Control Flow

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if / else if / else

let score = 82;

if (score >= 90) {
  console.log("A");
} else if (score >= 80) {
  console.log("B");  // ← this runs
} else if (score >= 70) {
  console.log("C");
} else {
  console.log("F");
}

---

switch Statement

let day = "Monday";

switch (day) {
  case "Monday":
  case "Tuesday":
  case "Wednesday":
  case "Thursday":
  case "Friday":
    console.log("Weekday");
    break;                    // IMPORTANT: without break, falls through!
  case "Saturday":
  case "Sunday":
    console.log("Weekend");
    break;
  default:
    console.log("Unknown day");
}

Always add break unless fall-through is intentional.

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for Loop

// Classic for loop
for (let i = 0; i < 5; i++) {
  console.log(i); // 0, 1, 2, 3, 4
}

// for...of loop (arrays, strings, iterables)
const fruits = ["apple", "banana", "cherry"];
for (const fruit of fruits) {
  console.log(fruit);
}

// for...in loop (object keys)
const person = { name: "Reet", age: 25, city: "Rajkot" };
for (const key in person) {
  console.log(`${key}: ${person[key]}`);
}

---

while and do...while

// while: checks condition FIRST
let count = 0;
while (count < 5) {
  console.log(count);
  count++;
}

// do...while: executes AT LEAST ONCE, then checks condition
let num = 10;
do {
  console.log(num);  // runs once even though condition is false
  num++;
} while (num < 5);
// Output: 10

---

break and continue

// break: exits the loop entirely
for (let i = 0; i < 10; i++) {
  if (i === 5) break;
  console.log(i); // 0, 1, 2, 3, 4
}

// continue: skips current iteration
for (let i = 0; i < 10; i++) {
  if (i % 2 === 0) continue;  // skip even numbers
  console.log(i); // 1, 3, 5, 7, 9
}

// Labeled break (for nested loops)
outer: for (let i = 0; i < 3; i++) {
  for (let j = 0; j < 3; j++) {
    if (j === 1) break outer;
    console.log(i, j);
  }
}
// Output: 0 0

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Truthy and Falsy Values

// Falsy values (6 total):
// false, 0, "", null, undefined, NaN

if (0)         { } // won't run
if ("")        { } // won't run
if (null)      { } // won't run
if (undefined) { } // won't run
if (NaN)       { } // won't run

// Truthy (everything else):
if (1)   console.log("runs");
if ("hello") console.log("runs");
if ([])  console.log("runs");  // empty array is truthy!
if ({})  console.log("runs");  // empty object is truthy!
if (-1)  console.log("runs");  // negative numbers are truthy!

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forEach, map, filter, reduce (Array iteration)

const nums = [1, 2, 3, 4, 5];

nums.forEach(n => console.log(n));       // iterate, no return value
nums.map(n => n * 2);                    // [2,4,6,8,10] — new array
nums.filter(n => n % 2 === 0);           // [2,4] — filtered array
nums.reduce((acc, n) => acc + n, 0);     // 15 — single value

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MCQ — Control Flow

Q1: What happens if you forget break in a switch case?

A) The switch exits automatically after each case B) Code falls through to the next case ✅ C) A SyntaxError is thrown D) The default case runs

Answer: B — Without break, execution continues into the next case (fall-through).

Q2: What does a do...while loop guarantee?

A) Nothing is logged if condition is false B) The body executes at least once ✅ C) An infinite loop D) undefined

Answer: B — do...while always executes at least once before checking the condition.

Q3: Which loop is best for iterating over object properties?

A) for...of B) while C) for...in ✅ D) forEach

Answer: C — for...in iterates over enumerable object keys.

Q4: What does continue do in a loop?

A) Exits the loop B) Exits the function C) Skips the current iteration and goes to the next ✅ D) Restarts the loop

Answer: C

Q5: Is [] truthy or falsy in JavaScript?

A) Falsy B) Truthy ✅ C) Error D) Depends on context

Answer: B — Empty arrays and empty objects are truthy! Only the 6 falsy values are falsy.

Q6: What is the difference between for...of and for...in?

A) They are identical B) for...of iterates values; for...in iterates keys ✅ C) for...in iterates values; for...of iterates keys D) for...of works only on objects

Answer: B

Q7: How many falsy values does JavaScript have?

A) 4 B) 5 C) 6 ✅ D) 7

Answer: C — false, 0, "", null, undefined, NaN.

Q8: What does this print?

for (let i = 0; i < 3; i++) {
  if (i === 1) continue;
  console.log(i);
}

A) 0, 1, 2 B) 0, 2 ✅ C) 1, 2 D) 0, 1

Answer: B — When i === 1, continue skips that iteration.