Module 1: Control Flow

If...Else & Switch

let age = 20;

if (age >= 18) {
  console.log("You can vote");
} else if (age >= 13) {
  console.log("You're a teenager");
} else {
  console.log("You're a child");
}

switch is a cleaner alternative when checking one value against many possibilities:

let day = "Mon";

switch (day) {
  case "Mon":
  case "Tue":
  case "Wed":
  case "Thu":
  case "Fri":
    console.log("Weekday");
    break;
  case "Sat":
  case "Sun":
    console.log("Weekend");
    break;
  default:
    console.log("Invalid day");
}

Don't forget break — without it, execution falls through into the next case, which is sometimes intentional (grouping Mon–Fri above) but usually a bug otherwise.

For, While & Do-While

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

let fruits = ["apple", "banana", "mango"];
for (let fruit of fruits) {  // for...of — loops over VALUES
  console.log(fruit);
}

let user = { name: "Chiru", age: 25 };
for (let key in user) {       // for...in — loops over KEYS, used for objects
  console.log(key, user[key]);
}

let i = 0;
while (i < 5) {
  console.log(i);
  i++;
}

let j = 10;
do {                    // runs the body at least ONCE, even if condition starts false
  console.log(j);
  j++;
} while (j < 5);

Break & Continue

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

for (let i = 0; i < 5; i++) {
  if (i === 2) continue; // skips to the next iteration
  console.log(i);          // 0, 1, 3, 4
}