JavaScript — Statements

Programs read top to bottom

A statement is one instruction. Scripts execute statements in order, one after another:

console.log("First");
console.log("Second");
console.log("Third");

What you see

First
Second
Third

No surprises yet — but this ordering is the foundation for everything (and the source of bugs when scripts run before HTML exists).

Statements end with semicolons… mostly

let a = 3;
let b = 4;
let sum = a + b;

Semicolons mark statement boundaries. Modern JavaScript also inserts them automatically ("automatic semicolon insertion"), so many style guides skip them:

let a = 3    // both styles exist in the wild

Pick a convention, stick to it. The classic hazard of skipping them:

const x = 10
[1, 2].forEach(console.log)   // parsed as 10[1,2] → crash!

Rare but real — starting a line with ( or [ right after an unterminated statement bites even experienced devs.

Whitespace is free

Spaces, tabs and newlines don't change meaning:

let name="Ada";          // cramped but valid

let greeting = "Hello";  // readable — do this

Format for the human who reads next. Consistent indentation (2 spaces is the JS convention) turns debugging from misery into scanning.

Blocks group statements

Curly braces bind statements into one unit:

{
    let local = "only visible inside";
    console.log(local);
}

You'll rarely write bare blocks — but they're the skeleton of every if, for and function:

if (age >= 18) {
    console.log("Adult");
    console.log("May proceed");
}

The braces make both prints conditional.

Case sensitivity — exact or nothing

JavaScript distinguishes upper from lower case ruthlessly:

let firstName = "Ada";
console.log(firstname);   // ReferenceError!
console.log(firstName);   // works

Same for keywords: getElementById, not getElementByID. When you meet "X is not defined" errors, first suspect spelling.

Comments

// single line comment

/* multi-line
   comment */

/**
 * Docblock — convention for describing functions
 */
function add(a, b) {
    return a + b;
}

Reserved words can't be names

let let = 5;        // SyntaxError
let class = "math"; // reserved — use className instead

Dozens of words (return, new, if, class…) are off-limits as variable names.

Mini Practice

1. Log five lines; predict output before running
2. Trigger the [1,2] semicolon crash on purpose; then fix it two ways (semicolon / leading semicolon)
3. Write an if-block containing two statements; move one outside the braces and compare behavior
4. Create myVar and myvar; confirm they're different variables
5. Comment out one line mid-program and verify the flow skips it

Next: syntax → (deepens naming/rules)