Can JSON have comments? What JSONC allows and how to clean a commented file

The rule comes from the grammar. RFC 8259 counts only four characters as insignificant whitespace: space, horizontal tab, line feed and carriage return. A comment has no place in that grammar, so a strict parser stops at the first slash. For a // comment on the second line, Node 24's JSON.parse reports Expected property name or '}' in JSON at position 4 (line 2 column 3). Python's json.loads rejects the same text.

Some tools accept comments anyway. RFC 8259 says, "A JSON parser MAY accept non-JSON forms or extensions." That's why a file can open in one editor and fail in your app. The VS Code JSON docs describe a JSON with Comments (jsonc) mode. VS Code uses it for settings.json, tasks.json and launch.json, and it accepts // and /* */ comments. It also accepts trailing commas, though it warns about them. This formatter makes a stricter call: comments yes, trailing commas no.

Turn on the Allow comments checkbox, and the formatter reads // line comments and /* */ block comments that sit outside strings, then leaves them out of the output. A .jsonc file loads with the option already on. The formatter checks each string before it looks for comments, so the // in the sample's URL stays. A find and replace on // would cut that value short, which is why a tool that reads the string rules is the better choice here. Here's the sample as input:

{
  // The port the server listens on.
  "port": 8080, /* tcp */
  "url": "https://example.com//api"
}

The formatted output has no comments, because JSON has nowhere to keep them. Here's what the sample gives:

{
  "port": 8080,
  "url": "https://example.com//api"
}

Some errors stay errors. A block comment that never closes gives an error that names the line and column, and a trailing comma still fails, as it does in plain JSON. An easy slip is commenting out the last item. The comma at the end of the item above it now has nothing after it, so delete that comma too. The error points at the closing brace, not at the comma. JSON parse errors covers the other usual mistakes.

When the consumer is strict and you still want notes, pick one of these:

• Keep the commented file as the source and generate a strict copy from it.
• Add a _comment key. It's a common habit, not a standard, and a schema that forbids extra keys will reject it.
• Use YAML when notes matter more than JSON compatibility. YAML supports # comments, and JSON vs YAML compares the two formats.