Common JSON Errors and How to Fix Them

JSON is a data format defined by RFC 8259, not a programming language. To keep it simple and unambiguous for every language that reads it, it allows only double-quoted strings, a fixed set of values (strings, numbers, objects, arrays, true, false and null) and no extras such as comments or trailing commas. Code that a browser happily accepts as a JavaScript object can therefore be rejected by a JSON parser.

The JSON Formatter checks your JSON as you type and points to the exact line and column of the first problem, with an explanation of the likely cause.

A comma after the last item of an object or array is the single most common error: {"a": 1, "b": 2,} or [1, 2, 3,]. JavaScript allows it, JSON does not.

Fix: delete the comma before the closing } or ].

Strings and property names must use double quotes. {'name': 'Ada'} and {name: "Ada"} are both invalid; the correct form is {"name": "Ada"}.

Fix: replace single quotes with double quotes and wrap every property name in double quotes. If a string itself contains a double quote, escape it as \".

When you add a line to a file by hand, it's easy to forget the comma at the end of the previous line. The parser then finds a new key where it expected a comma or a closing brace, and reports the error at the start of the new line, not at the end of the old one.

Fix: look at the line before the reported position and add the missing comma.

JSON has no comments: neither // line comments nor /* block */ comments. Configuration files that contain them are usually JSONC (JSON with comments, used by editors such as VS Code) or JSON5, which are different formats.

Fix: remove the comments, or move explanations into a separate "_comment" field if the program reading the file ignores unknown keys.

• undefined, NaN and Infinity exist in JavaScript but not in JSON. Use null instead, or store the value as a string.
• Numbers can't have leading zeros (007) or a trailing decimal point (1.). Write 7 and 1.0, or use a string for codes such as postal codes.
• Hexadecimal numbers (0xFF) are not allowed; write the decimal value.

A string can't span several lines or contain a raw tab character. Text pasted from a document often brings these along.

Fix: write line breaks as \n and tabs as \t inside the string. If you need to send binary data or a whole file inside JSON, encode it with Base64 first.

This one isn't a syntax error, which makes it dangerous. JavaScript stores every number as a 64-bit floating-point value, so integers above 9,007,199,254,740,991 lose precision when parsed: an ID such as 12345678901234567890 becomes 12345678901234567000. Many online formatters change it without warning.

Fix: send large IDs as strings. When you only need to read or reformat JSON, use a tool that keeps the original digits, such as the JSON Formatter.

• Double quotes around every string and every key.
• Commas between items, but never after the last one.
• No comments, undefined, NaN or leading zeros.
• Escape line breaks (\n), tabs (\t), quotes (\") and backslashes (\\) inside strings.
• Large IDs as strings.