Conditions

Why it matters

A program must be able to choose: show a "Log in" or "Log out" button, let someone into the game or say "too young", give a discount or not. That's what conditions are for — code that runs only if something is true.

How tasks with functions work

From this topic on, tasks look like this: the editor already contains a "blank" — a function:

function sign(n) {

}

A function is a named piece of code. Data comes into it through a parameter (here n), and it gives back a result with the word return. The checker will call the function with different numbers, e.g. sign(5) and sign(-2), and compare what it returned. Your job is to write the code inside the curly braces. We'll cover functions in detail in topic 7; for now this is enough.

function double(n) {
  return n * 2;
}
console.log(double(4));

Output:

8

if

const temperature = 30;
if (temperature > 25) {
  console.log("It's hot! Take some water.");
}
console.log("Have a nice day");

Output:

It's hot! Take some water.
Have a nice day

The parentheses hold the condition. If it's true, the block in curly braces runs. If it's false, the block is skipped and the program moves on.

else and else if

const score = 72;
if (score >= 90) {
  console.log("Excellent");
} else if (score >= 70) {
  console.log("Good");
} else if (score >= 50) {
  console.log("Satisfactory");
} else {
  console.log("Needs work");
}

Output:

Good

Conditions are checked from top to bottom, and only the first matching branch runs. Even though 72 >= 50 is true too, that check is never reached. So the order matters.

return stops the function

Inside a function, return immediately gives back the result and exits it. That's why you can skip else after return:

function canVote(age) {
  if (age >= 18) return "yes";
  return "no";
}
console.log(canVote(20), canVote(15));

Output:

yes no

If a block has a single command, the curly braces can be omitted, as here. With several commands the braces are required.

Logical operators

Operator	Meaning	Example
&&	AND — both conditions are true	age >= 18 && hasTicket
||	OR — at least one is true	isAdmin || isOwner
!	NOT — flips the value	!isLoggedIn

const age = 20;
const hasTicket = true;
console.log(age >= 18 && hasTicket); // true
console.log(age < 12 || age > 65);   // false
console.log(!hasTicket);             // false

You can group with parentheses: (a && b) || c.

The ternary operator ? :

A short "if-else" that returns a value:

const age = 16;
const status = age >= 18 ? "adult" : "teenager";
console.log(status);

Output:

teenager

Read it as: "condition ? value-if-yes : value-if-no". Handy for a simple choice between two options; for complex logic, if is better.

Default values: ?? and ||

const nickname = null;
console.log(nickname ?? "Guest"); // Guest

a ?? b returns b only if a is null or undefined. a || b returns b for any falsy a, including 0 and "". So for numbers ?? is safer: 0 ?? 10 → 0, but 0 || 10 → 10.

switch — choosing by value

const day = "sat";
switch (day) {
  case "sat":
  case "sun":
    console.log("Weekend");
    break;
  case "mon":
    console.log("Monday is hard");
    break;
  default:
    console.log("Weekday");
}

Output:

Weekend

switch compares the value strictly (===) with each case. break is required — without it execution "falls through" into the next case. default runs if nothing matched.

How it works inside

The condition in if doesn't have to be true/false — JavaScript converts it to a boolean using the "falsy" rule: 0, "", null, undefined, NaN are false, everything else is true. So if (name) means "if the name is not empty". And && and || are evaluated lazily: if the answer is already clear from the first part (false && … or true || …), the second part isn't even run.

Common mistakes

1. = instead of ===. if (x = 5) assigns 5 and is always true. Write if (x === 5).
2. Wrong else if order. If you check score >= 50 first, "Excellent" will never be given. Start with the strictest conditions.
3. A double comparison like in maths. 7 <= age <= 17 works incorrectly in JS. You need age >= 7 && age <= 17.
4. A missing return. A function without return returns undefined — the checker will show that it got undefined.
5. A missing break in switch. Several branches will run in a row.

Cheat sheet

What	How
If	if (condition) { … }
Else if / else	else if (…) { … } else { … }
AND / OR / NOT	&&, ||, !
Ternary	cond ? yes : no
Default	x ?? "fallback"
Choose by value	switch (x) { case …: …; break; default: … }
Return from a function	return value;