Decisions and Loops: The Picture

Zara's step counter keeps one number a day, and her goal is 8000 steps. She wants a line for each day of the week: met or short. Her first program checks day 1, then she copies the check for day 2, then for day 3. A week is seven copies, a month is thirty, and the copy for day 23 still says day 22. This lesson replaces every copy with one decision and one loop.

Zara's copied checks

Here is Zara's first program, for three days. Each check is the same five lines with a different day in it.

const day1 = 7500;
const day2 = 8000;
const day3 = 12030;
const goal = 8000;

if (day1 >= goal) {
  console.log("day 1: met");
} else {
  console.log("day 1: short");
}
if (day2 >= goal) {
  console.log("day 2: met");
} else {
  console.log("day 2: short");
}
if (day3 >= goal) {
  console.log("day 3: met");
} else {
  console.log("day 3: short");
}

day 1: short
day 2: met
day 3: met

It works, and it does not grow. Two ideas fix it. A decision runs some lines only when a condition holds: that is if. A loop runs the same lines again and again, once per day: that is for. Together they handle any number of days.

const input = require("fs").readFileSync(0, "utf8");
const tokens = input.split(/\s+/).filter(Boolean);
let at = 0;
const next = () => tokens[at++];
const nextInt = () => Number(next());
const out = [];

const GOAL = 8000;
const n = nextInt();
for (let day = 1; day <= n; day++) {
  const steps = nextInt();
  if (steps >= GOAL) {
    out.push("day " + day + ": met");
  } else {
    out.push("day " + day + ": short");
  }
}

console.log(out.join("\n"));

day 1: short
day 2: met
day 3: met
day 4: short
day 5: met
day 6: short
day 7: met

That output is for the input 7, then 7500 8000 12030 0 9100 4200 15000. The check is written once. The loop runs it seven times, and it would run it 365 times for a year without one extra line. So a loop repeats lines, and an if inside it decides what each repeat does.

if: lines that run only when a condition holds

An if statement has a condition in round brackets and a block in curly braces. A block is a group of lines that belong together. When the condition holds, the block runs. When it does not, the program skips the block and carries on after it.

const steps = 9100;
if (steps >= 8000) {
  console.log("Goal met. Well walked.");
}
console.log("Done for today.");

Goal met. Well walked.
Done for today.

Change steps to 500 and only the last line prints. An else block after the if runs in exactly the other case, so one of the two blocks always runs, never both.

The condition does not have to be a comparison. JavaScript reads any value as truthy or falsy, the rule from Module 2. The falsy values are false, 0 (with -0 and 0n), "", null, undefined and NaN, and everything else is truthy.

const name = "";
if (name) {
  console.log("Hello, " + name);
} else {
  console.log("Hello, stranger");
}

const unread = 3;
if (unread) {
  console.log(unread + " new messages");
}

Hello, stranger
3 new messages

The empty string is falsy, so the else block ran. The number 3 is truthy, so its block ran. So if (value) asks one question: is this value truthy? A comparison such as steps >= 8000 is just a value that is always true or false.

This track always writes the braces, even around one line. Common mistakes 2, near the end, shows what happens without them.

for: three parts, and the order they run in

The round brackets of a for loop hold three parts, separated by semicolons.

• The start, let i = 1, runs once, before anything else.
• The test, i <= 3, runs before every pass. When it is truthy, the body runs. When it is falsy, the loop ends.
• The step, i++, runs after every pass of the body. It adds 1 to i.

The body is the block in braces, and one run of it is a pass. Watch the order. Step through the loop below and keep an eye on the test column: it is checked before each pass.

Here is the same loop as a program. Each pass prints the value of i it saw.

let total = 0;
for (let i = 1; i <= 3; i++) {
  console.log("pass with i = " + i);
  total += i;
}
console.log("total " + total);

pass with i = 1
pass with i = 2
pass with i = 3
total 6

The test ran four times: three times it said yes, and the fourth time, with i at 4, it said no. The two pictures below put the if and the for side by side. The for is an if with an arrow back to the top.

So in a for loop, the start runs once, the test runs before every pass, and the step runs after every pass. The loop ends the first time the test is falsy.

Where the loop's i lives

A name declared with let in the start of a for belongs to the loop. It exists inside the round brackets and the body, and nowhere else. So two loops can each have their own i, and neither one sees the other's.

for (let i = 0; i < 2; i++) {
  console.log("first loop", i);
}
for (let i = 5; i < 7; i++) {
  console.log("second loop", i);
}

first loop 0
first loop 1
second loop 5
second loop 6

After the loop, i is gone, and reading it is an error (Common mistakes 4). Keep any value you need later in a variable declared before the loop, the way total was. So declare a loop's counter in its start, and a result before the loop.

for...of: one pass per value

Often the job is not "count to n" but "look at every value in this list". JavaScript keeps a list of values in an array, written in square brackets: [4200, 9900, 8000]. Module 7 teaches arrays properly. Here you only need two facts: an array holds values in order, and week.length is how many it holds.

