basics",
def: "Two ways to define functions with different hoisting behavior.",
expl: [
"Declaration: <code>function name() {}</code>. Hoisted fully.",
"Expression: <code>const name = function() {}</code>. Not hoisted (TDZ).",
"Expressions allow for anonymous functions and immediate use in HOFs."
],
code: `sayHi(); // Works\nfunction sayHi() { console.log('Hi'); }\n\nsayBye(); // Error\nconst sayBye = () => console.log('Bye');`,
output: "\"Hi\"\nUncaught ReferenceError: sayBye is not defined",
tip: "Use expressions (const + arrow) to enforce clean code order (declaration before use)."
},
{
id: 13,
title: "What are Arrow Functions?",
level: "basic",
category: "1. Basics",
def: "A shorter syntax for writing functions introduced in ES6.",
expl: [
"No <code>this</code>, <code>arguments</code>, or <code>super</code> bindings.",
"Cannot be used as constructors (new).",
"Implicit return for single-line expressions."
],
code: `const add = (a, b) => a + b;\nconsole.log(add(2, 3));`,
output: "5",
tip: "Arrow functions are perfect for callbacks and functional methods (map/filter)."
},
{
id: 14,
title: "Arrow vs Regular Functions",
level: "basic",
category: "1. Basics",
def: "Key differences in behavior, especially regarding the 'this' keyword.",
expl: [
"Regular: <code>this</code> is dynamic (depends on how it's called).",
"Arrow: <code>this</code> is lexical (inherited from parent scope).",
"Regular functions have an <code>arguments</code> object; arrows do not.",
"Arrows cannot be constructors; regulars can."
],
code: `const obj = {\n val: 10,\n reg: function() { console.log(this.val); },\n arr: () => console.log(this.val)\n};\nobj.reg(); // 10\nobj.arr(); // undefined`,
output: "10\nundefined",
tip: "Use regular functions for methods on objects, and arrows for internal callbacks."
},
{
id: 15,
title: "What is 'this' in JavaScript?",
level: "basic",
category: "1. Basics",
def: "A keyword referring to the context in which a function is executing.",
expl: [
"Global Context: Window (in browsers).",
"Method Call: The object before the dot.",
"Constructor Call: The newly created instance.",
"Arrow Function: Inherited from the surrounding scope."
],
code: `function checkThis() { console.log(this); }\ncheckThis(); // Window\n\nconst obj = { m: checkThis };\nobj.m(); // obj`,
output: "Window\n{ m: f }",
tip: "Master call(), apply(), and bind() to manually control 'this' context."
},
{
id: 16,
title: "Explain Template Literals",
level: "basic",
category: "1. Basics",
def: "Modern string syntax using backticks `` allowing for interpolation and multi-line strings.",
expl: [
"Use <code>${expression}</code> to inject variables.",
"Supports multi-line strings without escape characters.",
"Can be used for 'Tagged Templates' (advanced)."
],
code: `const name = 'JS';\nconsole.log(\`Hello, \${name}!\\nNext line.\`);`,
output: "\"Hello, JS!\\nNext line.\"",
tip: "Template literals are much cleaner than string concatenation with '+'."
},
{
id: 17,
title: "Map, Filter, and Reduce",
level: "basic",
category: "1. Basics",
def: "Higher-order array methods for functional data processing.",
expl: [
"Map: Returns a new array with transformed elements.",
"Filter: Returns a new array with elements that pass a test.",
"Reduce: Reduces array to a single value (sum, object, etc.).",
"None of these mutate the original array."
],
code: `const nums = [1, 2, 3];\nconst doubled = nums.map(n => n * 2);\nconst evens = nums.filter(n => n % 2 === 0);\nconst sum = nums.reduce((acc, n) => acc + n, 0);`,
output: "// doubled: [2,4,6], evens: [2], sum: 6",
tip: "Map is for 1:1 transformations; Filter is for subsets; Reduce is for anything else."
},
{
id: 18,
title: "forEach vs map",
level: "basic",
category: "1. Basics",
def: "Two ways to iterate over arrays with different return values.",
expl: [
"forEach: Just executes a callback; returns undefined.",
"map: Executes a callback and returns a NEW array.",
"Use forEach for side effects (logging); use map for data transformations."
