🔁 Lesson 19: Conditionals and Loops

Sometimes in your code, you want to make decisions — "Do this if the user is logged in", "Repeat this task 5 times", etc. JavaScript gives us tools to make those decisions and run repeated tasks.

🧠 Conditionals – Making Decisions

1. if...else

The simplest conditional. If the condition is true, run the first block. Otherwise, run the second.


let age = 20;

if (age >= 18) {
  console.log("Adult");
} else {
  console.log("Minor");
}

2. if...else if...else

You can chain multiple conditions. JavaScript checks them top to bottom and runs the first one that’s true.


let score = 75;

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

3. switch

A cleaner way to handle many exact matches. Useful when comparing one value to many possibilities.


let day = "Monday";

switch (day) {
  case "Monday":
    console.log("Start of the week!");
    break;
  case "Friday":
    console.log("Almost weekend!");
    break;
  case "Sunday":
    console.log("Rest day.");
    break;
  default:
    console.log("Just another day.");
}

💡 Use break to stop the switch from falling through to other cases.

🔁 Loops – Repeating Tasks

1. for loop

Great when you know how many times to repeat something.


for (let i = 0; i < 5; i++) {
  console.log("Number:", i);
}

This loop prints numbers 0 to 4. It has 3 parts:

let i = 0: Start
i < 5: Keep going while this is true
i++: Increase i after every loop

2. while loop

Keeps running as long as the condition is true.


let i = 0;

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

If you forget to increase i, it’ll loop forever. Be careful. ☠️

3. do...while loop

This loop always runs at least once, even if the condition is false.


let i = 0;

do {
  console.log(i);
  i++;
} while (i < 5);

Useful when you want to show something first, then check if you should continue.

✅ Recap

if checks a condition
else if lets you test more cases
switch is great for matching exact values
for is perfect for counting
while runs as long as something is true
do...while runs once, then checks

Next up, we'll learn how to organize logic into functions so we can reuse code and clean up our programs!

Lessons

Lesson 19: Conditionals and Loops