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JSON unexpected token

Invalid JSON syntax: the parser reached a character that cannot legally follow the previous token. The message often includes the offending token and a position (line/column in browsers, byte offset in some servers).

JSON is stricter than JavaScript object literals: no trailing commas, no comments, double quotes only for keys and string values, and no `undefined` or `NaN` literals.

This error usually appears when pasting from Stack Overflow, `.jsonc` config files, or JavaScript where trailing commas and single quotes are allowed.

Common causes

  • Trailing comma after the last property or array element (`{"a":1,}` or `[1,2,]`).
  • Unquoted or single-quoted keys (`{key: 1}` or `{'a': 1}`) — JSON requires double-quoted keys.
  • Single-quoted strings (`'text'`) — JSON strings must use double quotes.
  • JavaScript-style comments (`//` or `/* */`) — JSON has no comments.
  • BOM (byte order mark) or invisible Unicode at the start of the file confusing the first token.
  • Concatenated JSON documents or extra prose before/after the value (common when copying API responses into a file).
  • Using `undefined`, unquoted `true`/`false`/`null` in non-value positions, or a stray comma with no following value.

How to narrow it down

  • Note the exact character mentioned in the error (often `,`, `'`, `/`, `{`). Search for that pattern near the reported line.
  • If the position is “1” or early in the file, check for BOM, HTML pasted into a `.json` file, or multiple `{...}{...}` blobs.
  • Validate the smallest failing slice: paste into the JSON validator and bisect (remove half the file) until the error moves.

Examples

Trailing comma (invalid in JSON)
{
  "name": "demo",
  "ok": true,
}

Remove the comma after true (last property in the object).

Unquoted key (invalid)
{ name: "demo" }

Use double quotes around keys: { "name": "demo" }.

Single quotes (invalid)
{ 'a': 1 }

Use double quotes: { "a": 1 }.

How to fix

  • Remove trailing commas after the last element in every object and array.
  • Quote every key with double quotes and use double quotes for all string values.
  • Strip `//` and `/* */` comments or save as JSONC in an editor that strips them before production use.
  • Remove BOM: re-save as UTF-8 without BOM or use your editor’s “Save without BOM”.
  • If the payload is built in JavaScript, serialize with JSON.stringify instead of string concatenation.
  • Paste into ValidateThis JSON validator to get a precise first error location, then re-run after each fix.

Watch out

  • Some APIs return JSON with a `while(1);` prefix or JSONP — strip the wrapper before parsing.
  • Large config files may be JSON5 or YAML in disguise; don’t assume the `.json` extension means strict JSON.

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