],
code: `const arr = [1, 2];\nconst a = arr.forEach(x => x * 2);\nconst b = arr.map(x => x * 2);`,
output: "// a: undefined, b: [2, 4]",
tip: "Never use map if you don't intend to use the returned array."
},
{
id: 19,
title: "What are callbacks?",
level: "basic",
category: "1. Basics",
def: "Functions passed as arguments to other functions to be executed later.",
expl: [
"Used extensively in asynchronous operations (timers, events).",
"Allows for asynchronous execution in a single-threaded language.",
"Can lead to 'Callback Hell' if nested too deeply."
],
code: `function greet(name, callback) {\n console.log('Hello ' + name);\n callback();\n}\ngreet('User', () => console.log('Done'));`,
output: "\"Hello User\"\n\"Done\"",
tip: "Callbacks are the foundation, but modern JS prefers Promises and Async/Await."
},
{
id: 20,
title: "Higher-Order Functions",
level: "basic",
category: "1. Basics",
def: "Functions that take other functions as arguments or return them.",
expl: [
"Map, Filter, Reduce are HOFs.",
"Essential for functional programming patterns.",
"Allows for code abstraction and reusability."
],
code: `const multiplier = (factor) => (num) => num * factor;\nconst double = multiplier(2);\nconsole.log(double(5));`,
output: "10",
tip: "Explain that HOFs treat functions as 'first-class citizens'."
},
{
id: 21,
title: "What are Closures?",
level: "basic",
category: "1. Basics",
def: "A function bundled together with references to its surrounding state (lexical environment).",
expl: [
"Inner functions have access to variables defined in their outer scope.",
"This scope persists even after the outer function finishes execution.",
"Used for data privacy (encapsulation) and function factories."
],
mermaid: "graph LR\n Outer[Outer scope] -- Persists --> Inner[Inner Function]\n Inner -- Accesses --> OuterVar[Outer Variables]",
code: `function outer() {\n let secret = 'shh';\n return () => console.log(secret);\n}\nconst tell = outer();\ntell();`,
output: "\"shh\"",
tip: "Closures are the key to modular, private code in JavaScript."
},
{
id: 22,
title: "Explain Lexical Scope",
level: "basic",
category: "1. Basics",
def: "The scope of a variable is determined by its position in the source code.",
expl: [
"Inner functions can access variables in their parent's scope.",
"The engine looks 'upwards' through the scope chain.",
"It is 'static', meaning it doesn't change during runtime."
],
code: `const x = 10;\nfunction a() {\n console.log(x); // Accesses global x\n}\na();`,
output: "10",
tip: "Contrast this with 'Dynamic Scope' (which JS does not have) to impress interviewers."
},
{
id: 23,
title: "Spread Operator (...)",
level: "basic",
category: "1. Basics",
def: "Allows an iterable (array/object) to be expanded in places where zero or more arguments or elements are expected.",
expl: [
"Arrays: <code>const combined = [...a, ...b];</code>.",
"Objects: <code>const newObj = {...oldObj};</code> (Shallow Copy).",
"Function Calls: <code>Math.max(...nums);</code>."
],
code: `const arr = [1, 2];\nconst copy = [...arr, 3];\nconsole.log(copy);`,
output: "[1, 2, 3]",
tip: "Mention it only performs a shallow copy for nested elements."
},
{
id: 24,
title: "What is Destructuring?",
level: "basic",
category: "1. Basics",
def: "A syntax to extract values from arrays or properties from objects into distinct variables.",
expl: [
"Objects: <code>const { name, age } = user;</code>.",
"Arrays: <code>const [first, second] = list;</code>.",
"Can provide default values and aliases."
],
code: `const obj = { x: 1, y: 2 };\nconst { x: xCoord } = obj;\nconsole.log(xCoord);`,
output: "1",
tip: "Use destructuring in function parameters for cleaner, more readable code."
