Start with the idea

Statements describe actions. Strings need matching quotes, identifiers are case-sensitive, and braces group statements into a block.

Step-by-step lesson notes

JavaScript Syntax Basics

JavaScript syntax is the set of rules that define how a JavaScript program is written and interpreted. Understanding the basics prevents hard-to-find bugs.

Statements and Semicolons

JavaScript programs are made up of statements. A semicolon marks the end of a statement. JavaScript has Automatic Semicolon Insertion (ASI) — but relying on it can cause bugs.

// Each statement should end with ;
let x = 5;
let y = 10;
let sum = x + y;

// Multiple statements per line (avoid — hard to read)
let a = 1; let b = 2; let c = 3;

// ASI gotcha — never start a line with ( or [
const fn = () => console.log("hi")
(function() { /* this gets called as fn()! */ })()

Case Sensitivity

// JavaScript is CASE-SENSITIVE
let name = "Alice";
let Name = "Bob";    // completely different variable!
let NAME = "Charlie"; // yet another variable!

// Keywords must be lowercase
// Let x = 5;  // ❌ SyntaxError
let x = 5;     // ✅

Whitespace and Indentation

// JavaScript ignores extra whitespace
let x=5;       // works
let y = 5;     // works (readable!)
let z =   5;   // works

// Use 2 or 4 spaces for indentation (be consistent)
if (x > 0) {
  console.log("positive");
  if (x > 10) {
    console.log("large");
  }
}

Identifiers

An identifier is a name for a variable, function, or class.

A complete example

Run this complete index.js program in the JavaScript console editor below. The earlier reference fragments may need a browser page or other surrounding setup.

const course = "JavaScript";
if (course === "JavaScript") {
  console.log("Ready to learn");
}

Ready to learn

Make one change. See what happens.

Load the example, change a value, predict the output, and run it again. Use the console output to check your reasoning.

Wait for the editor to load, then load this example and press Run.

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Put the idea to work

Your learning checkpoint

• Explain the example in your own words.
• Complete the challenge before revealing its answer.
• Describe the common mistake and how to avoid it.