ts-npm-prettier-starter

TS + npm Prettier Starter (Agent-Friendly)

Use this when adding Prettier to a TypeScript npm project. In a monorepo, edit the workspace the user names — ask if it isn't named.

The defaults below treat the LLM agent as the primary editor, not human eyes. Prettier's human-tuned defaults (printWidth 80, etc.) reformat large regions when an agent edits a single line, shift surrounding line numbers, and silently invalidate old_string matches the agent is holding for upcoming edits in the same session. These settings shrink that failure surface.

Expected Outcome

• prettier is in devDependencies of the target package.json.
• .prettierrc.json exists with the agent-friendly settings below.
• .prettierignore exists covering node_modules and common build artifacts.
• package.json scripts include format (write) and format:check (read-only check).

Installation

npm add -D prettier

When prettier is already declared, keep its current version unless the user asks for a change.

Agent-Friendly Configuration

Create .prettierrc.json if it does not exist:

{
  "printWidth": 120,
  "trailingComma": "all",
  "arrowParens": "always",
  "semi": true,
  "endOfLine": "lf"
}

Why each value is chosen for an agent-primary workflow:

• printWidth: 120 — wider than the default 80, so a one-line agent edit rarely triggers auto-wrap. Stable surrounding line counts keep both old_string matches and conversational line-number references valid.
• trailingComma: "all" — appending an item to an array, object, or argument list becomes a single added-line diff instead of a two-line diff that also rewrites the previous line's ,.
• arrowParens: "always" — adding a type annotation to an arrow parameter doesn't also force inserting parens, so the annotation edit stays a localized change.
• semi: true — ASI quirks don't silently change behavior when an agent prepends a line that starts with [, (, +, -, or /.
• endOfLine: "lf" — avoids whole-file CRLF reformat noise when Windows and Unix contributors share the repo.

When .prettierrc.* already exists, keep it. Surface the diff against the values above so the user can decide whether to migrate, but do not overwrite.

Ignore Patterns

Create .prettierignore if it does not exist:

node_modules
dist
build
coverage
*.min.js
package-lock.json
*.md

*.md is in the list because Prettier's markdown formatter reflows paragraphs, list spacing, and table column widths across edits, which breaks old_string matches the same way reformatted JS/TS would — and markdown is content, not code, so the formatting consistency isn't worth the agent-edit cost.

When .prettierignore already exists, append only the missing entries from the list above and leave existing entries in place.

Scripts

Add to package.json:

{
  "scripts": {
    "format": "prettier --write .",
    "format:check": "prettier --check ."
  }
}

When format already targets a different formatter, leave format untouched and add format:prettier alongside it.

Verification

• npm run format runs without error across the repository.
• npm run format:check exits 0 after a one-time npm run format.
• .prettierrc.json parses as valid JSON.

References

• Prettier options: https://prettier.io/docs/en/options