// B3.1 · ~30 min · Beginner

if, else and truthiness

After this lesson you write if, else if and else chains that pick the right branch, know which values count as false, and use &&, ||, ! and ?? for conditions and defaults.

Lesson 1 of 5 in B3 Control flow and loops

Start of the moduleNext

You will be able to

Write if, else if and else chains with blocks, and predict which branch runs
Tell the eight falsy values from truthy ones such as "0", [] and {}
Predict the value that &&, ||, ! and ?? return, and choose || or ?? for a default

Warm-up · Activity 1 of 7
Warm-up: an if treats some values as false. Which of these count as false? Pick all that apply.
Select all that apply.
"0""" (an empty string)[] (an empty array)0
Predict · Activity 2 of 7
Predict before you read on: a form field gave you the text "0". What does this print?
```
const input = "0";
if (input) {
  console.log("got an answer");
} else {
  console.log("no answer");
}
```
no answerNothinggot an answergot an answer, no answer
Practice · Activity 3 of 7
The saved volume 0 is a real setting. Fill in the operator so that the program prints 0, and would use 50 only if nothing were saved (null or undefined).
```
const saved = 0;
const volume = saved ____ 50;
console.log(volume);
```
const volume = saved Your answer 50;

Practice · Activity 4 of 7

It is 25 degrees. What does this print?

const temp = 25;
if (temp > 10) {
  console.log("mild");
} else if (temp > 20) {
  console.log("warm");
} else {
  console.log("cold");
}

warmmildmild, warmcold

Practice · Activity 5 of 7
Match each expression to the value it gives.
"" || "Guest" — Match for “"" || "Guest"”
"Ada" && "admin" — Match for “"Ada" && "admin"”
0 ?? 5 — Match for “0 ?? 5”
!"hello" — Match for “!"hello"”
Brain teaser · Activity 6 of 7
Brain teaser. The shopping list is empty. What does this print?
```
const list = [];
if (list) {
  console.log("time to shop");
} else {
  console.log("nothing to buy");
}
```
time to shop, nothing to buynothing to buytime to shopNothing
Apply · Activity 7 of 7
Mini-task. Write a cinema till: children under 6 go free, under 18 pay 6 EUR, everyone else 10 EUR. Store the age in a constant and print the price with an if, else if and else chain. Then greet the guest with their nickname, or "movie fan" when the nickname is null. Try the ages 3, 6, 17, 18 and 40.
Check your work against this list
- Uses one chain of if, else if and else, with braces around every block
- Orders the tests so that each age reaches the right branch, including exactly 6 and 18
- Uses ?? for the nickname, so only null or undefined gets the default
- Prints two lines, such as Ticket: 6 EUR and Hello, movie fan

Build it yourself

Read the worked example, then write the exercises. Your code runs in your browser or on your computer and is never uploaded.

Worked example

A delivery checker

This program decides how an order is shipped. An else if chain picks the shipping price, ! turns a false into a true, an empty coupon counts as false, and ??, && and || fill in the remaining lines. Run it, then change total to 60 or 10, set coupon to "SPRING" or note to "ring twice", and predict each line before you run it again.

main.js

// Decide how an order is shipped.
const total = 42.5;
const express = false;
const coupon = "";
const note = null;

if (total >= 50) {
  console.log("Shipping: free");
} else if (total >= 20) {
  console.log("Shipping: 2.99 EUR");
} else {
  console.log("Shipping: 4.99 EUR");
}

if (!express) {
  console.log("Delivery: standard");
}

if (coupon) {
  console.log("Coupon:", coupon);
} else {
  console.log("Coupon: none");
}

console.log("Note:", note ?? "no note");
console.log("Gift wrap:", total > 40 && "offered");
console.log("Label:", coupon || "STANDARD");

Run it with

node main.js

Output

Shipping: 2.99 EUR
Delivery: standard
Coupon: none
Note: no note
Gift wrap: offered
Label: STANDARD

42.5 fails total >= 50 and passes total >= 20, so the second block runs and the else is skipped.
express is false, so !express is true.
The empty coupon is falsy: the else block runs, and || replaces it with "STANDARD".
total > 40 is true, so && goes on and returns its second value, "offered".

Change it and run it

Your code (main.js)

Tab indents and Shift+Tab outdents. To leave the editor with the keyboard, press Esc, then Tab.

The first run downloads the JavaScript runner (up to 0.1 MB) and keeps it cached. Your code runs in your browser’s own engine and stays on your device.

