JS References

JS Keywords Reference

Keywords are reserved words that have a special meaning in JavaScript. You cannot use them as variable names in the same context.

// Examples
let x = 10;
const y = 20;

if (x < y) {
  console.log("x is smaller");
}

Common keywords you will use often:

• let, const, var
• if, else, switch, case, default
• for, while, do, break, continue
• function, return, class, extends, new
• try, catch, finally, throw
• import, export, await, async
• typeof, instanceof, in, delete

JS Keywords Reserved

Reserved keywords and future-reserved words should not be used as identifiers (variable/function/class names). Some are only reserved in strict mode or in certain contexts.

Tip: If you need a name close to a keyword, add a suffix/prefix:

// Avoid: let class = 1; // invalid
let className = "Card";
let defaultValue = 0;

JS Operator Reference

Operators perform operations on values. Here are the main operator groups with short examples.

• Arithmetic: + - * / % **
• Assignment: = += -= *= /= ??= &&= ||=
• Comparison: == === != !== > >= < <=
• Logical: && || !
• Nullish: ?? (nullish coalescing)
• Ternary: condition ? a : b
• Optional chaining: ?.
• Bitwise: & | ^ ~ << >>

let a = 10;
let b = 3;

console.log(a + b);   // 13
console.log(a ** b);  // 1000

console.log(a === 10); // true

const user = null;
console.log(user ?? "Guest"); // Guest

JS Operator Precedence

Operator precedence decides which operations run first. When in doubt, use parentheses to make it explicit.

Common precedence (high to low):

• Parentheses: ( )
• Member access / call: ., [], (), optional chaining ?.
• Unary: !, typeof, + (unary), - (unary)
• Exponentiation: **
• Multiplication/division: *, /, %
• Addition/subtraction: +, -
• Comparisons: >, >=, <, <=, instanceof, in
• Equality: ==, ===, !=, !==
• Logical AND: &&
• Logical OR: ||
• Nullish coalescing: ?? (be careful mixing with &&/||)
• Ternary: ?:
• Assignment: =, +=, ??= ...

let x = 10 + 2 * 3;     // 16
let y = (10 + 2) * 3;   // 36

console.log(x, y);

// Precedence tip: be explicit with parentheses
const value = null;
const result = (value ?? 0) + 5;
console.log(result); // 5

Next Step

Continue with JS ECMAScript 2026 (or JS Versions) to understand how JavaScript evolves and which features are safe to use.