TypeScript

TypeScript code style + formatting.

MUST

• Arrow functions only. Why: one function form.
• All exports in a single block at end of file. Why: lint-enforced, scan-once.
• .tsx single component → export default; utilities/backend → named exports. Why: convention.
• ANY file with JSX uses .tsx — even a context provider <Ctx value={...}>. Why: biome parses .ts as non-JSX, misreads <Foo> as comparison/regex, fixer infinite-loops (90-min hang).
• for loops over reduce()/forEach(). Why: readability + perf.
• Exhaustive switch with default: never. Why: compile-time case coverage.
• Descriptive catch (error) state-var names (chatError, formError). Why: oxlint shadow rule.
• Short map callback names (t, m, i). Why: convention.
• Destructured object for 4+ args (max 3 positional). Why: call-site clarity.
• Co-locate components with their page; move to ~/components only when reused. Why: locality.
• Explicit imports from exact file paths; no barrel index.ts in app code (library packages use barrels for public API). Why: avoids barrel cycle + bloat.
• Prefer existing libraries over new dependencies. Why: minimize surface.
• node: prefix for Node builtins (import { join } from 'node:path'). Why: explicit builtin.
• interface over type where possible; properties sorted alphabetically. Why: lint-enforced.
• import type for type-only imports. Why: erased at build.

NEVER

• function declarations. Cost: violates arrow-only.
• Duplicate types. Cost: drift; single source of truth.
• import as aliases. Cost: rename the variable instead.
• Empty lines between statements. Cost: biome deletes them — wasted diff.
• Trailing comma single-line (keep it multi-line). Cost: format violation.

Formatting

• Single quotes, no semicolons; imports sorted alphabetically by source.