Syntax & Statements

Syntax is the set of rules for how JavaScript code is written: how statements end, how blocks are grouped, how names are spelled and how comments are marked. These rules are small in number, and getting comfortable with them now saves a lot of confusing errors later.

Statements

A program is a list of statements — instructions the engine runs one after another, top to bottom:

JS statements.js

let price = 20;
let quantity = 3;
let total = price * quantity;
console.log(total);

Output

Each line here is one statement. Statements usually end with a semicolon ;.

Semicolons

JavaScript has a feature called automatic semicolon insertion: if you leave a semicolon off at the end of a line, the engine usually adds one for you. So both of these work:

JS semicolons.js

let a = 1;
let b = 2
console.log(a + b)

Output

"Usually" is the important word. There are rare cases where a missing semicolon changes the meaning of the code. Many teams always write semicolons; others never do and rely on a formatter to catch the edge cases. Pick one style and let a tool like Prettier enforce it. This course uses semicolons.

Expressions vs statements

An expression is any piece of code that produces a value: 5, price * quantity, "Hi " + name. A statement performs an action, and often contains expressions. let total = price * quantity; is a statement containing the expression price * quantity.

Code blocks

Curly braces { } group statements into a block. Blocks belong to things like if statements, loops and functions:

JS blocks.js

let temperature = 31;

if (temperature > 30) {
  console.log("It's hot.");
  console.log("Drink some water.");
}

Output

It's hot.
Drink some water.

Indentation (usually two or four spaces) inside blocks doesn't change what the code does, but it makes structure visible at a glance. Always indent consistently.

Comments

Comments are notes for humans; the engine ignores them completely:

JS comments.js

// A single-line comment runs to the end of the line.

/*
  A multi-line comment
  can span several lines.
*/

let speed = 50; // comments can also follow code
// console.log("this line is switched off");
console.log(speed);

Output

Good comments explain why code exists, not what it does — the code already says what. "Commenting out" a line, as on the second-to-last line, is a quick way to disable it while testing.

Case sensitivity

JavaScript is case-sensitive: total, Total and TOTAL are three different names. Keywords such as let, if and function must be lowercase:

JS case.js

let score = 10;
console.log(Score);

Output

Uncaught ReferenceError: Score is not defined

Identifiers and naming

Names for variables and functions are called identifiers. They can contain letters, digits, _ and $, but can't start with a digit, and can't be a reserved word like if, class or return.

Style	Example	Used for
camelCase	`firstName`, `getUserById`	Variables and functions — the standard JavaScript style
PascalCase	`ShoppingCart`	Classes and constructors
UPPER_SNAKE_CASE	`MAX_RETRIES`	Fixed configuration constants

Whitespace

Extra spaces, tabs and line breaks between tokens are ignored, so let x=1+2; and let x = 1 + 2; are identical. Use whitespace to make code readable: a space around operators, and blank lines between logical sections.

Try it yourself

This code has a bug caused by capitalisation. Fix it so it prints the greeting:

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