cloudflare/workers-best-practices

Your knowledge of Cloudflare Workers APIs, types, and configuration may be outdated. Prefer retrieval over pre-training for any Workers code task — writing or reviewing.

Retrieval Sources

Fetch the latest versions before writing or reviewing Workers code. Do not rely on baked-in knowledge for API signatures, config fields, or binding shapes.

| Source | How to retrieve | Use for | |--------|----------------|---------| | Workers best practices | Fetch https://developers.cloudflare.com/workers/best-practices/workers-best-practices/ | Canonical rules, patterns, anti-patterns | | Workers types | See references/review.md for retrieval steps | API signatures, handler types, binding types | | Wrangler config schema | node_modules/wrangler/config-schema.json | Config fields, binding shapes, allowed values | | Cloudflare docs | Search tool or https://developers.cloudflare.com/workers/ | API reference, compatibility dates/flags |

FIRST: Fetch Latest References

Before reviewing or writing Workers code, retrieve the current best practices page and relevant type definitions. If the project's node_modules has an older version, prefer the latest published version.

# Fetch latest workers types
mkdir -p /tmp/workers-types-latest && \
  npm pack @cloudflare/workers-types --pack-destination /tmp/workers-types-latest && \
  tar -xzf /tmp/workers-types-latest/cloudflare-workers-types-*.tgz -C /tmp/workers-types-latest
# Types at /tmp/workers-types-latest/package/index.d.ts

Reference Documentation

• references/rules.md — all best practice rules with code examples and anti-patterns
• references/review.md — type validation, config validation, binding access patterns, review process

Rules Quick Reference

Configuration

| Rule | Summary | |------|---------| | Compatibility date | Set compatibility_date to today on new projects; update periodically on existing ones | | nodejs_compat | Enable the nodejs_compat flag — many libraries depend on Node.js built-ins | | wrangler types | Run wrangler types to generate Env — never hand-write binding interfaces | | Secrets | Use wrangler secret put, never hardcode secrets in config or source | | wrangler.jsonc | Use JSONC config for non-secret settings — newer features are JSON-only |

Request & Response Handling

| Rule | Summary | |------|---------| | Streaming | Stream large/unknown payloads — never await response.text() on unbounded data | | waitUntil | Use ctx.waitUntil() for post-response work; do not destructure ctx |

Architecture

| Rule | Summary | |------|---------| | Bindings over REST | Use in-process bindings (KV, R2, D1, Queues) — not the Cloudflare REST API | | Queues & Workflows | Move async/background work off the critical path | | Service bindings | Use service bindings for Worker-to-Worker calls — not public HTTP | | Hyperdrive | Always use Hyperdrive for external PostgreSQL/MySQL connections |

Observability

| Rule | Summary | |------|---------| | Logs & Traces | Enable observability in config with head_sampling_rate; use structured JSON logging |

Code Patterns

| Rule | Summary | |------|---------| | No global request state | Never store request-scoped data in module-level variables | | Floating promises | Every Promise must be awaited, returned, voided, or passed to ctx.waitUntil() |

Security

| Rule | Summary | |------|---------| | Web Crypto | Use crypto.randomUUID() / crypto.getRandomValues() — never Math.random() for security | | No passThroughOnException | Use explicit try/catch with structured error responses |

Anti-Patterns to Flag

| Anti-pattern | Why it matters | |-------------|----------------| | await response.text() on unbounded data | Memory exhaustion — 128 MB limit | | Hardcoded secrets in source or config | Credential leak via version control | | Math.random() for tokens/IDs | Predictable, not cryptographically secure | | Bare fetch() without await or waitUntil | Floating promise — dropped result, swallowed error | | Module-level mutable variables for request state | Cross-request data leaks, stale state, I/O errors | | Cloudflare REST API from inside a Worker | Unnecessary network hop, auth overhead, added latency | | ctx.passThroughOnException() as error handling | Hides bugs, makes debugging impossible | | Hand-written Env interface | Drifts from actual wrangler config bindings | | Direct string comparison for secret values | Timing side-channel — use crypto.subtle.timingSafeEqual | | Destructuring ctx (const { waitUntil } = ctx) | Loses this binding — throws "Illegal invocation" at runtime | | any on Env or handler params | Defeats type safety for all binding access | | as unknown as T double-cast | Hides real type incompatibilities — fix the design | | implements on platform base classes (instead of extends) | Legacy — loses this.ctx, this.env. Applies to DurableObject, WorkerEntrypoint, Workflow | | env.X inside platform base class | Should be this.env.X in classes extending DurableObject, WorkerEntrypoint, etc. |

Review Workflow

1. Retrieve — fetch latest best practices page, workers types, and wrangler schema
2. Read full files — not just diffs; context matters for binding access patterns
3. Check types — binding access, handler signatures, no any, no unsafe casts (see references/review.md)
4. Check config — compatibility_date, nodejs_compat, observability, secrets, binding-code consistency
5. Check patterns — streaming, floating promises, global state, serialization boundaries
6. Check security — crypto usage, secret handling, timing-safe comparisons, error handling
7. Validate with tools — npx tsc --noEmit, lint for no-floating-promises
8. Reference rules — see references/rules.md for each rule's correct pattern

Scope

This skill covers Workers-specific best practices and code review. For related topics:

• Durable Objects: load the durable-objects skill
• Workflows: see Rules of Workflows
• Wrangler CLI commands: load the wrangler skill

Principles

• Be certain. Retrieve before flagging. If unsure about an API, config field, or pattern, fetch the docs first.
• Provide evidence. Reference line numbers, tool output, or docs links.
• Focus on what developers will copy. Workers code in examples and docs gets pasted into production.
• Correctness over completeness. A concise example that works beats a comprehensive one with errors.