Chapter 3 - Loops (JavaScript)

Here's a JavaScript-flavoured version of the same concepts, with small JS examples for each idea.

while loops

A while loop repeats a block of code as long as its condition is true.

Example: print "Hello, world." five times.

let spam = 0;
while (spam < 5) {
  console.log("Hello, world.");
  spam = spam + 1;
}

An "annoying" while loop

You can keep asking for input until the user types exactly what you want.

Example: keep asking for "your name".

let name = "";
while (name !== "your name") {
  console.log("Please type your name.");
  name = prompt(">");
}
console.log("Thank you!");

Infinite loops and break

while (true) creates an infinite loop; break lets you exit early from inside the loop.

Example: same program using break.

while (true) {
  console.log("Please type your name.");
  const name = prompt(">");
  if (name === "your name") {
    break;
  }
}
console.log("Thank you!");

continue in loops

continue skips the rest of the loop body and jumps back to the top to recheck the condition.

Example: only "Joe" is asked for a password.

while (true) {
  console.log("Who are you?");
  const name = prompt(">");
  if (name !== "Joe") {
    continue;        // go back to start of loop
  }

  console.log("Hello, Joe. What is the password? (It is a fish.)");
  const password = prompt(">");
  if (password === "swordfish") {
    break;           // exit loop
  }
}
console.log("Access granted.");

Truthy and falsy values, Boolean()

In conditions, 0, "", null, undefined, and NaN are falsy; almost everything else is truthy. This is similar to Python but with more falsy values.

Example: use truthiness directly.

let name = "";
while (!name) {              // while name is empty
  console.log("Enter your name:");
  name = prompt(">");
}

console.log("How many guests will you have?");
const numOfGuests = Number(prompt(">"));

if (numOfGuests) {           // means "if numOfGuests !== 0"
  console.log("Be sure to have enough room for all your guests.");
}
console.log("Done");

You can test truthiness with Boolean():

Boolean(0)       // false
Boolean(42)      // true
Boolean("")      // false
Boolean("Hi")    // true

for loops

JavaScript's for loop uses an initializer, condition, and update expression instead of range().

Example: basic for loop.

console.log("Hello!");
for (let i = 0; i < 5; i++) {
  console.log("On this iteration, i is set to " + i);
}
console.log("Goodbye!");

Summing a series with for

You can accumulate a running total inside a loop.

Example: sum 0 to 100.

let total = 0;
for (let num = 0; num <= 100; num++) {
  total = total + num;
}
console.log(total);   // 5050

while equivalent of a for loop

Anything done with for can be done with while, though it's more verbose.

Example: while version of the loop.

console.log("Hello!");
let i = 0;
while (i < 5) {
  console.log("On this iteration, i is set to " + i);
  i = i + 1;
}
console.log("Goodbye!");

for loop arguments: start, stop, step

JavaScript's for loop handles start, stop, and step directly in its three expressions, equivalent to Python's range(start, stop, step).

Examples:

for (let i = 12; i < 16; i++) {       // 12, 13, 14, 15
  console.log(i);
}

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

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

Importing modules

In Node.js, you import modules with require() or ES module import. JavaScript has no built-in random module; use Math.random() instead.

Example: random integers in a loop.

function randInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

for (let i = 0; i < 5; i++) {
  console.log(randInt(1, 10));   // random int 1–10
}

You can import Node.js modules:

const fs = require("fs");
const path = require("path");

Ending a program early with process.exit()

In Node.js, process.exit() immediately terminates the program, even if you're inside a loop.

Example: loop until the user types exit.

while (true) {
  console.log("Type exit to exit.");
  const response = prompt(">");
  if (response === "exit") {
    process.exit();
  }
  console.log("You typed " + response + ".");
}

Short program: Guess the Number

This program uses Math.random(), a for loop, and break to let the user guess a secret number between 1 and 20, with up to 6 tries.

function randInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

const secretNumber = randInt(1, 20);
console.log("I am thinking of a number between 1 and 20.");

let guess;
let guessesTaken;
for (guessesTaken = 1; guessesTaken <= 6; guessesTaken++) {
  console.log("Take a guess.");
  guess = Number(prompt(">"));

  if (guess < secretNumber) {
    console.log("Your guess is too low.");
  } else if (guess > secretNumber) {
    console.log("Your guess is too high.");
  } else {
    break;  // correct guess
  }
}

if (guess === secretNumber) {
  console.log("Good job! You got it in " + guessesTaken + " guesses!");
} else {
  console.log("Nope. The number was " + secretNumber);
}

Short program: Rock, Paper, Scissors

This game uses nested while loops, Math.random(), and conditionals to play repeated rounds and track wins, losses, and ties.

function randInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

console.log("ROCK, PAPER, SCISSORS");
let wins = 0, losses = 0, ties = 0;

while (true) {   // main game loop
  console.log(`${wins} Wins, ${losses} Losses, ${ties} Ties`);

  // player input loop
  let playerMove;
  while (true) {
    console.log("Enter your move: (r)ock (p)aper (s)cissors or (q)uit");
    playerMove = prompt(">");
    if (playerMove === "q") {
      process.exit();
    }
    if (["r", "p", "s"].includes(playerMove)) {
      break;
    }
    console.log("Type one of r, p, s, or q.");
  }

  if (playerMove === "r") {
    console.log("ROCK versus...");
  } else if (playerMove === "p") {
    console.log("PAPER versus...");
  } else {
    console.log("SCISSORS versus...");
  }

  const moveNumber = randInt(1, 3);
  let computerMove;
  if (moveNumber === 1) {
    computerMove = "r";
    console.log("ROCK");
  } else if (moveNumber === 2) {
    computerMove = "p";
    console.log("PAPER");
  } else {
    computerMove = "s";
    console.log("SCISSORS");
  }

  if (playerMove === computerMove) {
    console.log("It is a tie!");
    ties++;
  } else if (
    (playerMove === "r" && computerMove === "s") ||
    (playerMove === "p" && computerMove === "r") ||
    (playerMove === "s" && computerMove === "p")
  ) {
    console.log("You win!");
    wins++;
  } else {
    console.log("You lose!");
    losses++;
  }
}

Overall idea of the chapter

The chapter's core ideas are: use while for "loop while condition is true," for for "loop a fixed number of times," control loops with break/continue, and combine all of this to build small interactive games. JavaScript uses C-style for loops instead of range(), Math.random() instead of random.randint(), and process.exit() instead of sys.exit().

while loops​

An "annoying" while loop​

Infinite loops and break​

continue in loops​

Truthy and falsy values, Boolean()​

for loops​

Summing a series with for​

while equivalent of a for loop​

for loop arguments: start, stop, step​

Importing modules​

Ending a program early with process.exit()​

Short program: Guess the Number​

Short program: Rock, Paper, Scissors​

Overall idea of the chapter​