Exercises

Exercise 1 of 2

Fix the weather report

The report says 24 degrees is mild and asks for an umbrella on a dry day. Fix the order of the else if chain, so that above 20 degrees it prints It is warm., and fix the second condition, which sets raining instead of reading it. The program should print It is warm. and then No umbrella needed.

Your code (main.js)

Tab indents and Shift+Tab outdents. To leave the editor with the keyboard, press Esc, then Tab.

The first run downloads the JavaScript runner (up to 0.1 MB) and keeps it cached. Your code runs in your browser’s own engine and stays on your device.

Hints

Hint 1
The chain stops at the first true condition. Is 24 > 10 true?
Hint 2
Put the narrower test, temperature > 20, first, and move It is mild. to the else if.
Hint 3
raining = true stores true in raining. To read the value, write if (raining).

Show a solution

One way to solve it. Yours can look different and still pass the checks.

// A weather report for one day.
const temperature = 24;
let raining = false;

if (temperature > 20) {
  console.log("It is warm.");
} else if (temperature > 10) {
  console.log("It is mild.");
} else {
  console.log("It is cold.");
}

if (raining) {
  console.log("Take an umbrella.");
} else {
  console.log("No umbrella needed.");
}

Run it on your computer

Install ECMAScript 2026 or newer. Save these files in one folder, open a terminal in that folder, and run the commands below.

main.js

// A weather report for one day.
const temperature = 24;
let raining = false;

if (temperature > 10) {
  console.log("It is mild.");
} else if (temperature > 20) {
  console.log("It is warm.");
} else {
  console.log("It is cold.");
}

if (raining = true) {
  console.log("Take an umbrella.");
} else {
  console.log("No umbrella needed.");
}

main.test.js

import {test} from 'node:test';
import assert from 'node:assert/strict';
import {runMain} from './learnrun.js';

const lines = async () => (await runMain()).trimEnd().split('\n');

test('prints two lines', async () => {
  const got = await lines();
  assert.equal(got.length, 2, `the program printed ${got.length} line(s), expected 2`);
});

test('24 degrees is warm', async () => {
  const got = await lines();
  assert.equal(got[0], 'It is warm.', `the first line is ${JSON.stringify(got[0])}, expected "It is warm."`);
});

test('a dry day needs no umbrella', async () => {
  const got = await lines();
  assert.equal(got[1], 'No umbrella needed.', `the second line is ${JSON.stringify(got[1])}, expected "No umbrella needed."`);
});

package.json

{
  "type": "module"
}

package.json tells Node.js that the .js files are modules; keep it in the folder.

Run the program:

node main.js

Run the checks (needs learnrun.js in the same folder):

node --test

Download learnrun.js

Exercise 2 of 2

Defaults that keep zero

A user saved the volume 0 and an empty name, and no theme. 0 is a real volume and must be kept; an empty name means no name, so it should become Guest. Change the operators so that the program prints volume: 0, then name: Guest, then theme: light. Do not change the saved values.

Your code (main.js)

Tab indents and Shift+Tab outdents. To leave the editor with the keyboard, press Esc, then Tab.

The first run downloads the JavaScript runner (up to 0.1 MB) and keeps it cached. Your code runs in your browser’s own engine and stays on your device.

Hints

Hint 1
Run it first: which line is wrong, and is its saved value 0, "" or undefined?
Hint 2
|| replaces every falsy value, 0 included. ?? replaces only null and undefined.
Hint 3
The volume needs ??, the name needs ||. For the theme, both work, because undefined is replaced by either.

Show a solution

One way to solve it. Yours can look different and still pass the checks.

// Settings saved by the user.
const savedVolume = 0;
const savedName = "";
const savedTheme = undefined;

const volume = savedVolume ?? 50;
const name = savedName || "Guest";
const theme = savedTheme ?? "light";

console.log("volume:", volume);
console.log("name:", name);
console.log("theme:", theme);

Run it on your computer

Install ECMAScript 2026 or newer. Save these files in one folder, open a terminal in that folder, and run the commands below.

