The BOM Error: Unexpected Token at Position 0

Quick answer: A byte-order mark (BOM) is three invisible bytes (EF BB BF, the character U+FEFF) that some editors prepend to UTF-8 files. JSON forbids it, so a BOM at the start of a file causes Unexpected token  in JSON at position 0 even though the file looks perfect. The fix is to strip the BOM before parsing, or save the file as "UTF-8 without BOM."

The BOM is genuinely invisible in most editors. You see:

{"ok": true}

…but the bytes on disk are EF BB BF 7B 22 6F .... Your parser reads the  first, which isn't a valid start to a JSON document, and throws at position 0. This is maddening precisely because nothing is visibly wrong — copying the text into a fresh file (which drops the BOM) often "fixes" it, hiding the real cause.

The JSON spec (RFC 8259, §8.1) is explicit: implementations must not add a BOM, and parsers may reject one. JSON.parse() rejects it.

If Unexpected token reports position 0 but the first visible character is clearly valid ({ or [), suspect a BOM. To check the raw bytes:

head -c 3 data.json | xxd
# 00000000: efbb bf                                  ...

ef bb bf confirms a UTF-8 BOM. You can also paste the file into the JSON Debugger — it flags an unexpected character at the very start when a BOM is present.

Strip it in code before parsing:

const clean = raw.replace(/^/, "");
JSON.parse(clean);

import json
with open("data.json", encoding="utf-8-sig") as f:   # -sig strips the BOM
    data = json.load(f)

Python's utf-8-sig codec transparently removes a leading BOM, which is the cleanest fix on that side.

Fix it at the source so it never comes back:

• In VS Code, click the encoding in the status bar → Save with Encoding → UTF-8 (not UTF-8 with BOM).
• In Notepad / Notepad++, choose UTF-8 without BOM when saving.
• In build pipelines, ensure tools emit BOM-less UTF-8 — it's the standard encoding for JSON.

After cleaning, confirm with the JSON Validator.

The BOM isn't the only character you can't see. Non-breaking spaces, zero-width spaces, and smart quotes pasted from documents cause similar "but it looks right!" errors. For how JSON handles such characters in general, see Escaping Special Characters, and for the broader list of subtle breakages, Common JSON Mistakes.

What is a BOM in a JSON file? A byte-order mark is an invisible character (U+FEFF, bytes EF BB BF in UTF-8) that some editors add to the start of a file. JSON disallows it, so it triggers a parse error at position 0.

How do I remove the BOM from JSON? Strip a leading  before parsing (raw.replace(/^/, "") in JS), open the file with Python's utf-8-sig codec, or re-save it as "UTF-8 without BOM."

Why does position 0 fail when the first character looks valid? Because the actual first character is the invisible BOM, not the { you see. The parser meets  and rejects it immediately.

How do I stop my editor from adding a BOM? Save files as UTF-8 without BOM. VS Code, Notepad++, and most editors offer this as an explicit encoding option in the save dialog or status bar.