},
{
id: 25,
title: "Default Parameters",
level: "basic",
category: "1. Basics",
def: "Allows formal parameters to be initialized with default values if no value or undefined is passed.",
expl: [
"Syntax: <code>function (a = 10) {}</code>.",
"Default values are evaluated at call time.",
"Only triggered by <code>undefined</code>, not <code>null</code> or <code>0</code>."
],
code: `function greet(name = 'Guest') { console.log(name); }\ngreet();\ngreet(undefined);\ngreet(null);`,
output: "\"Guest\"\n\"Guest\"\nnull",
tip: "Remember that null is considered a 'value' and won't trigger the default."
},
{
id: 26,
title: "Optional Chaining (?.)",
level: "basic",
category: "1. Basics",
def: "Allows reading the value of a property located deep within a chain of connected objects without checking each reference.",
expl: [
"Stops and returns <code>undefined</code> if a reference is nullish.",
"Prevents 'Cannot read property X of undefined' errors.",
"Works for function calls too: <code>obj.method?.()</code>."
],
code: `const user = {};\nconsole.log(user?.address?.city);`,
output: "undefined",
tip: "Essential for handling API responses where some fields might be missing."
},
{
id: 27,
title: "Nullish Coalescing (??)",
level: "basic",
category: "1. Basics",
def: "A logical operator that returns its right-hand side operand when its left-hand side is null or undefined.",
expl: [
"Difference from <code>||</code>: <code>||</code> fails for 0 and ''.",
"<code>??</code> only fails for <code>null</code> and <code>undefined</code>.",
"Great for setting default config values."
],
code: `const count = 0;\nconsole.log(count || 10); // 10\nconsole.log(count ?? 10); // 0`,
output: "10\n0",
tip: "Use ?? when you want to treat 0 or '' as valid values."
},
{
id: 28,
title: "Shallow vs Deep Copy",
level: "basic",
category: "1. Basics",
def: "Ways to copy objects with varying levels of independence for nested properties.",
expl: [
"Shallow: Only the top-level properties are copied. Nested objects share references.",
"Deep: All levels are copied recursively. Changes never affect the original.",
"Methods: <code>{...obj}</code> (Shallow), <code>structuredClone(obj)</code> (Deep)."
],
code: `const original = { a: { b: 1 } };\nconst shallow = { ...original };\nshallow.a.b = 2;\nconsole.log(original.a.b); // 2`,
output: "2",
tip: "Avoid JSON.parse(JSON.stringify()) for deep copies as it loses methods and Symbols."
},
{
id: 29,
title: "What is JSON?",
level: "basic",
category: "1. Basics",
def: "JavaScript Object Notation — a lightweight format for storing and transporting data.",
expl: [
"Language-independent data format.",
"JSON.stringify(): Object -> String.",
"JSON.parse(): String -> Object.",
"Requires double quotes for keys and strings."
],
code: `const str = '{"id": 1}';\nconst obj = JSON.parse(str);\nconsole.log(obj.id);`,
output: "1",
tip: "Remember that JSON cannot store functions or circular references."
},
{
id: 30,
title: "parseInt vs Number()",
level: "basic",
category: "1. Basics",
def: "Two ways to convert strings to numbers with different parsing logic.",
expl: [
"parseInt: Parses until it hits a non-digit character. Can specify radix (base).",
"Number(): Strict conversion. Returns NaN if the string contains non-numeric characters.",
"parseInt('10px') = 10; Number('10px') = NaN."
],
code: `console.log(parseInt('10.5'));\nconsole.log(Number('10.5'));`,
output: "10\n10.5",
tip: "Always specify the radix (usually 10) in parseInt() to avoid legacy octal issues."
},
// ── 2. ARRAYS & OBJECTS ──
{
id: 31,
title: "How do arrays work internally?",
level: "advanced",
category: "2. Arrays & Objects",
def: "JS Arrays are specialized objects that V8 optimizes for performance using different internal modes.",
expl: [
"SMI (Small Integer): Packed contiguous memory for integers.",
"Double: For floating point numbers.",
"Holey: When indices are missing (sparse).",
"Dictionary Mode: Slower hash-map based storage for very sparse or non-indexed properties."
