modern-javascript

Modern JavaScript (ES6-ES2026)

Write clean, performant, maintainable JavaScript using modern language features. This skill covers ES6 through ES2026, emphasizing immutability, functional patterns, and expressive syntax — and it pins each feature to the ECMAScript edition and runtimes that actually support it, because misattributed versions are the most common way modern-JS advice breaks.

Quick Decision Trees

"Which array method should I use?"

What do I need?
├─ Transform each element           → .map()
├─ Keep some elements               → .filter()
├─ Find one element                 → .find() / .findLast()
├─ Check if condition met           → .some() / .every()
├─ Reduce to single value           → .reduce()
├─ Get last element                 → .at(-1)
├─ Sort without mutating            → .toSorted()
├─ Reverse without mutating         → .toReversed()
├─ Group by property                → Object.groupBy() / Map.groupBy()
├─ Collect an async iterable        → Array.fromAsync()
└─ Flatten nested arrays            → .flat() / .flatMap()

"How do I handle nullish values?"

Nullish handling?
├─ Safe property access              → obj?.prop / obj?.[key]
├─ Safe method call                  → obj?.method?.()
├─ Default for null/undefined only   → value ?? 'default'
├─ Default for any falsy             → value || 'default'
├─ Assign if null/undefined          → obj.prop ??= 'default'
└─ Check property exists             → Object.hasOwn(obj, 'key')

"Should I mutate or copy?"

Always prefer non-mutating methods:
├─ Sort array      → .toSorted()    (not .sort())
├─ Reverse array   → .toReversed()  (not .reverse())
├─ Splice array    → .toSpliced()   (not .splice())
├─ Update element  → .with(i, val)  (not arr[i] = val)
├─ Add to array    → [...arr, item] (not .push())
└─ Merge objects   → {...obj, key}  (not Object.assign())

ES Version Quick Reference

Editions are set by which proposals reach TC39 Stage 4 before the spring cutoff, so a feature's edition often lags its browser availability (e.g. Array.fromAsync shipped in browsers in 2023-24 but is spec'd in ES2026). Cite editions from this table, not from memory.

Already Stage 4 and slated for ES2027: Temporal, explicit resource management (using/await using), Atomics.pause(), Iterator.zip(). Decorators remain Stage 3 (transpiler only). Records & Tuples (#{}/#[]) was withdrawn — never emit that syntax. See references/UPCOMING.md.

Runtime Support Rules of Thumb

State the baseline when reaching for newer features; suggesting .toSorted() to someone on Node 18 produces a runtime TypeError: arr.toSorted is not a function.

• Safe almost everywhere (Baseline widely available): everything through ES2023, plus ES2024 grouping/Promise.withResolvers and Array.fromAsync (Node 21-22+, Safari 16.4-17.4+).
• Needs a modern baseline (2024-2025 browsers, Node 22-23+): Set methods, iterator helpers, import attributes, Promise.try, duplicate named capture groups.
• Cutting edge (Node 24+, 2025+ browsers; Safari partly behind flags): RegExp.escape, Float16Array, using/await using, Error.isError, Uint8Array base64.
• Native but not universal: Temporal (Firefox 139+, Chrome 144+, Node 26+; polyfill elsewhere). Decorators always need Babel or TypeScript.

Per-feature version tables live at the end of references/ES2024.md, references/ES2025.md, and references/ES2026.md.

Modernization Patterns

Array Access

// ❌ Legacy
const last = arr[arr.length - 1];
const secondLast = arr[arr.length - 2];

// ✅ Modern (ES2022)
const last = arr.at(-1);
const secondLast = arr.at(-2);

Non-Mutating Array Operations

// ❌ Mutates original array (and returns the same reference)
const sorted = arr.sort((a, b) => a - b);
const reversed = arr.reverse();

// ✅ Returns new array (ES2023)
const sorted = arr.toSorted((a, b) => a - b);
const reversed = arr.toReversed();
const updated = arr.with(2, 'new value');
const removed = arr.toSpliced(1, 1);

String Replacement

// ❌ Legacy with regex
const result = str.replace(/foo/g, 'bar');

// ✅ Modern (ES2021) — no escaping worries for literal strings
const result = str.replaceAll('foo', 'bar');

Grouping Data

// ❌ Manual grouping
const grouped = items.reduce((acc, item) => {
  const key = item.category;
  acc[key] = acc[key] || [];
  acc[key].push(item);
  return acc;
}, {});

// ✅ Modern (ES2024)
const grouped = Object.groupBy(items, item => item.category);

Gotcha: Object.groupBy returns a null-prototype object — property access and destructuring work, but grouped.hasOwnProperty(...) throws. Use Object.hasOwn(). Use Map.groupBy when keys are objects or non-strings. There is no Array.prototype.group() — that earlier shape of the proposal never shipped.