A for...of loop hands you the values one at a time. There is no counter to start, test or step: the loop takes the next value on each pass and stops after the last one.

const week = [4200, 9900, 8000, 7999, 15000];
let count = 0;
for (const steps of week) {
  if (steps >= 8000) {
    count++;
  }
}
console.log(count + " of " + week.length + " days met the goal");

3 of 5 days met the goal

steps is a const, and that is allowed: each pass gets a fresh steps with the next value, so nothing is ever reassigned. A string works the same way, one character per pass. Module 3 shows where "one character" gets subtle.

for (const letter of "Zara") {
  console.log(letter);
}

Z
a
r
a

So for...of is the loop for "every value of this list", and a for with a counter is the loop for "count from here to there". Lesson 04 turns that into a flowchart.

Why a loop stops, and when it never does

A loop stops because its test finally answers falsy. In a for loop the step moves i toward that answer on every pass. If the test is falsy from the very first check, the body runs zero times, and that is not an error.

const n = 0;
for (let i = 1; i <= n; i++) {
  console.log("never printed");
}
console.log("the loop ran 0 times");

the loop ran 0 times

If nothing moves the test toward false, the loop never stops. That is an infinite loop. The while loop below, from lesson 02, forgot to add 1 to i, so i < 5 stays true for ever.

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

Nothing prints. On the Playground, signed out, this program ran for about 10 seconds and then stopped with the verdict Time Limit Exceeded (a run on 2026-10-07). The judge does the same to a submitted program. So every loop needs something inside it that moves its test toward false.

if (condition) {
  // runs when condition is truthy
} else {
  // runs when condition is falsy (the else part is optional)
}

for (start; test; step) {
  // the body: runs while test is truthy
}

for (const value of list) {
  // one pass per value of the list, in order
}

const celsius = 31;
if (celsius >= 30) {
  console.log("hot: carry water");
} else {
  console.log("not hot");
}

hot: carry water

let saved = 0;
for (let day = 1; day <= 7; day++) {
  saved += day * 10;
  console.log("day " + day + ": " + saved);
}

day 1: 10
day 2: 30
day 3: 60
day 4: 100
day 5: 150
day 6: 210
day 7: 280

const input = require("fs").readFileSync(0, "utf8");
const tokens = input.split(/\s+/).filter(Boolean);
let at = 0;
const next = () => tokens[at++];
const nextInt = () => Number(next());
const out = [];

const n = nextInt();
let total = 0;
let restDays = 0;
for (let day = 1; day <= n; day++) {
  const steps = nextInt();
  total += steps;
  if (steps < 1000) {
    restDays++;
  }
}
out.push("total " + total);
out.push("rest days " + restDays);

console.log(out.join("\n"));

total 52230
rest days 2

const steps = 500;
if (steps >= 8000); {
  console.log("Goal met");
}

Goal met

const steps = 500;
if (steps >= 8000)
  console.log("Goal met");
  console.log("Badge unlocked");

Badge unlocked

for (const i = 0; i < 3; i++) {
  console.log(i);
}

0
TypeError: Assignment to constant variable.

for (let i = 0; i < 3; i++) {
  console.log(i);
}
console.log("finished at " + i);

0
1
2
ReferenceError: i is not defined

for (let i = 10; i > 0; i -= 3) {
  console.log(i);
}

const input = require("fs").readFileSync(0, "utf8");
const tokens = input.split(/\s+/).filter(Boolean);
let at = 0;
const next = () => tokens[at++];
const nextInt = () => Number(next());
const out = [];

// your code: read with next() and nextInt(), push every line of output to out

console.log(out.join("\n"));

const input = require("fs").readFileSync(0, "utf8");
const tokens = input.split(/\s+/).filter(Boolean);
let at = 0;
const next = () => tokens[at++];
const nextInt = () => Number(next());
const out = [];

// your code: read with next() and nextInt(), push every line of output to out

console.log(out.join("\n"));

const input = require("fs").readFileSync(0, "utf8");
const tokens = input.split(/\s+/).filter(Boolean);
let at = 0;
const next = () => tokens[at++];
const nextInt = () => Number(next());
const out = [];

// your code: read with next() and nextInt(), push every line of output to out

console.log(out.join("\n"));

const input = require("fs").readFileSync(0, "utf8");
const tokens = input.split(/\s+/).filter(Boolean);
let at = 0;
const next = () => tokens[at++];
const nextInt = () => Number(next());
const out = [];

// your code: read with next() and nextInt(), push every line of output to out

console.log(out.join("\n"));

Next, lesson 02 takes every statement one at a time: else if, switch, while, do...while, for...in, break and continue.