Episode 7 — DSA with JavaScript / 7.1 — Conditional Statements

7.1 — Interview Questions: Conditional Statements

Beginner

Q1. What is the difference between `==` and `===` in JavaScript?

Answer: == (loose equality) performs type coercion before comparing, while === (strict equality) compares both value and type without coercion.

0 == ""       // true  — both coerced to 0
0 === ""      // false — number vs string
null == undefined   // true
null === undefined  // false
1 == "1"      // true
1 === "1"     // false

Best practice: Always use === and !== to avoid subtle bugs.

C++ note: C++ only has == which compares values of the same type. No coercion happens between unrelated types (compiler error instead).

Q2. What are falsy values in JavaScript? List all of them.

Answer: There are exactly 7 falsy values in JavaScript:

Value	Type
`false`	boolean
`0`, `-0`	number
`0n`	BigInt
`""`	string (empty)
`null`	null
`undefined`	undefined
`NaN`	number

Everything else is truthy, including [], {}, "0", and "false".

// Common gotcha
if ([]) console.log("truthy");    // prints! Empty array is truthy
if ("0") console.log("truthy");   // prints! Non-empty string is truthy

Q3. When should you use `switch` instead of `if-else`?

Answer: Use switch when:

You are comparing one variable against multiple constant values
There are many branches (5+ cases)
You need fall-through behavior (grouping cases)

Use if-else when:

You need range checks (e.g., x > 10)
Conditions involve different variables
Conditions have complex boolean expressions

// switch is cleaner here
switch (command) {
    case "start": start(); break;
    case "stop":  stop();  break;
    case "reset": reset(); break;
    default: unknown();
}

// if-else is better here (range check)
if (score >= 90) grade = "A";
else if (score >= 80) grade = "B";

Q4. What is short-circuit evaluation? Give an example.

Answer: Short-circuit evaluation means the second operand of && or || is only evaluated if needed.

&&: If the left side is falsy, the right side is never evaluated
||: If the left side is truthy, the right side is never evaluated

// Safe property access (short-circuit prevents crash)
const name = user && user.name;

// Default value
const port = config.port || 3000;

// Conditional function call
isReady && initialize();

Why it matters: Prevents runtime errors (accessing properties of null) and enables concise patterns.

Q5. What is the difference between `||` and `??` in JavaScript?

Answer:

|| returns the right side when the left is falsy (includes 0, "", false)
?? returns the right side only when the left is null or undefined

const a = 0 || 10;    // 10 (0 is falsy)
const b = 0 ?? 10;    // 0  (0 is not null/undefined)

const c = "" || "default";   // "default"
const d = "" ?? "default";   // ""

const e = null ?? "fallback";  // "fallback"

Use ?? when 0, "", or false are valid values.

Intermediate

Q6. Explain the ternary operator. When should you avoid using it?

Answer: The ternary operator condition ? valueIfTrue : valueIfFalse is a concise alternative to if-else for expressions that produce a value.

Use it for:

Simple value assignment based on a condition
Inline values in template literals or JSX

Avoid when:

Logic is complex (multi-step operations)
Nesting would be needed (nested ternaries are hard to read)
Side effects are involved

// GOOD
const label = count === 1 ? "item" : "items";

// BAD — too complex, use if-else
const result = a > b ? (c > d ? (e > f ? 1 : 2) : 3) : 4;

Q7. How do you handle multiple conditions efficiently without deep nesting?

Answer: Three techniques:

1. Guard clauses (early return):

function process(data) {
    if (!data) return null;
    if (!data.isValid) return { error: "Invalid" };
    if (data.isProcessed) return data.result;
    return compute(data);
}

2. Object/Map lookup:

const handlers = {
    create: handleCreate,
    read: handleRead,
    update: handleUpdate,
    delete: handleDelete,
};
const handler = handlers[action];
if (handler) handler(payload);

3. Array of conditions:

const rules = [
    { test: (v) => v.length < 3, msg: "Too short" },
    { test: (v) => v.length > 50, msg: "Too long" },
    { test: (v) => !/[A-Z]/.test(v), msg: "Need uppercase" },
];
const errors = rules.filter(r => r.test(value)).map(r => r.msg);

Q8. What is a dangling else problem? How do you prevent it?

Answer: In C/C++, when you omit braces, an else binds to the nearest if, which may not be what you intended:

// Misleading indentation
if (a > 0)
    if (b > 0)
        cout << "both positive";
else
    cout << "a is not positive";  // Actually: "b is not positive"!

