Running JavaScript

JavaScript can be executed in two primary environments: the web browser and Node.js, a server-side runtime environment for JavaScript. To run JavaScript in a browser, you can use the browser’s built-in developer console, which allows you to interact with web pages and execute JavaScript code directly. In Node.js, you can create JavaScript files with a .js extension and run them using the node command in your terminal.

Let’s start with the classic “Hello World” example to illustrate how to run JavaScript:

// In a web browser console
console.log("Hello, world!");

// In Node.js
console.log("Hello, world!");

Variables and Data Types

Variables are used to store data in your programs. Every variable has a label, it’s data and it’s data types. The label is the name you assign to it, and the data is on the right hand side of the =. The data type is a bit more complicated, and will be explained below.

Integers, Floats, and Strings

JavaScript is a dynamically typed language, which means you don’t need to specify a data type when declaring a variable. Here’s how you declare and assign values to variables for different data types:

let integerNumber = 42;
let floatNumber = 3.14;
let greeting = "Hello, JavaScript!";

Collections (Arrays)

Arrays are used to store collections of data. They can hold various data types, including numbers, strings, or even other arrays:

let fruits = ["apple", "banana", "cherry"];
let mixedArray = [1, "two", 3.0, ["four", 5]];


Functions in JavaScript allow you to encapsulate and reuse blocks of code. Here’s how you define a simple function:

function sayHello(name) {
  console.log("Hello, " + name + "!");

sayHello("Alice"); // Output: Hello, Alice!

Control Flow

If Statements

Conditional statements like if are used to make decisions in your code:

let temperature = 25;

if (temperature > 30) {
  console.log("It's hot outside!");
} else if (temperature > 20) {
  console.log("It's a pleasant day.");
} else {
  console.log("It's cold.");

Truthiness and Boolean Logic Comparisons

JavaScript evaluates values for truthiness. In conditional statements, values like 0, null, undefined, false, "" (empty string), and NaN are considered falsy, while other values are considered truthy.

let isRaining = true;
let isSunny = false;

if (isRaining || isSunny) {
  console.log("The weather is unpredictable!");

if (!isRaining && !isSunny) {
  console.log("The weather is clear.");

Nested If Statements

You can nest if statements to create more complex conditional logic:

let hour = 15;

if (hour < 12) {
  console.log("Good morning!");
} else {
  if (hour < 18) {
    console.log("Good afternoon!");
  } else {
    console.log("Good evening!");


For Loops

For loops are used to iterate over a range of values:

for (let i = 0; i < 5; i++) {
  console.log("Count: " + i);

Foreach Loops with Arrays

You can use for...of loops to iterate through arrays:

let colors = ["red", "green", "blue"];

for (let color of colors) {

While Loops

While loops repeat code while a condition is true:

let count = 0;

while (count < 5) {
  console.log("Count: " + count);

Loop Control Statements (break and continue)

You can use break to exit a loop prematurely and continue to skip the current iteration and move to the next one:

for (let i = 0; i < 10; i++) {
  if (i === 5) {
    break; // Exit the loop when i equals 5
  if (i === 3) {
    continue; // Skip the iteration when i equals 3
  console.log("Value of i: " + i);

Defining Your Own Types (Classes)

JavaScript supports object-oriented programming (OOP) and allows you to define your own types using classes. Here’s a simple example:

class Person {
  constructor(name, age) { = name;
    this.age = age;

  sayHello() {
    console.log(`Hello, my name is ${} and I am ${this.age} years old.`);

let alice = new Person("Alice", 30);
alice.sayHello(); // Output: Hello, my name is Alice and I am 30 years old.

This article covers the foundational aspects of JavaScript, providing you with a solid starting point for your programming journey. JavaScript is a vast language with numerous advanced features and libraries, so don’t hesitate to explore further and build exciting web applications and beyond. Happy coding!