JavaScript Optimizations & Best Practices

30 JavaScript Optimizations (The Pro Checklist)

Variables & Scope

#	Optimization	Why
1	Use const by default, let when needed	Communicates intent, enables engine optimizations
2	Never use var	Function-scoped, hoisted to undefined, causes bugs
3	Avoid global variables	Pollute namespace, slow lookups, prevent GC
4	Use Set for uniqueness checks	set.has() is O(1) vs array.indexOf() O(n)
5	Use literal notation ({}, [])	Faster than new Object(), new Array()

DOM & Events

#	Optimization	Why
6	Cache DOM references	Avoid repeated querySelector calls
7	Use DocumentFragment	Batch DOM updates into one reflow
8	Use Event Delegation	One listener on parent vs N on children
9	Debounce input/resize events	Prevent excessive function calls
10	Use requestAnimationFrame	Sync with browser paint cycle for smooth animations

Async & Network

#	Optimization	Why
11	Use Promise.all for parallel fetches	Don't await sequentially for independent requests
12	Use AbortController	Cancel stale/unnecessary requests
13	Use Web Workers for CPU-heavy tasks	Keep the main thread responsive
14	Use async/await over callbacks	Readable, debuggable, proper error handling
15	Use IntersectionObserver	Efficient lazy loading without scroll listeners

Memory & Performance

#	Optimization	Why
16	Avoid eval()	Security risk, kills engine optimizations
17	De-reference large objects	Set to null to help GC
18	Avoid closures in loops	Can accidentally trap large data in memory
19	Use WeakMap for caches	Entries are GC'd when keys are no longer referenced
20	Keep object shapes consistent	V8 Hidden Classes optimize consistent structures

Code Quality

#	Optimization	Why
21	Use Optional Chaining (?.)	Reduces verbose null checks
22	Use Nullish Coalescing (??)	Only defaults on null/undefined (not 0 or "")
23	Use static import	Enables tree shaking by bundlers
24	Keep functions small	Small functions are more likely to be JIT-inlined
25	Avoid try/catch inside loops	Prevents JIT optimization of the loop
26	Profile with Chrome DevTools	Don't guess — measure with Performance & Memory tabs
27	Use for/while for huge arrays	Faster than forEach for 100K+ elements
28	Pre-allocate array size	Avoids dynamic resizing overhead
29	Use short-circuit evaluation	&&, || for concise conditional logic
30	Use JSON.parse sparingly	Synchronous, blocking — use streaming for large JSON