JavaScript Guide 2026: The Modern Handbook
A modern JavaScript handbook — ES2026 features, async patterns, modules, and the runtime concepts every engineer needs.
Modules
ESM everywhere. type="module" in browsers, package.json {"type":"module"} in Node.
Named imports/exports; dynamic import() for code splitting.
Top-level await lets modules await at module scope.
Async Patterns
async/await over then chains.
Promise.all for parallel, Promise.allSettled when partial failures OK, Promise.any for first-success.
AbortController for cancellation of fetch, listeners, and async workflows.
for await...of for async iterables (streams, paginated APIs).
Collections & Utilities
Map/Set over object-as-map.
Array.prototype.at, findLast, groupBy, toSorted/toReversed for immutable ops.
Object.hasOwn over hasOwnProperty.
structuredClone for deep copies.
Async APIs cheat sheet
| Need | API |
|---|---|
| Run in parallel | Promise.all |
| Wait for first success | Promise.any |
| Collect all outcomes | Promise.allSettled |
| Cancel a fetch | AbortController |
| Iterate a stream | for await...of |
| Timeout a promise | Promise.race with setTimeout |
Error Handling
Error causes: throw new Error('msg', { cause }).
try/catch around await points; propagate meaningful errors.
Global handlers: window.onerror + unhandledrejection.
TypeScript by Default
TS is the industry standard for anything beyond a script.
strict: true, noUncheckedIndexedAccess, exactOptionalPropertyTypes.
Zod / Valibot for runtime schema validation.
Runtime Understanding
Event loop: sync stack → microtasks → macrotasks.
Await schedules a microtask — matters for ordering.
Memory: watch for closures capturing large objects; use WeakMap/WeakRef when needed.
2026 Landscape
Temporal (Date replacement) — Stage 3, shipping soon.
Records/Tuples (immutable primitives) — Stage 3.
Import Attributes (JSON/CSS modules).
Explicit Resource Management (using declarations).
Frequently Asked Questions
Should I still learn Promises before async/await?+
Yes — async/await is sugar over Promises. Understanding the underlying primitives helps debugging.
Var, let, or const?+
const by default. let only when reassigned. Never var in modern code.
Is TypeScript required?+
For any real project, yes. Even solo work benefits from strict types.
Is jQuery still relevant?+
No. Modern DOM APIs (querySelector, fetch, addEventListener) cover its use cases.
What runtime should I use for backend JS?+
Node LTS for stability. Bun for speed. Deno for security-first setups.