],
mermaid: "graph LR\n P[Packed SMI] --> PD[Packed Double]\n PD --> PH[Packed Holey]\n PH --> D[Dictionary Elements]",
code: `const arr = [1, 2, 3]; // SMI\narr.push(4.5); // Transition to Double\narr[100] = 10; // Transition to Holey`,
output: "// V8 internal state transition",
tip: "Maintain 'Packed' arrays by avoiding holes and consistent types for max speed."
},
{
id: 32,
title: "Mutable vs Immutable Operations",
level: "intermediate",
category: "2. Arrays & Objects",
def: "Difference between changing an existing object versus creating a new one.",
expl: [
"Mutable: push(), pop(), splice(), sort(), reverse().",
"Immutable: concat(), slice(), map(), filter(), spread (...).",
"Immutability is preferred in state management (Redux/React)."
],
code: `const a = [1];\na.push(2); // Mutated\nconst b = [...a, 3]; // Immutable`,
output: "// a: [1, 2], b: [1, 2, 3]",
tip: "Always use immutable operations when dealing with React state to ensure proper re-rendering."
},
{
id: 33,
title: "Remove duplicates from an array",
level: "basic",
category: "2. Arrays & Objects",
def: "Techniques to filter unique elements.",
expl: [
"Set: <code>[...new Set(arr)]</code> (Fastest/Cleanest).",
"Filter: <code>arr.filter((val, i) => arr.indexOf(val) === i)</code>.",
"Reduce: Building an accumulator object/map."
],
code: `const arr = [1, 1, 2, 2, 3];\nconst unique = [...new Set(arr)];`,
output: "[1, 2, 3]",
tip: "Set is O(n) while filter + indexOf is O(n²) Alexandr prefer Set for performance."
},
{
id: 34,
title: "How to flatten nested arrays?",
level: "intermediate",
category: "2. Arrays & Objects",
def: "Reducing multi-dimensional arrays into a single dimension.",
expl: [
"flat(): <code>arr.flat(depth)</code> (Modern).",
"flatMap(): Map and then flatten by 1 level.",
"Recursion: For very old environments or custom logic."
],
code: `const nested = [1, [2, [3]]];\nconsole.log(nested.flat(2));`,
output: "[1, 2, 3]",
tip: "Use Infinity as depth to flatten an array of any depth: arr.flat(Infinity)."
},
{
id: 35,
title: "What is Object Destructuring?",
level: "basic",
category: "2. Arrays & Objects",
def: "Extracting multiple properties from an object into variables in a single statement.",
expl: [
"Saves lines of code.",
"Allows for default values: <code>const { x = 0 } = obj;</code>.",
"Supports renaming: <code>const { x: xPos } = obj;</code>."
],
code: `const user = { name: 'Ali', role: 'Dev' };\nconst { name, role } = user;`,
output: "// name: 'Ali', role: 'Dev'",
tip: "Use it to pluck exactly what you need from large API response objects."
},
{
id: 36,
title: "Object.freeze() vs Object.seal()",
level: "intermediate",
category: "2. Arrays & Objects",
def: "Methods to restrict modifications to an object.",
expl: [
"Seal: Cannot add/delete properties, but CAN modify existing ones.",
"Freeze: Cannot add, delete, OR modify existing properties. (Read-only).",
"Both are shallow—they don't protect nested objects automatically."
],
code: `const obj = { a: 1 };\nObject.freeze(obj);\nobj.a = 2; // Fails silently (or error in strict mode)`,
output: "// obj.a remains 1",
tip: "Freeze is useful for constants and configuration objects that must never change."
},
{
id: 37,
title: "What is Object.create()?",
level: "advanced",
category: "2. Arrays & Objects",
def: "Creates a new object using an existing object as the prototype.",
expl: [
"Allows for explicit prototype linking.",
"Can create an object with NO prototype (Object.create(null)).",
"Useful for inheritance patterns without classes."
],
code: `const proto = { greet: () => 'Hi' };\nconst obj = Object.create(proto);\nconsole.log(obj.greet());`,
output: "\"Hi\"",
tip: "Use Object.create(null) for 'Dictionary' objects to avoid prototype pollution risks."