main.js

// Settings saved by the user.
const savedVolume = 0;
const savedName = "";
const savedTheme = undefined;

const volume = savedVolume || 50;
const name = savedName ?? "Guest";
const theme = savedTheme || "light";

console.log("volume:", volume);
console.log("name:", name);
console.log("theme:", theme);

main.test.js

import {test} from 'node:test';
import assert from 'node:assert/strict';
import {runMain} from './learnrun.js';

const lines = async () => (await runMain()).trimEnd().split('\n');

test('keeps the volume 0', async () => {
  const got = await lines();
  assert.equal(got[0], 'volume: 0', `the first line is ${JSON.stringify(got[0])}, expected "volume: 0"`);
});

test('replaces the empty name with Guest', async () => {
  const got = await lines();
  assert.equal(got[1], 'name: Guest', `the second line is ${JSON.stringify(got[1])}, expected "name: Guest"`);
});

test('uses the light theme when none is saved', async () => {
  const got = await lines();
  assert.equal(got[2], 'theme: light', `the third line is ${JSON.stringify(got[2])}, expected "theme: light"`);
});

test('prints exactly three lines', async () => {
  const got = await lines();
  assert.equal(got.length, 3, `the program printed ${got.length} line(s), expected 3`);
});

package.json

{
  "type": "module"
}

package.json tells Node.js that the .js files are modules; keep it in the folder.

Run the program:

node main.js

Run the checks (needs learnrun.js in the same folder):

node --test

Download learnrun.js

Common mistakes

elseif in one word

const size = 8;
if (size > 10) {
  console.log("large");
} elseif (size > 5) {
  console.log("medium");
}

What Node.js prints

SyntaxError: Unexpected token '{'

Why, and the fix

JavaScript has no elseif keyword: it reads elseif (size > 5) as a call of something named elseif, and then a { cannot follow. Because the file cannot be read, no line runs. Write else if as two words: } else if (size > 5) {.

A condition without parentheses

const size = 8;
if size > 5 {
  console.log("big");
}

What Node.js prints

SyntaxError: Unexpected identifier 'size'

Why, and the fix

The condition of an if must sit in parentheses: if (size > 5). Without them, JavaScript expects a ( right after if and stops at the name size. Other languages allow the bare form; JavaScript does not.

?? mixed with || without parentheses

const saved = null;
const name = saved || undefined ?? "guest";
console.log(name);

What Node.js prints

SyntaxError: Unexpected token '??'

Why, and the fix

JavaScript refuses to guess whether || or ?? should go first, so it does not let you mix them, or ?? with &&, without parentheses. The whole file then fails to load. Add parentheses to say what you mean: (saved || undefined) ?? "guest", or use one operator only.

JavaScript in the browser: your browser’s own engine, in a sandboxed worker. Syntax errors are located with acorn 8.18.0, MIT. Licence and source

Exit ticket

5 questions, no hints. Score 80% or more to complete the lesson.

Finish every activity above to unlock the exit ticket.

Key ideas

The first true condition wins

if (condition) { … } runs the block in curly braces only when the condition is true; else { … } runs when it is not. Chain more tests with else if: JavaScript checks the conditions from top to bottom, runs the block of the first one that is true, and skips all the rest. So put the narrowest test first: with if (points >= 50) above else if (points >= 90), 95 points never reach the second test. There is no elseif in one word, and the condition always sits in parentheses. Always write the braces: without them, only the next single statement belongs to the if.

Eight falsy values; everything else is truthy

A condition does not have to be true or false: JavaScript converts any value to a boolean when it needs one. Exactly eight values count as false, the falsy values: false, 0, -0, 0n, "" (the empty string), null, undefined and NaN. Every other value is truthy, including the string "0", the string "false", a string with one space, the empty array [] and the empty object {}. So if (list) is true even for an empty list; ask if (list.length > 0) instead. Boolean(value) shows what a value counts as.

&&, || and ?? return one of their values

! gives a real boolean: !"" is true, !"hi" is false. && and || return one of their two values, and they stop as soon as the answer is known (short-circuit). a && b gives a if a is falsy, otherwise b: "Ada" && "admin" is "admin", 0 && "admin" is 0. a || b gives a if a is truthy, otherwise b: "" || "Guest" is "Guest". That makes || a default, but it also replaces 0 and "". a ?? b replaces only null and undefined: 0 ?? 50 is 0. Mixing ?? with && or || needs parentheses, or the file does not load.

Sources

Last reviewed October 1, 2026

if, else and truthiness

Warm-up: an if treats some values as false. Which of these count as false? Pick all that apply.

Predict before you read on: a form field gave you the text "0". What does this print?

The saved volume 0 is a real setting. Fill in the operator so that the program prints 0, and would use 50 only if nothing were saved (null or undefined).

It is 25 degrees. What does this print?

Match each expression to the value it gives.

Brain teaser. The shopping list is empty. What does this print?

Build it yourself

A delivery checker

Exercises

Fix the weather report

Defaults that keep zero

Common mistakes

elseif in one word

A condition without parentheses

?? mixed with || without parentheses

Exit ticket

Key ideas

The first true condition wins

Eight falsy values; everything else is truthy

&&, || and ?? return one of their values

Sources