javascript

JavaScript

Clarity is the highest JavaScript virtue. If your code requires a comment to explain its control flow, rewrite it.

JavaScript rewards explicit, readable code. Prefer boring patterns that are easy to understand over clever tricks that save characters.

References

• Functions, closures, composition → [${CLAUDE_SKILL_DIR}/references/functions.md] — Arrow function examples, closure patterns, early return, parameter destructuring
• Async patterns, error handling, concurrency → [${CLAUDE_SKILL_DIR}/references/async.md] — Promise.all/race/any examples, cancellation, custom error classes, for-await
• Objects, arrays, iteration, Map/Set → [${CLAUDE_SKILL_DIR}/references/objects-and-arrays.md] — Iteration decision table, destructuring patterns, immutable updates, generators
• ES modules, imports, barrel files → [${CLAUDE_SKILL_DIR}/references/modules.md] — Import ordering, barrel file rationale, directory import pitfalls, dynamic imports
• JSDoc typing, full tag catalog → [${CLAUDE_SKILL_DIR}/references/jsdoc.md] — Full tag reference (@callback, @template, @enum), type assertions, class modifiers
• General JS idioms and edge cases → [${CLAUDE_SKILL_DIR}/references/idioms.md] — Variable/naming examples, equality coercion table, modern syntax patterns

Variables and Declarations

• const by default. Use let only when reassignment is required. Never var.
• const prevents reassignment, not mutation. Objects and arrays declared with const can still be mutated.
• Block scope only. let/const are block-scoped; var is function-scoped and hoists — this causes bugs in loops and conditionals.
• One declaration per line. Never chain const a = 1, b = 2.
• Group declarations. const first, then let.

Naming

Entity	Style	Examples
Variables, functions	camelCase	userName, fetchData
Classes, constructors	PascalCase	UserService, HttpClient
True compile-time constants	SCREAMING_SNAKE_CASE	MAX_RETRIES, API_BASE_URL
Private fields/methods	# prefix (class)	#count, #validate()
Booleans	is/has/can/should prefix	isValid, hasAccess
File names	kebab-case or camelCase	user-service.js, userService.js

• SCREAMING_SNAKE_CASE is for true constants only — values known at compile time, never computed at runtime. A variable holding a function return value uses camelCase.
• Descriptive names. userCount not n. Short names (i, x) only in tiny scopes (loop indices, simple arrow callbacks).
• Accepted abbreviations: url, id, err, ctx, req, res — universally understood. Avoid all others.
• No redundant context. car.make not car.carMake.
• Consistent vocabulary. Use the same word for the same concept throughout a codebase — getUser() everywhere, not getUserInfo() / getClientData() / getCustomerRecord().

Equality and Safety

• Always === and !==. Never == except for value == null (checks both null and undefined).
• ?? over || for defaults — || treats 0, "", false as falsy.
• ?. for optional access. Don't overuse — missing data you expect should throw, not silently return undefined.
• Know the falsy values: false, 0, -0, 0n, "", null, undefined, NaN. Everything else is truthy, including [], {}, and "0".

Ternary Operator

• One-liners or split-per-branch only. Ternaries are acceptable in two forms:

  // OK — fits on one line
  const label = isActive ? 'Active' : 'Inactive';
  
  // OK — each branch on its own line
  const label = isActive
    ? buildActiveLabel(user)
    : buildInactiveLabel(user);
  

  Any ternary that doesn't fit one of these two patterns must be rewritten as if/else or early return.

• Nested ternaries are banned. No exceptions. Use if/else, early returns, or a lookup object.

Modern Syntax

• Template literals for string interpolation: `Hello, ${name}`. Don't use template literals for strings without interpolation — use plain quotes.
• Spread for copies: { ...obj } and [...arr]. Never Object.assign.
• Rest parameters to collect remaining: const { id, ...rest } = user.
• Shorthand properties: { name, age } not { name: name, age: age }. Group shorthand properties at the top of object literals.
• Computed property names: { [key]: value, [${key}Date]: new Date() }.
• Logical assignment operators: opts.timeout ??= 5000 (assign if nullish), opts.name ||= "default" (assign if falsy), opts.handler &&= wrap(opts.handler) (assign if truthy).

