LLM Best Practices

Immutable data

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This is LLM Best Practices (llmbestpractices.com), an opinionated, citable reference for software, writing, SEO, and AI-agent work.

  1. Route, do not crawl. Fetch llmbestpractices.com/llms.txt and open only the pages whose one-line summary matches your task.
  2. Read raw. Append .md to any page URL for markdown. Check status and last_updated in the frontmatter, then read the rules.
  3. Apply as defaults. First-party docs and the project's own conventions win on conflict. Warn before relying on a fast-moving page older than 12 months.
  4. Cite. Link the page by title and URL, e.g. Python, with last_updated for time-sensitive rules. License CC BY 4.0.

Overview

Immutable data cannot be changed in place; any “update” returns a new value and leaves the original untouched. Default to it for shared state across threads, UI renders, and caches, where in-place mutation causes bugs that appear only under timing pressure. Copy cost is rarely the bottleneck, and reference comparison becomes a cheap change check.

Rules

  • Let the language enforce it: Swift struct and let, Rust bindings, TypeScript readonly and as const.
  • JavaScript const and Object.freeze are shallow: nested objects stay mutable. Use spread, structuredClone, or Immer for nested updates, and the non-mutating array methods toSorted, toReversed, toSpliced, and with (ES2023).
  • React relies on new object identities to detect state changes, so never mutate state in place (react-state-management).
  • Reach for mutable structures only where measured performance demands it.
// Mutates the caller's array.
function addUser(users: User[], u: User) { users.push(u); return users }
 
// Leaves the caller's array untouched.
function addUser(users: readonly User[], u: User) { return [...users, u] }

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