== vs === and truthy/falsy traps

JSjunior

New file in the project — a colleague is creating a utility for filtering products and checking stock. It passes the tests, but it already crashed in prod on "weird" products. Walk the diff and find every loose-equality and truthy/falsy trap.

PR: lesson.javascript +16 −04 проблем

1-// products-utils.js — new file

1+export function filterValidProducts(products) {

2+ return products.filter((p) => {

3+ if (p.available == true) {

4+ if (p.price && p.name) {

5+ if (p.discount != null) {

6+ return p.price - p.discount > 0

7+ }

8+ return true

9+ }

10+ }

11+ return false

12+ })

13+}

14+

15+export function hasStock(product) {

16+ return product.stock ? 'yes' : 'no'

17+}

Шаг 1 из 6

The main JavaScript traps

JavaScript has two huge bug categories: 1. **Loose equality (`==`, `!=`)** — comparison with type coercion. `'0' == 0 == false` is `true`. Hell. 2. **Truthy/falsy coercion** — when you write `if (x)`, JS turns `x` into a boolean by non-obvious rules. `0`, `''`, `null`, `undefined`, `NaN`, `false` — all falsy, everything else truthy. Rule: always `===`, always explicit checks. Walk the code and find every trap.

ArchCase

ArchCase

== vs === and truthy/falsy traps

The main JavaScript traps