Common JSON syntax errors and how to fix them

The rules a strict JSON parser enforces

JSON is a data format, not a JavaScript object literal. It deliberately supports a smaller set of syntax so different systems can exchange the same data reliably.

A document must contain one complete JSON value. Objects use curly brackets, arrays use square brackets, and every opening bracket must have a matching closing bracket.

• Property names must use double quotes.
• String values must use double quotes.
• Allowed values are objects, arrays, strings, numbers, true, false, and null.
• Comments and trailing commas are not part of standard JSON.

Trailing commas after the final item

A comma separates one property or array item from the next. A comma after the final item tells the parser to expect another value, but the object or array closes instead.

{
  "name": "Bacodev",
  "private": true,
}

{
  "name": "Bacodev",
  "private": true
}

Single quotes and unquoted property names

Strict JSON requires double quotes around property names and string values. Single quotes and bare identifiers may be accepted by JavaScript, JSON5, or a custom parser, but they are not standard JSON.

{name: 'Bacodev', private: true}

{"name": "Bacodev", "private": true}

Comments inside JSON

Standard JSON has no comment syntax. Remove comments before parsing or use a format that explicitly supports them, such as JSON5, when the receiving system accepts that format.

{
  // keep local
  "private": true
}

{
  "private": true
}

Unsupported values such as undefined, NaN, and Infinity

JSON supports finite numbers, but it does not support JavaScript values such as undefined, NaN, or Infinity. Choose null, a quoted label, or another valid representation that matches the intended meaning.

Missing brackets, commas, or colons

A missing delimiter can make the parser report an error near a later line. Check the property or array item immediately before the reported location and verify that every structure closes correctly.

• Every property needs a colon between its name and value.
• Every neighboring property or array item needs a comma.
• Objects and arrays must close in the reverse order in which they opened.

{
  "user": {
    "name": "Ada"
  ,
  "active": true
}

{
  "user": {
    "name": "Ada"
  },
  "active": true
}

Unescaped quotation marks and control characters

Quotation marks inside a JSON string must be escaped. Literal line breaks, tabs, and certain control characters also need valid escape sequences.

{"message":"She said "hello""}

{"message":"She said \"hello\""}

Duplicate object keys

Some parsers accept duplicate keys and silently keep the last value. Others reject them or preserve behavior that is difficult to predict. Validating for duplicate keys prevents data from being overwritten unexpectedly.

{"role":"viewer","role":"admin"}

{"roles":["viewer","admin"]}

A safe JSON repair checklist

Automatic repair is useful, but every repair changes text. Review the result before using it in an API request, configuration file, database import, or production deployment.

1. 1

   Keep a copy of the original input.

2. 2

   Repair one class of syntax issue at a time.

3. 3

   Validate the repaired document.

4. 4

   Compare important values and data types.

5. 5

   Test the result in the target system.