Control Flow and Loops

If/Else Statements

The if statement lets your program make decisions. If a condition is true, a block of code runs. Unlike Python which uses indentation to define blocks, JavaScript uses curly braces { }.

const age = 18;

// Simple if
if (age >= 18) {
    console.log("You are an adult.");
}

// if...else
if (age >= 18) {
    console.log("You can vote.");
} else {
    console.log("You cannot vote yet.");
}

// if...else if...else
const score = 85;

if (score >= 90) {
    console.log("Grade: A");
} else if (score >= 80) {
    console.log("Grade: B");
} else if (score >= 70) {
    console.log("Grade: C");
} else if (score >= 60) {
    console.log("Grade: D");
} else {
    console.log("Grade: F");
}
// Output: "Grade: B"

Logical Operators in Conditions

const age = 25;
const hasLicense = true;
const isSuspended = false;

// AND (&&) — both conditions must be true
if (age >= 16 && hasLicense) {
    console.log("You can drive.");
}

// OR (||) — at least one condition must be true
if (age < 13 || age > 65) {
    console.log("Discounted ticket.");
}

// NOT (!) — inverts a boolean
if (!isSuspended) {
    console.log("Account is active.");
}

// Combining operators
if (age >= 16 && hasLicense && !isSuspended) {
    console.log("You are cleared to drive.");
}

Switch Statements

Use switch when you need to compare a single value against many possible options. It is cleaner than a long chain of if...else if statements.

const day = "Monday";

switch (day) {
    case "Monday":
        console.log("Start of the work week.");
        break;
    case "Tuesday":
    case "Wednesday":
    case "Thursday":
        console.log("Midweek.");
        break;
    case "Friday":
        console.log("Almost the weekend!");
        break;
    case "Saturday":
    case "Sunday":
        console.log("Weekend!");
        break;
    default:
        console.log("Not a valid day.");
}
// Output: "Start of the work week."

// Practical example: HTTP status codes
const status = 404;

switch (status) {
    case 200:
        console.log("OK — request succeeded.");
        break;
    case 301:
        console.log("Moved Permanently.");
        break;
    case 404:
        console.log("Not Found.");
        break;
    case 500:
        console.log("Internal Server Error.");
        break;
    default:
        console.log(`Unknown status: ${status}`);
}
// Output: "Not Found."

Ternary Operator

The ternary operator is a shorthand for simple if...else statements. It fits on one line and is useful for assigning values conditionally:

// Syntax: condition ? valueIfTrue : valueIfFalse

const age = 20;
const status = age >= 18 ? "adult" : "minor";
console.log(status); // "adult"

// Equivalent if...else:
// let status;
// if (age >= 18) {
//     status = "adult";
// } else {
//     status = "minor";
// }

// Common uses
const score = 85;
const grade = score >= 90 ? "A" : score >= 80 ? "B" : score >= 70 ? "C" : "F";

const items = 3;
console.log(`You have ${items} item${items === 1 ? "" : "s"}.`);
// "You have 3 items."

// In template literals
const loggedIn = true;
console.log(`Welcome${loggedIn ? ", user" : ""}!`);
// "Welcome, user!"

For Loops

The classic for loop repeats a block of code a specific number of times. It has three parts: initialization, condition, and update.

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

// Counting by twos
for (let i = 0; i <= 10; i += 2) {
    console.log(i);
}
// 0, 2, 4, 6, 8, 10

// Counting backwards
for (let i = 5; i > 0; i--) {
    console.log(i);
}
// 5, 4, 3, 2, 1

// Looping through an array by index
const fruits = ["apple", "banana", "cherry"];
for (let i = 0; i < fruits.length; i++) {
    console.log(`${i}: ${fruits[i]}`);
}
// "0: apple"
// "1: banana"
// "2: cherry"

Practical Example: Building a Multiplication Table

const number = 7;
for (let i = 1; i <= 10; i++) {
    console.log(`${number} x ${i} = ${number * i}`);
}
// 7 x 1 = 7
// 7 x 2 = 14
// ... up to 7 x 10 = 70

While and Do-While Loops

while Loop

A while loop repeats as long as its condition is true. Use it when you don't know in advance how many times you need to loop.

