JavaScript Part 1: Syntax, Statements, Comments & Variables

1. JavaScript Syntax

Syntax is simply the set of rules that define how JavaScript code must be written just like grammar rules in English.

JavaScript is case-sensitive. This means name, Name, and NAME are all treated as different things.

let name = "Karthick";   // ✅ valid
let Name = "Ravi";       // also valid, but different variable

Whitespace (spaces, tabs, blank lines) is mostly ignored by JavaScript, but using it properly makes your code readable.

// Hard to read
let x=5;let y=10;let z=x+y;

// Easy to read
let x = 5;
let y = 10;
let z = x + y;

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2. JavaScript Statements

A statement is a single instruction that tells JavaScript to do something. Think of it like a sentence in English — it expresses a complete action.

let age = 25;             // declares a variable
console.log(age);         // prints the value to the console
alert("Hello, World!");   // shows a popup in the browser

Semicolons

Statements usually end with a semicolon ;. While JavaScript can often figure out where a statement ends without it (a feature called ASI — Automatic Semicolon Insertion), it's good practice to always include them.

let city = "Chennai";   // ✅ with semicolon — recommended
let city = "Chennai"    // also works, but avoid relying on this

Code Blocks

Multiple statements can be grouped inside curly braces {}. You'll see this a lot with functions and conditions.

if (age > 18) {
  console.log("Adult");
  console.log("Can vote");
}

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3. JavaScript Comments

Comments are notes you write in your code for yourself or other developers. JavaScript completely ignores them when running the code.

There are two types:

Single-line comments

Use // to comment out a single line.

// This is a single-line comment
let price = 500; // price is in rupees

Multi-line comments

Use /* ... */ to comment across multiple lines.

/*
  This function calculates the total price
  including GST.
  Written by: Karthick
*/
function calculateTotal(price) {
  return price * 1.18;
}

💡 Tip: Use comments to explain why you did something, not just what you did. The code itself shows what — the comment should explain why.

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4. JavaScript Variables

A variable is like a labelled box — it stores a value that your program can use later.

Declaring a variable with var

var is the old way to declare variables (before 2015). You'll still see it in older code.

var greeting = "Hello!";
console.log(greeting); // Hello!

var has some quirky behavior (like being accessible outside the block it was defined in), which is why modern JavaScript uses let and const instead — covered in Part 2!

Naming variables

Variable names in JavaScript must follow these rules:

• Must start with a letter, underscore _, or dollar sign $
• Cannot start with a number
• Cannot use reserved keywords like let, return, function
• Are case-sensitive

let firstName = "Priya";    // ✅ valid
let _count = 0;             // ✅ valid
let $total = 100;           // ✅ valid
let 1stName = "Ravi";       // ❌ invalid — starts with a number
let let = "something";      // ❌ invalid — reserved keyword

Assigning and updating values

var score = 10;
console.log(score); // 10

score = 20;         // updating the value
console.log(score); // 20

Variables without a value

If you declare a variable but don't assign a value, it holds the special value undefined.

var myVariable;
console.log(myVariable); // undefined

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Quick Recap

Concept	What it means
Syntax	Rules for writing valid JavaScript
Statement	A single instruction (often ends with ;)
Comment	Notes in code ignored by JavaScript
Variable	A named container to store values