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javascript.md

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+# JavaScript Patterns and Traps
+
+## Type Coercion
+
+- **`==` vs `===`** — Always use `===`; `"0" == false` is true, `"0" === false` is false
+- **`typeof null`** — Returns `"object"` (bug from JS v1); check with `=== null`
+- **`NaN !== NaN`** — Use `Number.isNaN()` not `=== NaN`
+- **Array in boolean context** — Empty array `[]` is truthy; check `.length` for emptiness
+
+## Async
+
+- **`forEach` doesn't await** — Use `for...of` loop or `Promise.all(arr.map(async...))` for parallel
+- **Unhandled rejection** — Always `.catch()` or wrap in try/catch; uncaught rejections crash Node
+- **`async` function returns Promise** — Even if you return a value, caller gets a Promise
+- **Race conditions** — Multiple `setState` calls can overwrite; use functional updates or refs
+
+## DOM
+
+- **`querySelector` returns null** — Check before accessing properties; `document.querySelector('.x').classList` crashes if `.x` missing
+- **Event delegation** — Add listener to parent, check `e.target`; better than listeners on each child
+- **`innerHTML` security** — Never insert user content with `innerHTML`; use `textContent` or sanitize
+- **Live vs static NodeLists** — `getElementsByClassName` is live (updates); `querySelectorAll` is static
+
+## Objects/Arrays
+
+- **Shallow copy** — `{...obj}` and `[...arr]` are shallow; nested objects share references
+- **Array holes** — `Array(5)` creates holes; `.map()` skips them; use `Array(5).fill()` instead
+- **`delete` on array** — Creates hole, doesn't shift; use `.splice()` to remove elements
+- **Object key order** — Guaranteed insertion order for string keys; numeric keys sort ascending
+
+## Functions
+
+- **`this` in arrow functions** — Lexically bound; can't be changed with `.bind()`, `.call()`, `.apply()`
+- **Default parameters evaluate** — `fn(x = Date.now())` evaluates on each call, not definition
+- **Rest parameters must be last** — `fn(...rest, other)` is syntax error