},
{
id: 38,
title: "in operator vs hasOwnProperty()",
level: "intermediate",
category: "2. Arrays & Objects",
def: "Different ways to check if a property exists on an object.",
expl: [
"in: Checks the object AND its prototype chain.",
"hasOwnProperty: Checks ONLY the object itself (not inherited).",
"Modern alternative: Object.hasOwn(obj, prop)."
],
code: `const obj = { a: 1 };\nconsole.log('a' in obj);\nconsole.log('toString' in obj);\nconsole.log(obj.hasOwnProperty('toString'));`,
output: "true\ntrue\nfalse",
tip: "Always use hasOwnProperty or Object.hasOwn when iterating over properties to avoid inherited junk."
},
{
id: 39,
title: "How does Object Reference work?",
level: "basic",
category: "2. Arrays & Objects",
def: "Objects are stored in memory and variables hold a pointer (reference) to that memory address.",
expl: [
"Copying an object variable only copies the reference.",
"Modifying through one variable affects all references.",
"Comparison (==) checks if both point to the SAME memory address."
],
code: `const a = { x: 1 };\nconst b = a;\nb.x = 2;\nconsole.log(a.x); // 2`,
output: "2",
tip: "This is why you need 'Shallow' or 'Deep' copies to create independent objects."
},
{
id: 40,
title: "Pass by Value vs Pass by Reference",
level: "intermediate",
category: "2. Arrays & Objects",
def: "How data is passed into functions.",
expl: [
"Primitives (Value): Function gets a copy. Changing it inside doesn't affect outside.",
"Objects (Reference): Function gets a reference. Mutating properties affects outside.",
"Wait: Reassigning the whole object inside doesn't affect outside reference."
],
code: `function mut(o) { o.x = 2; }\nconst obj = { x: 1 };\nmut(obj);\nconsole.log(obj.x);`,
output: "2",
tip: "Technically, JS is 'Call by Sharing'. Values are copies, but for objects, the value IS the reference."
},
{
id: 41,
title: "Maps and Sets",
level: "intermediate",
category: "2. Arrays & Objects",
def: "Modern collection types introduced in ES6.",
expl: [
"Map: Key-value pairs where keys can be ANY type (objects, functions).",
"Set: Collection of UNIQUE values.",
"Both preserve insertion order and are iterable."
],
code: `const m = new Map(); m.set({}, 'Val');\nconst s = new Set([1, 1, 2]);`,
output: "// m.size is 1, s.size is 2",
tip: "Use Map for heavy additions/removals and non-string keys."
},
{
id: 42,
title: "Object vs Map",
level: "intermediate",
category: "2. Arrays & Objects",
def: "Comparison of the two key-value storage methods.",
expl: [
"Keys: Object keys are strings/symbols; Map keys can be anything.",
"Size: Map has a .size property; Objects require manual counting.",
"Performance: Map is optimized for frequent additions/removals.",
"Prototypes: Objects have default keys (toString, etc.); Maps are clean."
],
code: `const map = new Map();\nmap.set(123, 'Number key');\nconst obj = { 123: 'Stringified key' };`,
output: "// map[123] vs obj['123']",
tip: "Default to Object for simple data; use Map for complex logic or large datasets."
},
{
id: 43,
title: "How to clone an object deeply?",
level: "intermediate",
category: "2. Arrays & Objects",
def: "Creating a completely independent copy of a nested structure.",
expl: [
"Modern: structuredClone(obj).",
"Legacy/Safe: JSON.parse(JSON.stringify(obj)) - limited.",
"Library: _.cloneDeep (lodash).",
"Manual: Recursive function (complex edge cases)."
],
code: `const original = { a: { b: 1 } };\nconst deep = structuredClone(original);\ndeep.a.b = 99;\nconsole.log(original.a.b); // 1`,
output: "1",
tip: "structuredClone() is built into the platform now. Use it!"
