JSON Dialect Detector

They are three grammars that produce nearly identical-looking files. Plain JSON is <a href="https://datatracker.ietf.org/doc/html/rfc8259">RFC 8259</a>: double-quoted keys and strings, decimal numbers, no comments, no trailing commas. JSONC is that plus <code>//</code> and <code>/* */</code> comments and trailing commas - it is the dialect Visual Studio Code reads for <code>tsconfig.json</code>, <code>settings.json</code> and <code>launch.json</code>. JSON5 goes furthest: unquoted identifier keys, single-quoted strings, hexadecimal numbers, leading and trailing decimal points, <code>Infinity</code> and <code>NaN</code>, and multi-line strings. Each is a strict superset of the one before it.

Because your editor is almost certainly parsing it as JSONC while <code>JSON.parse</code> parses it as strict JSON. VS Code ships a JSONC reader for the well-known config filenames, so a comment or a trailing comma in <code>tsconfig.json</code> looks completely fine in the editor and throws the moment a Node script reads the same bytes. The file extension is <code>.json</code> in both cases, which is what makes this confusing - the extension tells you nothing about which parser will be used.

No. The parser is plain TypeScript bundled into the page and it runs on the page thread. There is no <code>fetch</code> call, no upload, and no service worker involved, so a config containing internal hostnames or keys never leaves the tab. You can confirm it by opening the Network tab in DevTools before pasting, or by loading the page once and then going offline.

Because most languages that developers move between allow it. Python, JavaScript array and object literals, Rust, Go and modern C all tolerate a trailing comma, precisely so that adding a line to a list produces a one-line diff. JSON deliberately does not, which means reordering or appending a key in a strict JSON file always dirties two lines instead of one. That friction is why JSONC exists at all.

They become <code>null</code>, and the tool warns you when that happens. Strict JSON has no production for non-finite numbers, and JavaScript's <code>JSON.stringify</code> resolves them to <code>null</code> rather than failing. If those values are meaningful in your data, encode them explicitly as strings such as <code>"Infinity"</code> or use a sentinel number your consumer understands, then convert on the way back in.

For plain JSON, the built-in <code>JSON.parse</code>. For JSONC, either <code>jsonc-parser</code> (the package VS Code itself uses) or <code>strip-json-comments</code> followed by <code>JSON.parse</code> - the former is preferable because it also handles trailing commas and reports error positions. For JSON5, the <code>json5</code> package, whose <code>JSON5.parse</code> is a drop-in replacement. Never write your own comment-stripping regex: a <code>//</code> inside a string value such as a URL will be mangled by it.