How to Read and Fix JSON Syntax Errors

JSON looks simple until a single misplaced comma breaks an import, an API request or a configuration file. The good news is that almost every JSON error comes from a short list of causes. This guide shows how to read parser error messages and fix the mistakes that cause them.

The rules JSON actually enforces

• Strings and object keys use double quotes. Single quotes are invalid.
• No trailing commas after the last item in an object or array.
• No comments. Neither // nor /* */ is allowed.
• Values are only strings, numbers, true, false, null, objects and arrays. No undefined, NaN, Infinity or functions.
• Numbers have no leading zeros (007 is invalid) and no leading plus sign.
• Special characters inside strings must be escaped: ” for a quote, \ for a backslash, n for a newline.

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Reading the error message

Parsers report where they gave up, which is often just after the real mistake. Different tools word it differently:

Message	Usually means
Unexpected token } in JSON at position 52	A trailing comma before the closing brace.
Unexpected token ‘ in JSON	Single quotes instead of double quotes.
Expected property name or ‘}’	An unquoted key, or a trailing comma in an object.
Unexpected end of JSON input	The text was cut off: a missing closing bracket or brace.
Unexpected token u in JSON at position 0	You parsed the string “undefined”, typically an empty variable in JavaScript.
Unexpected token < in JSON at position 0	The server returned HTML (often an error page) instead of JSON.
Bad control character in string literal	A raw line break or tab inside a string; escape it as n or t.

Paste the JSON into the JSON validator; it converts “position” into a line and column and highlights the spot.

The five fixes that solve most errors

1. Remove the trailing comma

{ "name": "WPDV", "tags": ["seo", "tools",] }   ✗
{ "name": "WPDV", "tags": ["seo", "tools"] }    ✓

2. Use double quotes everywhere

{ 'title': 'Hello' }   ✗
{ "title": "Hello" }   ✓

3. Quote every key

{ title: "Hello" }     ✗  (valid JavaScript, invalid JSON)
{ "title": "Hello" }   ✓

4. Escape quotes and backslashes inside strings

{ "html": "<a href="/">Home</a>" }     ✗
{ "html": "<a href="/">Home</a>" }   ✓
{ "path": "C:Usersme" }               ✗
{ "path": "C:\Users\me" }             ✓

5. Close every bracket

Count opening and closing braces and brackets, or format the JSON so the indentation shows where a block is not closed. The JSON formatter indents valid JSON and points at the line where structure breaks.

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Invisible problems

• Smart quotes. Word processors and some chat apps turn ” into “ ”. Retype them in a code editor.
• A byte order mark (BOM) at the start of a file saved by some Windows editors. Save as “UTF-8 without BOM”.
• Non-breaking spaces copied from web pages. They look like spaces but are not whitespace to a JSON parser.
• Duplicate keys. Technically allowed by the spec, but most parsers silently keep only the last one, which causes confusing bugs.

When the JSON comes from a server

If JSON.parse fails on an API response, look at the raw response first (the Network tab in your browser’s developer tools). An HTML error page, a PHP warning printed before the JSON, or a redirect to a login page all produce “Unexpected token <“. Fix the server error rather than the parser.

In WordPress, a common cause is a plugin or theme printing a notice. Turn on WP_DEBUG_LOG and turn off WP_DEBUG_DISPLAY so warnings go to the log, not into REST and AJAX responses.

Need comments? Use another format

If a configuration file needs comments, JSON is the wrong tool. YAML supports comments and is easier to read, and many tools accept it. The YAML ⇄ JSON converter converts between the two, with line-numbered errors on both sides.

Checklist

• Double quotes only, on keys and strings.
• No trailing commas, no comments.
• Escape ” and \ inside strings.
• Every { and [ has a matching } and ].
• Check the raw response when parsing server output.