prettier 3.x

Complete instructions

Prettier 3 project formatting

Use the installed formatter and the project's package manager. The fixture targets Prettier 3.9.9. Preserve existing options and plugin versions instead of adding a competing configuration for one edited file.

Choose the file and existing configuration

Use the project's format script or invoke the local CLI on owned paths. check does not write; write mutates source. Keep generated files and dependencies under the project's ignore policy, and review the resulting diff before delivery.

prettier --check src/component.tsx
prettier --write src/component.tsx

Check which configuration applies when editor and CLI results differ. Config search starts from the source file; a command's working directory, explicit config or ignore path can change the effective behavior. A new configuration should be an intentional project decision rather than a workaround for one file.

Use the asynchronous API correctly

import { readFile } from "node:fs/promises";
import * as prettier from "prettier";

export async function formatFile(path) {
  const options = await prettier.resolveConfig(path);
  const source = await readFile(path, "utf8");
  return prettier.format(source, { ...options, filepath: path });
}

The public Prettier 3 APIs are asynchronous. Supply filepath to infer a parser, or parser explicitly for in-memory content. format does not independently skip ignored files; an editor integration needs getFileInfo with its intended ignore policy before deciding to format.

Install a plugin only for a language the project needs and keep its compatibility with the installed Prettier version explicit. The removed custom-parser function option is not the Prettier 3 plugin API.

Verify the formatting workflow

CLI check returns 0 when formatted, 1 for formatting differences and 2 for an execution error. Distinguish a formatter failure from an ordinary unformatted file. Run check after write and ensure formatting is stable on a second pass.

Prettier handles representation and width; a green result does not establish one-statement-per-line policies, correct logic, lint cleanliness or module design. Keep those project rules and checks alongside the formatter.

The fixture exercises configuration lookup, format idempotence, a failing CLI check, write→check success and ignored malformed input. It does not install plugins or rewrite the consumer's entire repository.

Sources

• Prettier API
• CLI and exit codes
• Configuration
• Ignoring code