// Count up to a threshold
let count = 0;
while (count < 5) {
    console.log(count);
    count++;
}
// 0, 1, 2, 3, 4

// Practical: halving a number until it's below 1
let value = 100;
let steps = 0;
while (value >= 1) {
    value = value / 2;
    steps++;
}
console.log(`Took ${steps} steps to go below 1.`);
// "Took 7 steps to go below 1."

do...while Loop

A do...while loop always runs at least once, then checks the condition before repeating:

// Runs at least once even if condition is false
let input;
do {
    input = prompt("Enter 'quit' to exit:");
    console.log(`You entered: ${input}`);
} while (input !== "quit");

// Practical: generating a random number until we get one above 0.9
let random;
let attempts = 0;
do {
    random = Math.random();
    attempts++;
} while (random < 0.9);
console.log(`Got ${random.toFixed(4)} after ${attempts} attempts.`);

for...of and for...in

for...of — Iterate Over Values

// Arrays — get each value directly
const fruits = ["apple", "banana", "cherry"];
for (const fruit of fruits) {
    console.log(fruit);
}
// "apple", "banana", "cherry"

// Strings — iterate over characters
const word = "Hello";
for (const char of word) {
    console.log(char);
}
// "H", "e", "l", "l", "o"

// With array destructuring
const entries = [["name", "Alice"], ["age", 25]];
for (const [key, value] of entries) {
    console.log(`${key}: ${value}`);
}
// "name: Alice", "age: 25"

for...in — Iterate Over Object Keys

// Objects — get each property name
const user = {
    name: "Alice",
    age: 25,
    city: "London"
};

for (const key in user) {
    console.log(`${key}: ${user[key]}`);
}
// "name: Alice"
// "age: 25"
// "city: London"

// Why NOT to use for...in with arrays:
const colors = ["red", "green", "blue"];
for (const index in colors) {
    console.log(typeof index); // "string" — not a number!
    console.log(index);        // "0", "1", "2" — string keys
}
// Use for...of instead for arrays

Break and Continue

break exits a loop entirely. continue skips the current iteration and moves to the next one.

break — Exit Early

// Find the first negative number
const numbers = [3, 7, -2, 8, -5, 1];
let firstNegative = null;

for (const num of numbers) {
    if (num < 0) {
        firstNegative = num;
        break; // stop searching
    }
}
console.log(firstNegative); // -2

// Search for a user
const users = ["Alice", "Bob", "Charlie", "Diana"];
for (let i = 0; i < users.length; i++) {
    if (users[i] === "Charlie") {
        console.log(`Found Charlie at index ${i}`);
        break;
    }
}

continue — Skip This Iteration

// Print only even numbers
for (let i = 0; i < 10; i++) {
    if (i % 2 !== 0) {
        continue; // skip odd numbers
    }
    console.log(i);
}
// 0, 2, 4, 6, 8

// Skip blank entries
const data = ["Alice", "", "Bob", "", "", "Charlie"];
for (const name of data) {
    if (name === "") {
        continue; // skip empty strings
    }
    console.log(`Processing: ${name}`);
}
// "Processing: Alice"
// "Processing: Bob"
// "Processing: Charlie"

Labeled Statements (Advanced)

// Break out of nested loops using labels
outer: for (let i = 0; i < 3; i++) {
    for (let j = 0; j < 3; j++) {
        if (i === 1 && j === 1) {
            break outer; // breaks the outer loop, not just inner
        }
        console.log(`i=${i}, j=${j}`);
    }
}
// "i=0, j=0"
// "i=0, j=1"
// "i=0, j=2"
// "i=1, j=0"
// (stops here)

Now Do It Yourself: Five Steps

Decisions and loops, plus the comparison rule that causes more JavaScript bugs than anything else. Every output below is exactly what Node printed.

const score = 72;
if (score >= 90) {
  console.log("A");
} else if (score >= 70) {
  console.log("B");
} else {
  console.log("C");
}

console.log(0 == "0");
console.log(0 === "0");
console.log([] == false);

for (const name of ["Alice", "Bob"]) {
  console.log("Hello,", name);
}

let total = 0;
for (let i = 1; i <= 5; i++) {
  total += i;
}
console.log(total);

const scores = [95, 72, 58, 88];
for (const s of scores) {
  if (s >= 90) {
    console.log(s, "A");
  } else if (s >= 70) {
    console.log(s, "B");
  } else {
    console.log(s, "C");
  }
}

Summary

• if/else if/else — make decisions based on conditions
• switch — compare one value against many options (don't forget break!)
• condition ? a : b — ternary operator for inline conditionals
• for loop — repeat a known number of times
• while — repeat while a condition is true
• do...while — always runs at least once
• for...of — iterate over values (arrays, strings)
• for...in — iterate over keys (objects)
• break — exit a loop early; continue — skip to next iteration