Common JSON Errors and How to Debug Them

JSON errors can be frustrating, especially when error messages are cryptic. This guide covers the most common JSON errors and provides practical solutions for debugging them.

1. Missing or Mismatched Quotes

Error: Property names and string values must use double quotes.

// WRONG
{
  name: "John",
  'email': 'john@example.com'
}

// CORRECT
{
  "name": "John",
  "email": "john@example.com"
}

2. Trailing Commas

Error: JSON does not allow trailing commas after the last item in arrays or objects.

// WRONG
{
  "items": [1, 2, 3,],
  "name": "John",
}

// CORRECT
{
  "items": [1, 2, 3],
  "name": "John"
}

3. Missing Commas

Error: Commas are required between items in arrays and objects.

// WRONG
{
  "name": "John"
  "age": 30
}

// CORRECT
{
  "name": "John",
  "age": 30
}

4. Unescaped Special Characters

Error: Special characters in strings must be escaped.

// WRONG
{
  "message": "He said "Hello"",
  "path": "C:\Users\Documents"
}

// CORRECT
{
  "message": "He said \"Hello\"",
  "path": "C:\\Users\\Documents"
}

5. Invalid Number Formats

Error: Numbers must be valid JSON numbers.

// WRONG
{
  "price": 19.99.99,
  "hex": 0xFF,
  "nan": NaN,
  "infinity": Infinity
}

// CORRECT
{
  "price": 19.99,
  "hex": 255,
  "nan": null,
  "infinity": null
}

6. Mismatched Brackets

Error: All brackets must be properly matched and nested.

// WRONG
{
  "users": [
    {"name": "John"}
  ,
  "count": 1
}

// CORRECT
{
  "users": [
    {"name": "John"}
  ],
  "count": 1
}

7. Using undefined

Error: undefined is not a valid JSON value.

// WRONG
{
  "name": undefined
}

// CORRECT
{
  "name": null
}

// Or omit the field entirely
{}

Debugging Tips

1. Use Online Validators

Use our JSON Doctor tool to quickly validate and format your JSON. It will highlight errors and show you exactly where the problem is.

2. Check Character by Character

function findJsonError(json) {
  try {
    JSON.parse(json);
    return "Valid JSON";
  } catch (e) {
    const match = e.message.match(/position (\d+)/);
    if (match) {
      const pos = parseInt(match[1]);
      const context = json.substring(Math.max(0, pos - 20), pos + 20);
      return `Error at position ${pos}: ...${context}...`;
    }
    return e.message;
  }
}

3. Use a Linter

Install a JSON linter in your IDE to catch errors as you type.

4. Pretty Print for Clarity

const ugly = '{"name":"John","age":30}';
const pretty = JSON.stringify(JSON.parse(ugly), null, 2);
console.log(pretty);

Common Error Messages

• Unexpected token: Usually a syntax error like missing quotes or commas
• Unexpected end of JSON: Missing closing brackets or incomplete JSON
• Unexpected number: Invalid number format
• Expected property name: Missing or invalid property name