Part 29 — Cheat Sheets

Truthy / Falsy#

Falsy (all 8): false, 0, -0, 0n, "", null, undefined, NaN. Everything else truthy (incl. "0", "false", [], {}).

Equality quick table#

Array methods — mutate vs return-new#

Mutate: push pop shift unshift splice sort reverse fill copyWithin. Return new / non-mutating: map filter reduce slice concat flat flatMap find findIndex some every includes indexOf join at toSorted toReversed toSpliced with.

String methods#

length at slice substring toUpperCase toLowerCase trim includes startsWith endsWith indexOf replace replaceAll split padStart padEnd repeat match matchAll normalize codePointAt localeCompare.

Object methods#

keys values entries fromEntries assign create freeze seal preventExtensions hasOwn getOwnPropertyNames getPrototypeOf setPrototypeOf defineProperty groupBy · structuredClone for deep clone.

Promise APIs#

then catch finally · Promise.resolve/reject · Promise.all all any(→ AggregateError) race allSettled · Promise.withResolvers · async/await.

Loops#

for · while · do…while · for…of (values) · for…in (keys, incl. inherited) · for await…of (async). forEach/map/filter/reduce — no break, ignore await.

this rules#

new > call/apply/bind > obj.method() > default(undefined/global); arrows = lexical.

Time complexity (arrays)#

index O(1) · push/pop O(1) · shift/unshift O(n) · splice O(n) · search O(n) · sort O(n log n).

Script loading#

defer = parse-then-run, ordered, non-blocking (🟢). async = run-when-ready, unordered. modules = deferred.

DOM essentials#

querySelector(All) · createElement · append/prepend/remove · textContent (safe) vs innerHTML (XSS) · addEventListener(type, fn, {capture,once,passive}) · e.target/currentTarget · e.preventDefault()/stopPropagation().

Conversions#

Number() parseInt(str, radix) parseFloat() String() Boolean() / !! Array.from() JSON.parse/stringify (numbers).toString(radix).