.prettierrc.json and .prettierrc.toml reject a leading UTF-8 BOM

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
68/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
javascript, node.js
Domain
cli, tooling

Research direction

Start with the CLI entry point bin/prettier.cjs and reproduce the issue using the provided .prettierrc.json and .prettierrc.toml commands. Trace how these textual configuration files are read, then verify that a single leading UTF-8 BOM is ignored and example.js formats with the expected output.

Written by the indexing model from the issue text.

Description

Environments:

  • Prettier Version: 3.10.0-dev (commit 1a153120)
  • Running Prettier via: CLI
  • Runtime: Node.js v25.6.0
  • Operating System: Windows 11 64-bit
  • Prettier plugins (if any): None

Steps to reproduce:

Create a JavaScript file:

// example.js
const message = "hello";

Create a .prettierrc.json file with a leading UTF-8 BOM:

node -e "require('node:fs').writeFileSync('.prettierrc.json', '\uFEFF{\"semi\":false}\n')"

Run Prettier:

node ./bin/prettier.cjs --config .prettierrc.json example.js

The same issue can be reproduced with TOML:

node -e "require('node:fs').writeFileSync('.prettierrc.toml', '\uFEFFsemi = false\n')"

node ./bin/prettier.cjs --config .prettierrc.toml example.js

For comparison, configuration files containing the same leading BOM work when using JSON5 or YAML.

Expected behavior:

Prettier should ignore a single leading UTF-8 BOM when reading textual configuration files and format the JavaScript file as:

const message = "hello"

This would make the behavior consistent across JSON, TOML, JSON5, and YAML configuration files.

RFC 8259 also allows JSON parsers to ignore a leading BOM for interoperability:

https://www.rfc-editor.org/rfc/rfc8259.html#section-8.1

Actual behavior:

The JSON configuration exits with status code 2:

Invalid configuration for file "example.js":
JSON Error in .prettierrc.json:
Unexpected token "" (\u{feff})

The TOML configuration also exits with status code 2:

Invalid configuration for file "example.js":
TOML Error in .prettierrc.toml:
Invalid TOML document: only letters, numbers, dashes and underscores are allowed in keys

1:  semi = false
    ^
Dominant language
JavaScript
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