The else binds to if (b > 0), not if (a > 0).

Prevention: Always use braces {}:

if (a > 0) {
    if (b > 0) {
        cout << "both positive";
    }
} else {
    cout << "a is not positive";
}

Q9. Implement a function that classifies a triangle by its sides. Handle all edge cases.

Answer:

function classifyTriangle(a, b, c) {
    // Validate inputs
    if (a <= 0 || b <= 0 || c <= 0) return "Invalid: sides must be positive";
    // Triangle inequality theorem
    if (a + b <= c || a + c <= b || b + c <= a) return "Not a triangle";
    // Classify
    if (a === b && b === c) return "Equilateral";
    if (a === b || b === c || a === c) return "Isosceles";
    return "Scalene";
}

string classifyTriangle(double a, double b, double c) {
    if (a <= 0 || b <= 0 || c <= 0) return "Invalid: sides must be positive";
    if (a + b <= c || a + c <= b || b + c <= a) return "Not a triangle";
    if (a == b && b == c) return "Equilateral";
    if (a == b || b == c || a == c) return "Isosceles";
    return "Scalene";
}

Edge cases tested: zero/negative sides, degenerate triangle (a+b=c), floating-point equality issues.

Q10. What is the Luhn algorithm? Implement it using conditionals.

Answer: The Luhn algorithm validates credit card numbers. From the right, double every second digit; if the result > 9, subtract 9. Sum all digits. Valid if sum % 10 === 0.

function luhnCheck(cardNumber) {
    const digits = cardNumber.replace(/\s/g, "").split("").reverse();
    let sum = 0;
    for (let i = 0; i < digits.length; i++) {
        let d = parseInt(digits[i], 10);
        if (i % 2 === 1) {
            d *= 2;
            if (d > 9) d -= 9;
        }
        sum += d;
    }
    return sum % 10 === 0;
}

console.log(luhnCheck("4539 1488 0343 6467")); // true

#include <string>
using namespace std;

bool luhnCheck(const string& card) {
    int sum = 0, count = 0;
    for (int i = card.size() - 1; i >= 0; i--) {
        if (card[i] == ' ') continue;
        int d = card[i] - '0';
        if (count % 2 == 1) {
            d *= 2;
            if (d > 9) d -= 9;
        }
        sum += d;
        count++;
    }
    return sum % 10 == 0;
}

Advanced

Q11. How do you implement a rule engine using conditionals?

Answer: A rule engine evaluates a set of conditions and applies corresponding actions.

const rules = [
    {
        name: "High-value order",
        condition: (order) => order.total > 1000,
        action: (order) => { order.discount = 0.1; order.priority = "high"; },
    },
    {
        name: "Returning customer",
        condition: (order) => order.customer.orderCount > 5,
        action: (order) => { order.discount = (order.discount || 0) + 0.05; },
    },
    {
        name: "Bulk order",
        condition: (order) => order.items.length > 20,
        action: (order) => { order.shippingFree = true; },
    },
];

function applyRules(order) {
    for (const rule of rules) {
        if (rule.condition(order)) {
            rule.action(order);
        }
    }
    return order;
}

This replaces a massive if-else chain with a data-driven approach.

Q12. Explain how conditional logic is used in state machines. Design a traffic light.

Answer:

States: RED → GREEN → YELLOW → RED → ...

    ┌──────┐     ┌───────┐     ┌────────┐
    │ RED  │────▶│ GREEN │────▶│ YELLOW │──┐
    └──────┘     └───────┘     └────────┘  │
        ▲                                   │
        └───────────────────────────────────┘

function nextLight(current) {
    switch (current) {
        case "RED":    return "GREEN";
        case "GREEN":  return "YELLOW";
        case "YELLOW": return "RED";
        default: throw new Error(`Invalid state: ${current}`);
    }
}

// Simulate
let state = "RED";
for (let i = 0; i < 6; i++) {
    console.log(state);
    state = nextLight(state);
}
// RED → GREEN → YELLOW → RED → GREEN → YELLOW

#include <iostream>
#include <string>
using namespace std;

string nextLight(const string& current) {
    if (current == "RED")    return "GREEN";
    if (current == "GREEN")  return "YELLOW";
    if (current == "YELLOW") return "RED";
    throw runtime_error("Invalid state");
}

Q13. How do you handle conditional logic in a functional programming style?

