JavaScript Syntax

JavaScript syntax defines how we write statements, expressions, identifiers, comments, and literals. Mastering these basics makes later topics like variables, functions, and DOM much easier.

1) Statements & Semicolons (ASI)

Each instruction is a statement. Semicolons are optional because of Automatic Semicolon Insertion (ASI), but keeping semicolons avoids rare edge cases.


console.log('Hello'); // statement 1
console.log('World'); // statement 2
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Gotcha (ASI): Lines that start with (, [, +, -, etc. can join with the previous line unexpectedly. Keep semicolons or begin such lines with one.

2) Comments

Use comments to document intent or temporarily disable code.


// Single-line comment
console.log('Visible'); // runs

/* Multi-line
   comment block */
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3) Identifiers & Keywords

Identifiers (names) can contain letters, digits (not first), _, $. They are case-sensitive. Reserved keywords cannot be used as variable names.


let userName = 'Sonu';  // ok
let $count = 10, _total1 = 20; // ok
// let class = 1; // ❌ reserved keyword
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4) Whitespace & New Lines

Whitespace usually doesn’t matter. Format code for readability. Avoid ASI pitfalls by using semicolons.


let a=1, b=2;
console.log(
  a + b
); // 3
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5) Code Blocks { ... } (Scope Preview)

Blocks group statements. Variables declared with let/const are block-scoped; we’ll go deeper on the next page.


if (true) {
  let msg = 'inside';
  console.log(msg);
}
// msg is not accessible here
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6) Literals (Numbers, Strings, Booleans, null, undefined)

Write values directly with literals. Numbers are 64-bit floating point; strings are in quotes; booleans are true/false.


const n = 42, pi = 3.14;
const s1 = 'single', s2 = "double";
const ok = true, bad = false;
const empty = null, missing = undefined;
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7) Strings: Escaping & Template Literals

Escape characters with backslashes; use backticks (`) for interpolation and multi-line strings.


const quote = 'It\'s fine';
const name = 'Sonu';
const msg = `Hello, ${name}! 2+2=${2+2}`;
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8) Expressions vs Statements

An expression produces a value; a statement performs an action. Many statements contain expressions.


// expression:
(2 + 3) * 4

// statement (declaration + expression):
let sum = 2 + 3;
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9) Strict Mode

Add 'use strict' at the top of a file or function to catch common mistakes. ES modules are strict by default.


'use strict';
// Following would throw ReferenceError (x not declared):
// x = 10;
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10) Script Placement & defer/async

Put scripts before </body> or use defer/async so HTML parses first.


<!-- Recommended: parsed first, executed after HTML -->
<script src="app.js" defer></script>

<!-- async runs as soon as it downloads (order may vary) -->
<script src="analytics.js" async></script>
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11) Reserved Words (Cannot Be Identifiers)

Some words are part of the language grammar and cannot be used as variable names.


// Examples: let, const, var, function, class, return, if, else, switch, case,
// default, for, while, do, break, continue, try, catch, finally, throw,
// import, export, new, delete, typeof, instanceof, in, of, await, yield
Next Step: You now know the building blocks of JS syntax. Continue to Variables to learn how to declare and use values safely with let and const.