Functions

• Arrow functions for callbacks and anonymous functions. Use function declarations only when hoisting or this binding is needed.
• Prefer parentheses around arrow function parameters even for single params — smaller diffs when adding/removing parameters.
• Implicit return for single expressions (no braces). Explicit return (braces) for multi-statement bodies.
• Arrow functions capture lexical this — they do NOT have their own this. Never use arrow functions as object methods or on prototypes.
• Destructured options for 3+ parameters. Self-documenting and order-independent.
• Default parameters over || or manual checks. Defaults are evaluated left-to-right and can reference earlier params.
• Rest parameters over arguments object. arguments is array-like, not a real Array.
• Early return. Guard clauses first, happy path flat. Reduce nesting.
• One function, one job. If the name contains "and", split it.
• Keep functions under ~30 lines. Extract helpers. Use composition over complex branching.
• Prefer pure functions (same input = same output, no side effects). Isolate side effects (DOM, network, logging) — don't hide them inside data transformations.
• Closures retain references to outer variables, not copies. Be cautious with large objects captured unintentionally — they won't be garbage collected until the closure is released.

Async

• async/await over .then() chains for sequential operations.
• Always await promises. Missing await = floating promise = silent failures.
• return await only inside try blocks where you need to catch the awaited error. Otherwise just return promise — no need for async wrapper.
• Promise.all for independent parallel work. Rejects on first rejection.
• Promise.allSettled when all results matter regardless of individual failures.
• Promise.race for timeouts. Promise.any for fallbacks (rejects only when ALL reject).
• Avoid sequential awaits in loops. Use Promise.all(items.map(...)) for parallel. Use a concurrency limiter (e.g. p-map) for large arrays.
• Throw Error objects, never strings or plain objects — strings lose stack traces.
• Custom error classes when callers need to distinguish errors: extend Error, set this.name, add context properties.
• Never swallow errors. Every catch must handle, rethrow, or report. Empty catch blocks hide bugs. console.log(err) alone is not handling.
• Let errors propagate to a top-level handler when possible. Don't wrap every await in try/catch — only where you need to handle at that level.
• Attach .catch() to non-awaited promise chains. Unhandled rejections crash Node.js. Fire-and-forget: fetchData().catch(reportError).
• Only use new Promise() to wrap callback-based APIs. Most async code should compose existing promises with async/await.
• AbortController for cancellable async operations: pass { signal } to fetch and other APIs.
• for await...of for async iterables (streams, async generators).

Modules

• ES modules only. import/export for all new code. CommonJS (require) is legacy — use only when runtime requires it.
• Named exports over default exports. Default exports cause inconsistent naming across importers. Exception: default exports acceptable when required by framework convention (Next.js pages, Remix routes).
• Don't export mutable let bindings. Export accessor functions instead: export function getCount() not export let count.
• Imports at the top, grouped with blank lines: built-in (node:fs), external (express), internal (./utils).
• Always include file extensions in import paths — "./user.js", not "./user". Extensionless imports vary across runtimes.
• No directory imports. Import from the file directly, not from a folder that resolves to index.js.
• No barrel files in subdirectories. index.js re-exports create indirection and hurt tree-shaking. Acceptable only as a standalone package entry point where the runtime can enforce the boundary via package.json exports.
• No circular dependencies. Extract shared code to a third module, merge tightly coupled modules, or use dependency injection.
• No wildcard re-exports. Explicit re-exports only — wildcards bypass tree-shaking.
• Merge imports from the same module into a single statement.
• Namespace imports (import * as dateFns) for large modules (5+ items). Prefer named imports when importing fewer than ~5 items.
• Dynamic imports (import()) for code splitting and lazy loading: routes loaded on navigation, large conditional dependencies, feature flags.
• One concern per module. If a module exports unrelated functionality, split it.
• Side-effect imports (import "./polyfill.js") should be rare. Document why.