Answer: Use pure functions that map inputs to outputs without mutation:

// Imperative (mutates)
function applyDiscount(order) {
    if (order.total > 100) {
        order.total *= 0.9;  // mutation!
    }
    return order;
}

// Functional (no mutation)
function applyDiscount(order) {
    const discount = order.total > 100 ? 0.9 : 1;
    return { ...order, total: order.total * discount };
}

Use pipe or compose for chains:

const pipe = (...fns) => (x) => fns.reduce((v, f) => f(v), x);

const processOrder = pipe(
    validateOrder,
    applyDiscount,
    calculateTax,
    formatReceipt,
);

Q14. What are optional chaining and nullish coalescing? When do they replace conditionals?

Answer:

// Before (verbose conditionals)
let city;
if (user && user.address && user.address.city) {
    city = user.address.city;
} else {
    city = "Unknown";
}

// After (optional chaining + nullish coalescing)
const city = user?.address?.city ?? "Unknown";

?. short-circuits to undefined if any part of the chain is null/undefined. ?? provides a fallback only for null/undefined (not for 0, "", or false).

When to use: Property access chains on potentially null objects. When NOT to use: When you need to distinguish between "not found" and "empty string."

Q15. Compare error handling strategies: conditionals vs try-catch. When to use each?

Answer:

Strategy	Use case	Example
Conditionals	Expected invalid states	`if (!data) return null;`
try-catch	Unexpected runtime errors	`try { JSON.parse(str) } catch {}`

// Conditional: expected case
function safeDivide(a, b) {
    if (b === 0) return { error: "Division by zero" };
    return { value: a / b };
}

// try-catch: unexpected parsing failure
function parseConfig(json) {
    try {
        return JSON.parse(json);
    } catch (e) {
        console.error("Invalid config JSON:", e.message);
        return null;
    }
}

Rule: Use conditionals for control flow, try-catch for error recovery.

Quick-fire table

#	Question	One-line answer
1	Falsy values in JS?	`false, 0, -0, 0n, "", null, undefined, NaN`
2	`==` vs `===`?	`==` coerces types, `===` does not
3	Default operator?	`
4	Ternary limit?	Avoid nesting more than one level
5	Switch fall-through?	Execution continues without `break`
6	Guard clause?	Early return for invalid input
7	Dangling else?	`else` binds to nearest `if`; use braces
8	`&&` short-circuit?	Right side skipped if left is falsy
9	Object lookup vs switch?	Object lookup when returning values
10	Luhn check?	Double every 2nd digit from right, sum mod 10 = 0

Additional rapid-fire cards

RC-001

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-002

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-003

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-004

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-005

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-006

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-007

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-008

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-009

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-010

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-011

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-012

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-013

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-014

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-015

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-016

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-017

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-018

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-019

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-020

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

RC-021

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-022

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-023

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-024

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-025

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-026

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-027

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-028

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-029

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-030

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-031

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-032

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-033

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-034

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-035

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-036

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-037

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-038

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-039

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-040

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

RC-041

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-042

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-043

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-044

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-045

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-046

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-047

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-048

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-049

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-050

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-051

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-052

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-053

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-054

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-055

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-056

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-057

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-058

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-059

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-060

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

RC-061

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-062

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-063

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-064

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-065

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-066

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-067

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-068

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-069

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-070

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-071

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-072

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-073

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-074

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-075

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-076

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-077

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-078

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-079

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-080

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

RC-081

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-082

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-083

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-084

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-085

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-086

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-087

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-088

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-089

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-090

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-091

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-092

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-093

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-094

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-095

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-096

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-097

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-098

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-099

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-100

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

RC-101

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-102

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-103

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-104

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-105

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-106

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-107

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-108

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-109

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-110

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-111

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-112

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-113

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-114

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-115

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-116

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-117

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-118

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-119

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-120

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

RC-121

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-122

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-123

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-124

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-125

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-126

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-127

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-128

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-129

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-130

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-131

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-132

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-133

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-134

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-135

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-136

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-137

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-138

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-139

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-140

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

RC-141

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-142

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-143

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-144

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-145

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-146

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-147

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-148

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-149

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-150

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-151

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-152

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-153

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-154

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-155

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-156

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-157

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-158

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-159

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-160

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

RC-161

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-162

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-163

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-164

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-165

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-166

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-167

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-168

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-169

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-170

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-171

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-172

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-173

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-174

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-175

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-176

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-177

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-178

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-179

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-180

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

RC-181

Q: What happens when you use = instead of == in an if condition?
A: Assignment returns the assigned value, which is truthy if non-zero — always use ===.

