Values, Variables and Operators

Declaring variables

const taxRate = 0.2;      // cannot be reassigned
let total = 0;            // can be reassigned
total = total + 10;

Use const by default and let when the value must change. Avoid the old var: its scoping rules cause bugs.

Primitive types

const count = 42;              // number (integers and decimals share one type)
const price = 19.99;
const name = "Ada";            // string
const isAdmin = false;         // boolean
let phone = null;              // intentionally empty
let nickname;                  // undefined: never assigned
const big = 9007199254740993n; // bigint

typeof tells you the type: typeof "x" is "string". (Quirk: typeof null is "object".)

Numbers and money

0.1 + 0.2 is 0.30000000000000004, as in most languages. Store money as integer cents and format at the end:

const cents = 1999 * 3;
(cents / 100).toFixed(2);   // "59.97"
Number("42");               // 42
Number("abc");              // NaN (not a number)
Number.isNaN(Number("abc"));// true

Strings

const product = "Hoodie";
`${product} costs $${(3900 / 100).toFixed(2)}`;   // template literal
product.toUpperCase();
product.includes("ood");      // true
product.slice(0, 3);          // "Hoo"
"a,b,c".split(",");           // ["a", "b", "c"]
"  hi  ".trim();              // "hi"

Equality: always use ===

0 == "0"     // true  (coerces types: confusing)
0 === "0"    // false (strict: compares type and value)
null == undefined   // true
null === undefined  // false

Use === and !== everywhere.

Truthy and falsy

Falsy values: false, 0, "", null, undefined, NaN. Everything else is truthy, including "0" and [].

Useful operators

const displayName = user.nickname || user.name;   // first truthy value
const limit = settings.limit ?? 20;               // only null/undefined fall back (0 is kept)
const city = user.address?.city;                  // optional chaining: no error if address is missing
count += 1;  count++;

?? is safer than || for numbers and empty strings that are valid values.

Exercise

Write a function-free script that takes const priceCents = 2599 and const qty = 3, computes a 20% tax, and logs a receipt line using a template literal and toFixed(2).

Check yourself

• When should you use let instead of const?
• Why is === preferred over ==?
• What is the difference between || and ???