},
// ── 3. FUNCTIONS ──
{
id: 44,
title: "What is Currying?",
level: "intermediate",
category: "3. Functions",
def: "Technique of converting a function that takes multiple arguments into a sequence of functions that each take a single argument.",
expl: [
"Allows for partial application of arguments.",
"Helps in creating reusable utility functions.",
"Foundation for functional composition."
],
code: `const sum = a => b => a + b;\nconst addFive = sum(5);\nconsole.log(addFive(10));`,
output: "15",
tip: "Currying makes code more modular and declarative."
},
{
id: 45,
title: "What is Function Composition?",
level: "advanced",
category: "3. Functions",
def: "Combining two or more functions to produce a new function.",
expl: [
"Math: f(g(x)).",
"Pipes: Data flows through a series of transformations.",
"Reduces intermediate variable noise."
],
code: `const compose = (f, g) => (x) => f(g(x));\nconst add1 = x => x + 1;\nconst double = x => x * 2;\nconst addThenDouble = compose(double, add1);\nconsole.log(addThenDouble(5));`,
output: "12",
tip: "composition is better than nesting functions deeply."
},
{
id: 46,
title: "Explain IIFE",
level: "intermediate",
category: "3. Functions",
def: "Immediately Invoked Function Expression.",
expl: [
"Executed as soon as it is defined.",
"Creates a private scope, preventing global namespace pollution.",
"Used before ES6 modules to encapsulate code."
],
code: `(function() {\n const secret = 'IIFE';\n console.log('Running!');\n})();`,
output: "\"Running!\"",
tip: "Mostly replaced by ESM, but still useful for one-off async initializations."
},
{
id: 47,
title: "What is Memoization?",
level: "advanced",
category: "3. Functions",
def: "An optimization technique that caches the results of expensive function calls.",
expl: [
"Checks if the result for given arguments is already in the cache.",
"Returns cached result if hit; computes and caches if miss.",
"Requires pure functions (same input = same output)."
],
code: `const cache = {};\nfunction slow(n) {\n if (n in cache) return cache[n];\n const res = n * 2; // Expensive work\n cache[n] = res;\n return res;\n}`,
output: "// Second call is O(1)",
tip: "Use memoization for heavy UI calculations or recursive algorithms like Fibonacci."
},
{
id: 48,
title: "Debounce vs Throttle",
level: "advanced",
category: "3. Functions",
def: "Rate-limiting techniques for performance-heavy callbacks (resize, scroll, search).",
expl: [
"Debounce: Wait until the user STOPS for X ms before firing.",
"Throttle: Fire at most once every X ms, regardless of activity.",
"Analogy: Debounce is 'Elevator wait'; Throttle is 'Staggered traffic light'."
],
mermaid: "graph LR\n E[Events] --> D[Debounce]\n D --> |Last Only| F[Fire]\n E --> T[Throttle]\n T --> |Every X ms| F",
code: `// High frequency events\nwindow.addEventListener('resize', debounce(cb, 200));\nwindow.addEventListener('scroll', throttle(cb, 100));`,
output: "// Significant CPU usage reduction",
tip: "Use Debounce for search inputs and Throttle for scroll tracking."
},
{
id: 49,
title: "Implement Debounce Manually",
level: "advanced",
category: "3. Functions",
def: "Coding a debounce utility from scratch.",
expl: [
"Uses a closure to track the timer.",
"Clears previous timer on every call.",
"Starts a new timer that fires the callback."
],
code: `function debounce(fn, delay) {\n let timer;\n return (...args) => {\n clearTimeout(timer);\n timer = setTimeout(() => fn(...args), delay);\n };\n}`,
output: "// Closure holds 'timer' state",
tip: "Be prepared to write this on a whiteboard; it's a very common live coding task."
},
{
id: 50,
title: "Implement Throttle Manually",
level: "advanced",
category: "3. Functions",
def: "Coding a throttle utility from scratch.",
expl: [
"Uses a flag (boolean) or timestamp to track 'cool down'.",
"Only fires if enough time has passed.",
"Ignores calls made during the cool down period."
