JavaScript Statements and Syntax

A JavaScript program is a list of instructions called statements, written using a few simple rules called syntax.

Statements

Each statement tells the browser to do one thing. The browser runs them one by one, from top to bottom.

<!DOCTYPE html>
<html>
<body>

<p id="demo"></p>

<script>
let price = 250;                 // statement 1
let quantity = 3;                // statement 2
let bill = price * quantity;     // statement 3
document.getElementById("demo").innerHTML = "Bill: Rs " + bill;  // statement 4
</script>

</body>
</html>

Bill: Rs 750

Semicolons

A semicolon ; ends a statement. JavaScript can often guess where a statement ends without it, but adding semicolons is a good habit and avoids strange bugs.

let a = 5; let b = 10; let c = a + b;

Several statements can be on one line if they are separated by semicolons, but one statement per line is easier to read.

White Space and Line Length

JavaScript ignores extra spaces. These two lines do the same thing:

let city="Karachi";      // hard to read
let town = "Karachi";    // easier to read

Add spaces around operators like = and + to make code readable. Keep lines short (under about 80 characters).

Code Blocks

Statements can be grouped inside curly brackets { }. A group like this is called a code block. You will see blocks in functions, conditions and loops.

function welcome() {
  let name = "Ali";
  console.log("Welcome, " + name);
}

Values: Literals and Variables

JavaScript works with two kinds of values:

• Literals are fixed values written directly, like 100, 3.5 or "Hello"
• Variables are named boxes that store values, like price or name

Text (a string) is written inside double or single quotes: "Karachi" or 'Karachi'.

Keywords

Some words have a special meaning in JavaScript and cannot be used as names.

Comments

Comments are notes for humans. JavaScript ignores them.

// This is a one line comment
let fee = 5000; // a comment at the end of a line

/*
  This is a comment
  on several lines
*/

Comments are also handy for switching off a line while testing.

Identifiers (Names)

Names you give to variables and functions are called identifiers.

• They can contain letters, digits, _ and $
• They cannot start with a digit
• They are case sensitive: total and Total are different

Most JavaScript developers use camelCase: the first word is lowercase and each next word starts with a capital letter, like studentName or totalFee.

Exercise