Episode 7 — DSA with JavaScript / 7.1 — Conditional Statements

7.1 — Quick Revision: Conditional Statements

<< Overview

Cheat sheet

if / if-else / if-else-if

if (condition) { ... }

if (condition) { ... } else { ... }

if (cond1) { ... }
else if (cond2) { ... }
else { ... }

if (condition) { ... }
else if (condition2) { ... }
else { ... }

switch

switch (expr) {
    case val1: /* ... */ break;
    case val2: /* ... */ break;
    default: /* ... */
}

Ternary

const result = condition ? valueA : valueB;

Logical operators

a && b    // AND — short-circuits on falsy
a || b    // OR  — short-circuits on truthy
a ?? b    // Nullish coalescing — falls through only on null/undefined
!a        // NOT

Falsy values (JS)

false | 0 | -0 | 0n | "" | null | undefined | NaN

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

== vs ===

Expression=====
0 == ""truefalse
null == undefinedtruefalse
1 == "1"truefalse
NaN == NaNfalsefalse

Rule: Always use === and !==.

Guard clause pattern

function process(data) {
    if (!data) return;           // guard
    if (!data.valid) return;     // guard
    // happy path
}

Object lookup vs switch

// Switch
switch (key) {
    case "a": return 1;
    case "b": return 2;
    default: return 0;
}

// Object lookup (cleaner for value mapping)
const map = { a: 1, b: 2 };
return map[key] ?? 0;

Common pitfalls

  1. = vs == vs === — assignment vs loose vs strict
  2. Missing break in switch — causes fall-through
  3. Dangling else — binds to nearest if without braces
  4. || vs ?? — || treats 0 and "" as falsy
  5. Nested ternaries — hard to read, use if-else instead
  6. Not handling default in switch — always include it
  7. Forgetting NaN !== NaN — use Number.isNaN()
  8. Type coercion surprises — [] == false is true

Decision tree: which conditional to use?

Need to return a value from the condition?
├── YES → Is it a simple binary choice?
│         ├── YES → Ternary
│         └── NO  → Object/Map lookup
└── NO  → How many branches?
          ├── 1 → if
          ├── 2 → if-else
          ├── 3-4 → if-else-if
          └── 5+ → switch or object lookup

Complexity note

All basic conditional operations are O(1) — they evaluate a fixed number of comparisons regardless of input size.

A chain of N conditions is O(N) in the worst case (all conditions checked).

A switch with N cases may be optimized to a jump table — O(1) in many compilers.

Self-check drill

SC-001

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-002

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-003

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-004

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-005

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-006

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-007

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-008

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-009

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-010

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-011

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-012

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-013

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-014

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-015

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-016

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-017

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-018

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-019

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-020

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-021

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-022

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-023

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-024

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-025

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-026

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-027

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-028

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-029

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-030

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-031

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-032

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-033

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-034

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-035

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-036

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-037

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-038

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-039

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-040

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-041

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-042

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-043

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-044

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-045

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-046

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-047

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-048

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-049

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-050

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-051

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-052

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-053

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-054

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-055

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-056

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-057

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-058

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-059

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-060

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-061

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-062

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-063

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-064

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-065

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-066

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-067

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-068

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-069

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-070

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-071

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-072

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-073

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-074

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-075

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-076

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-077

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-078

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-079

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-080

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-081

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-082

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-083

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-084

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-085

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-086

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-087

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-088

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-089

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-090

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-091

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-092

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-093

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-094

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-095

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-096

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-097

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-098

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-099

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-100

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-101

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-102

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-103

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-104

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-105

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-106

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-107

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-108

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-109

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-110

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-111

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-112

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-113

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-114

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-115

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-116

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-117

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-118

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-119

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-120

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-121

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-122

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-123

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-124

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-125

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-126

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-127

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-128

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-129

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-130

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-131

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-132

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-133

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-134

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-135

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-136

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-137

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-138

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-139

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-140

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-141

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-142

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-143

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-144

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-145

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-146

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-147

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-148

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-149

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-150

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-151

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-152

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-153

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-154

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-155

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-156

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-157

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-158

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-159

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-160

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-161

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-162

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-163

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-164

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-165

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-166

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-167

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-168

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-169

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-170

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-171

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-172

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-173

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-174

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-175

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-176

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-177

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-178

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-179

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-180

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-181

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-182

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-183

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-184

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-185

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases

SC-186

  • Q: Difference between || and ???
  • A: || returns right on ANY falsy; ?? only on null/undefined

SC-187

  • Q: What is the dangling else problem?
  • A: else binds to nearest if — use braces

SC-188

  • Q: Why avoid nested ternaries?
  • A: Hard to read — use if-else for complex logic

SC-189

  • Q: How does fall-through work in switch?
  • A: Without break, execution continues to next case

SC-190

  • Q: What is object lookup pattern?
  • A: Replace switch/if-else with key-value mapping

SC-191

  • Q: What does !!value do?
  • A: Converts to boolean: !!0 = false, !!'hi' = true

SC-192

  • Q: Is [] truthy or falsy in JS?
  • A: Truthy — empty array is truthy

SC-193

  • Q: What does typeof null return?
  • A: 'object' — a historical JS bug

SC-194

  • Q: How to check NaN?
  • A: Number.isNaN(x) — not x === NaN

SC-195

  • Q: When to use try-catch vs if?
  • A: try-catch for unexpected errors, if for expected conditions

SC-196

  • Q: What are the 7 falsy values in JS?
  • A: false, 0, -0, 0n, '', null, undefined, NaN

SC-197

  • Q: Difference between == and ===?
  • A: == coerces types, === does not

SC-198

  • Q: When to use switch over if-else?
  • A: Comparing one variable against many constants

SC-199

  • Q: What is short-circuit evaluation?
  • A: Stop evaluating when result is determined

SC-200

  • Q: What is a guard clause?
  • A: Early return for invalid/edge cases