Objects and Arrays

• Literal syntax. {} and [], never new Object() / new Array().
• Use method shorthand on objects: greet() { } not greet: function() { }.
• Spread for copies. { ...obj } and [...arr]. Prefer over Object.assign.
• Destructure to extract properties. Prefer parameter destructuring.
• Dot notation for static properties, brackets for dynamic: user.name vs user[dynamicKey].
• Object.hasOwn(obj, key) instead of obj.hasOwnProperty(key).
• Functional array methods (map, filter, find, some, every, flatMap, reduce) over imperative loops for data transformation.
• Always return in map, filter, reduce callbacks.
• Array.from(arrayLike) for array-like objects (not spread). Array.from(iterable, mapFn) instead of [...iterable].map(mapFn) — avoids intermediate array.
• Don't mutate inputs. Return new objects/arrays. Immutable update patterns: add [...arr, item], remove arr.filter(...), update arr.map(...).
• for...of for side-effect loops. Never for...in on arrays.
• Return objects for multiple values, not arrays — callers don't depend on order.
• Never extend built-in prototypes (Array.prototype, Object.prototype). Use utility functions or subclasses.

Prefer for...of for side-effect loops, Array.prototype methods (.map, .filter, .reduce, .find, .some, .every) for data transforms, for for index-needed loops. See ${CLAUDE_SKILL_DIR}/references/objects-and-arrays.md for the full iteration decision table.

Use Map when keys aren't strings or are user-provided (avoids prototype pollution). Use Set for dedup ([...new Set(items)]). Use generators (function*) for lazy sequences and deferred computation.

Classes

• ES class syntax only. No function constructors or prototype manipulation.
• #private fields for encapsulation. Not _ convention.
• Composition over inheritance. Use extends only for true "is-a" relationships.
• No empty constructors. If the constructor only calls super(), omit it.
• Static methods for operations that don't need instance state.
• Methods can return this for fluent/chainable APIs.
• Don't force classes when plain functions and objects suffice. A class with one method is a function in disguise.

JSDoc Typing

For pure JavaScript projects that don't use TypeScript, use JSDoc annotations to provide type safety through editor tooling. Enable // @ts-check at file top or checkJs in jsconfig.json.

Core tags: @type, @param, @returns, @typedef (with @property), @template. See ${CLAUDE_SKILL_DIR}/references/jsdoc.md for the full tag catalog including @callback, @enum, class modifiers, and type import syntax.

JSDoc Best Practices

• Annotate public API boundaries — exported functions, classes, module-level variables. Internal code often needs fewer annotations; types flow from context.
• Prefer inline TypeScript syntax in JSDoc types: {string | number} over {(string|number)}.
• Use @typedef for shared shapes — define once near file top or in types.js.
• Don't annotate the obvious — if const x = 5 is clearly a number, skip @type.

Application

When writing JavaScript code:

• Apply all conventions silently — don't narrate each rule being followed.
• If an existing codebase contradicts a convention, follow the codebase and flag the divergence once.

When reviewing JavaScript code:

• Cite the specific violation and show the fix inline.
• Don't lecture or quote the rule — state what's wrong and how to fix it.

Bad review comment:
  "According to best practices, you should use const instead of let
   when the variable is never reassigned."

Good review comment:
  "`let` -> `const` — `config` is never reassigned."

Code Navigation — LSP Required

A typescript-language-server LSP server is configured for all JS/TS file types (.js, .jsx, .ts, .tsx, .mjs, .cjs, .mts, .cts). Always use LSP tools for code navigation instead of Grep or Glob. LSP understands module resolution, type inference, scope rules, and project boundaries — text search does not.

Tool Routing

• Find where a function/class/variable is defined → goToDefinition — Resolves imports, re-exports, aliases
• Find all usages of a symbol → findReferences — Scope-aware, no false positives from string matches
• Get type signature, docs, or return types → hover — Instant type info without reading source files
• List all exports/symbols in a file → documentSymbol — Structured output vs grepping for function/class/export
• Find a symbol by name across the project → workspaceSymbol — Searches all modules
• Find implementations of an interface → goToImplementation — Knows the type system
• Find what calls a function → incomingCalls — Precise call graph across module boundaries
• Find what a function calls → outgoingCalls — Structured dependency map

Grep/Glob remain appropriate for: text in comments, string literals, log messages, TODO markers, config values, env vars, CSS classes, file name patterns, URLs, error message text.

When spawning subagents for JS/TS codebase exploration, instruct them to use LSP tools. Subagents have access to the same LSP server.

Integration

The coding skill governs workflow; this skill governs JavaScript implementation choices. For TypeScript projects, the typescript skill extends this one.