RC-182

Q: Can switch compare strings in JavaScript?
A: Yes, switch uses strict comparison (===) for each case.

RC-183

Q: What is the output of Boolean([])?
A: true — empty array is truthy in JavaScript.

RC-184

Q: How does optional chaining (?.) work?
A: Returns undefined instead of throwing if a property chain has null/undefined.

RC-185

Q: What does NaN === NaN return?
A: false — NaN is not equal to anything, including itself.

RC-186

Q: How do you check for null AND undefined at once?
A: Use value == null (loose) or value === null || value === undefined.

RC-187

Q: Can you use return inside a switch?
A: Yes, return exits the function and implicitly breaks out of the switch.

RC-188

Q: What is a guard clause?
A: An early return at the top of a function to handle invalid or edge cases.

RC-189

Q: How do you compare objects in JavaScript?
A: By reference — two different objects are never === even with same contents.

RC-190

Q: What is the output of typeof null?
A: 'object' — this is a known JavaScript bug from the first version.

RC-191

Q: Can ternary operators have side effects?
A: Yes, but it's a bad practice. Use if-else for side effects.

RC-192

Q: How does C++ handle switch on enums?
A: C++ switch works on integral types and enums, not strings.

RC-193

Q: What is the purpose of default in switch?
A: Handles any case not matched by other case labels.

RC-194

Q: How do you test multiple conditions with &&?
A: Both sides must be truthy; if the first is falsy, the second is not evaluated.

RC-195

Q: What is nullish coalescing operator (??)?
A: Returns right operand only when left is null or undefined (not 0, '', false).

RC-196

Q: How do you avoid deeply nested if statements?
A: Use guard clauses, extract functions, use lookup objects, or use early returns.

RC-197

Q: Can switch handle ranges in JavaScript?
A: No, switch only matches exact values. Use if-else for ranges.

RC-198

Q: What does !!value do?
A: Double negation converts any value to its boolean equivalent.

RC-199

Q: How do you implement early returns?
A: Check invalid conditions first and return immediately, keeping the happy path unindented.

RC-200

Q: What is the difference between if-else and ternary?
A: Ternary is an expression (returns value), if-else is a statement.

7.1 — Interview Questions: Conditional Statements

Beginner

Q1. What is the difference between == and === in JavaScript?

Q2. What are falsy values in JavaScript? List all of them.

Q3. When should you use switch instead of if-else?

Q4. What is short-circuit evaluation? Give an example.

Q5. What is the difference between || and ?? in JavaScript?

Intermediate

Q6. Explain the ternary operator. When should you avoid using it?

Q7. How do you handle multiple conditions efficiently without deep nesting?

Q8. What is a dangling else problem? How do you prevent it?

Q9. Implement a function that classifies a triangle by its sides. Handle all edge cases.

Q10. What is the Luhn algorithm? Implement it using conditionals.

Advanced

Q11. How do you implement a rule engine using conditionals?

Q12. Explain how conditional logic is used in state machines. Design a traffic light.

Q13. How do you handle conditional logic in a functional programming style?

Q14. What are optional chaining and nullish coalescing? When do they replace conditionals?

Q15. Compare error handling strategies: conditionals vs try-catch. When to use each?

Quick-fire table

Additional rapid-fire cards

RC-001

RC-002

RC-003

RC-004

RC-005

RC-006

RC-007

RC-008

RC-009

RC-010

RC-011

RC-012

RC-013

RC-014

RC-015

RC-016

RC-017

RC-018

RC-019

RC-020

RC-021

RC-022

RC-023

RC-024

RC-025

RC-026

RC-027

RC-028

RC-029

RC-030

RC-031

RC-032

RC-033

RC-034

RC-035

RC-036

RC-037

RC-038

RC-039

RC-040

RC-041

RC-042

RC-043

RC-044

RC-045

RC-046

RC-047

RC-048

RC-049

RC-050

RC-051

RC-052

RC-053

RC-054

RC-055

RC-056

RC-057

RC-058

RC-059

Q1. What is the difference between `==` and `===` in JavaScript?

Q3. When should you use `switch` instead of `if-else`?

Q5. What is the difference between `||` and `??` in JavaScript?