Nullish Handling

// ❌ Falsy check (0, '', false are valid values)
const value = input || 'default';
const name = user && user.profile && user.profile.name;

// ✅ Nullish check (only null/undefined)
const value = input ?? 'default';
const name = user?.profile?.name;

Property Existence

// ❌ Can be fooled by prototype or overwritten hasOwnProperty
if (obj.hasOwnProperty('key')) { }

// ✅ Modern (ES2022) — also works on null-prototype objects
if (Object.hasOwn(obj, 'key')) { }

Logical Assignment

// ❌ Verbose assignment
if (obj.prop === null || obj.prop === undefined) {
  obj.prop = 'default';
}

// ✅ Modern (ES2021)
obj.prop ??= 'default';  // Assign if null/undefined
obj.count ||= 0;         // Assign if falsy
obj.enabled &&= check(); // Assign if truthy

Async Patterns

Promise Combinators

// Wait for all, fail if any fails
const [users, posts] = await Promise.all([fetchUsers(), fetchPosts()]);

// Wait for all, get status of each
const results = await Promise.allSettled([fetchA(), fetchB()]);
results.forEach(r => {
  if (r.status === 'fulfilled') console.log(r.value);
  else console.error(r.reason);
});

// First to succeed
const fastest = await Promise.any([fetchFromCDN1(), fetchFromCDN2()]);

// First to settle
const winner = await Promise.race([fetchData(), timeout(5000)]);

Promise.withResolvers (ES2024)

// ❌ Legacy pattern
let resolve, reject;
const promise = new Promise((res, rej) => {
  resolve = res;
  reject = rej;
});

// ✅ Modern (ES2024)
const { promise, resolve, reject } = Promise.withResolvers();

Top-Level Await (ES2022)

// In ES modules, await at top level
const config = await fetch('/config.json').then(r => r.json());
const db = await connectDatabase(config);

export { db };

Functional Patterns

Immutable Object Updates

// Add/update property
const updated = { ...user, age: 31 };

// Remove property
const { password, ...userWithoutPassword } = user;

// Nested update
const newState = {
  ...state,
  user: { ...state.user, name: 'New Name' }
};

Array Transformations

// Chain transformations (ES2023)
const result = users
  .filter(u => u.active)
  .map(u => u.name)
  .toSorted();

// Using flatMap for filter+map (single pass)
const activeNames = users.flatMap(u => u.active ? [u.name] : []);

// ES2024: Group then process
const byStatus = Object.groupBy(users, u => u.active ? 'active' : 'inactive');
const activeUserNames = byStatus.active?.map(u => u.name) ?? [];

Composition

const pipe = (...fns) => x => fns.reduce((v, f) => f(v), x);
const compose = (...fns) => x => fns.reduceRight((v, f) => f(v), x);

const processUser = pipe(
  user => ({ ...user, name: user.name.trim() }),
  user => ({ ...user, email: user.email.toLowerCase() }),
  user => ({ ...user, createdAt: new Date() })
);

Destructuring Patterns

Object Destructuring

// Basic with rename and default
const { name: userName, age = 18 } = user;

// Nested
const { address: { city, country } } = user;

// Rest
const { id, ...userData } = user;

Array Destructuring

// Skip elements
const [first, , third] = array;

// Rest
const [head, ...tail] = array;

// Swap variables
[a, b] = [b, a];

// Function returns
const [x, y] = getCoordinates();

Anti-Patterns

Best Practices

1. Use const by default — Only use let when reassignment is needed
2. Prefer arrow functions — Especially for callbacks and short functions
3. Use template literals — Instead of string concatenation
4. Destructure early — Extract what you need at function start
5. Avoid mutations — Use .toSorted(), .toReversed(), spread operator
6. Use optional chaining — Prevent "Cannot read property of undefined"
7. Use nullish coalescing — ?? for defaults, not || (unless intentional)
8. Prefer array methods — .map(), .filter(), .find() over loops
9. Use async/await — Instead of .then() chains
10. State the runtime baseline — Note the Node/browser floor when using post-ES2023 features

Reference Documentation

Open the reference that matches the task; SKILL.md alone is enough for everyday syntax choices.

ES Version References

Pattern References

Resources

Specifications

• ECMAScript Specification: https://tc39.es/ecma262/ (living standard)
• TC39 Proposals: https://github.com/tc39/proposals (stage status; finished-proposals.md maps features to editions)
• TC39 Process: https://tc39.es/process-document/ (how features are added)

Documentation

• MDN Web Docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript
• JavaScript.info: https://javascript.info/

Compatibility

• Can I Use: https://caniuse.com (browser support tables)
• Node.js ES Compatibility: https://node.green/