Part 3 — Syntax

Comments#

// single line
/* multi
   line */
/** JSDoc — tooling reads these for types & hints
 * @param {number} x
 * @returns {number}
 */

Semicolons and ASI#

JavaScript has Automatic Semicolon Insertion. You can omit most semicolons, but the rules bite in a few cases:

// ⚠️ ASI trap 1: return with a newline
function f() {
  return
    42;       // returns undefined! ASI inserts ; after return
}

// ⚠️ ASI trap 2: line starting with ( [ ` / + -
const a = 1
const b = 2
[a, b].forEach(x => console.log(x))  // parsed as: const b = 2[a, b].forEach(...) → crash

🟢 Best practice: pick a style and let Prettier enforce it. If you omit semicolons, know the "leading ([ ⁠" gotcha and prefix such lines with ;`.

Identifiers, keywords, naming#

• Identifiers may contain letters, digits, _, $, and start with anything but a digit.
• Reserved words (if, for, class, return, await, yield, …) can't be identifiers.
• Case-sensitive: name ≠ Name.

Conventions:

Strict mode#

"use strict";   // top of file or top of a function

Why it exists: to remove sloppy, error-hiding behaviours. In strict mode:

• Assigning to an undeclared variable throws (instead of creating a global).
• this in a plain function call is undefined, not the global object.
• Duplicate parameter names and octal literals are errors.
• Silent assignment failures (to read-only props) throw.

🟢 ES modules and class bodies are always strict — you rarely need to write "use strict" in modern code.

Literals, expressions, statements#

• Literal — a value written directly: 42, "hi", true, [1,2], {a:1}, /re/g.
• Expression — produces a value: 2 + 2, f(), x ? a : b.
• Statement — performs an action: if, for, let x = 1;. Statements don't produce values you can assign.

⚠️ Trap: {} at the start of a line is a block, not an object. {} + [] parses {} as an empty block, then +[] → 0. Wrap in parens to force object context: ({}) + [].