],
code: `function throttle(fn, limit) {\n let wait = false;\n return (...args) => {\n if (wait) return;\n fn(...args);\n wait = true;\n setTimeout(() => wait = false, limit);\n };\n}`,
output: "// Closure holds 'wait' state",
tip: "Explain that throttle ensures the UI stays responsive without overloading the main thread."
},
{
id: 51,
title: "Sync vs Async Functions",
level: "basic",
category: "3. Functions",
def: "The execution flow of code relative to time.",
expl: [
"Synchronous: Line-by-line. Next line waits for the current one to finish.",
"Asynchronous: Task starts now, finishes later. Doesn't block the rest of the code.",
"JS uses the Event Loop to make async possible on a single thread."
],
code: `console.log(1);\nsetTimeout(() => console.log(2), 0);\nconsole.log(3);`,
output: "1\n3\n2",
tip: "Async is non-blocking, which is why the browser stays responsive during API calls."
},
{
id: 52,
title: "What are Pure Functions?",
level: "intermediate",
category: "3. Functions",
def: "Functions that produce the same output for the same input and have no side effects.",
expl: [
"Predictable and easy to test.",
"Doesn't modify variables outside its scope.",
"Fundamental for React components and Redux reducers."
],
code: `const add = (a, b) => a + b; // Pure\n\nlet total = 0;\nconst addT = (n) => total += n; // Impure (Side effect)`,
output: "// add(2,2) is always 4",
tip: "Purity enables 'Referential Transparency', where a function call can be replaced by its result."
},
{
id: 53,
title: "What are Side Effects?",
level: "intermediate",
category: "3. Functions",
def: "Any interaction with the outside world from within a function.",
expl: [
"Modifying global variables.",
"Console logging or UI updates.",
"API calls (Network I/O).",
"Reading/Writing to the DOM."
],
code: `function updateUI() {\n document.body.innerText = 'Changed';\n}`,
output: "// State outside function changed",
tip: "In React, use useEffect to isolate side effects from the pure rendering cycle."
},
// ── 4. ASYNCHRONOUS JAVASCRIPT ──
{
id: 54,
title: "What is the Event Loop?",
level: "advanced",
category: "4. Async",
def: "The mechanism that allows JS to perform non-blocking I/O operations despite being single-threaded.",
expl: [
"Monitors the Call Stack and the Callback Queue.",
"If the stack is empty, it pushes the first task from the queue to the stack.",
"Consists of Macrotask and Microtask queues."
],
mermaid: "graph TD\n S[Call Stack] --> W[Web APIs]\n W --> Q[Task Queue]\n Q --> |Event Loop| S",
code: `console.log('Start');\nsetTimeout(() => console.log('Timeout'), 0);\nconsole.log('End');`,
output: "\"Start\"\n\"End\"\n\"Timeout\"",
tip: "Explain that the Event Loop is part of the browser environment (Libuv in Node), not the JS engine (V8) itself."
},
{
id: 55,
title: "Explain Call Stack",
level: "basic",
category: "4. Async",
def: "A LIFO (Last-In, First-Out) structure that keeps track of function calls.",
expl: [
"When a function is called, it's pushed to the stack.",
"When it returns, it's popped off.",
"Synchronous code runs entirely on the stack."
],
code: `function a() { b(); }\nfunction b() { console.log('in b'); }\na();`,
output: "\"in b\"",
tip: "A 'Stack Overflow' occurs when you have too many recursive calls without a base case."
},
{
id: 56,
title: "What are Web APIs?",
level: "intermediate",
category: "4. Async",
def: "External functionalities provided by the browser environment.",
expl: [
"Examples: setTimeout, Fetch, DOM Events, Geolocation.",
"They run outside the JS engine and notify the engine via the Task Queue.",
"This is how 'multi-tasking' happens in a single-threaded language."
],
code: `fetch('https://api.com').then(res => console.log(res));`,
output: "// Request handled by browser network thread",
tip: "Distinguish between JS core features (V8) and Browser environment features (Web APIs)."
},
{
id: 57,
title: "Macro task vs Micro task?",
level: "advanced",
category: "4. Async",
def: "Two different queues for asynchronous callbacks with different priorities.",