JSON Troubleshooting Guide 2025

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

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

Error Message: "Unexpected token '}' in JSON"

Solution: Remove the comma after the last property in objects and arrays.

{
  'name': 'John',
  'age': 30
}

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

Error Message: "Unexpected token ''' in JSON"

Solution: Always use double quotes for strings and property names in JSON.

{
  name: "John",
  age: 30,
  active: true
}

{
  "name": "John",
  "age": 30,
  "active": true
}

Error Message: "Unexpected token 'n' in JSON"

Solution: All property names must be enclosed in double quotes.

{
  "message": "He said "Hello" to me"
}

{
  "message": "He said \"Hello\" to me"
}

Common Escape Sequences:

• \" - Double quote
• \\ - Backslash
• \n - Newline
• \t - Tab
• \r - Carriage return

{
  "users": [
    {
      "name": "John",
      "age": 30
    // Missing closing brace
    {
      "name": "Jane",
      "age": 25
    }
  ]
// Missing closing brace

{
  "users": [
    {
      "name": "John",
      "age": 30
    },
    {
      "name": "Jane",
      "age": 25
    }
  ]
}

Error Message: "Unexpected end of JSON input"

Solution: Ensure every opening bracket/brace has a matching closing bracket/brace.

JSON only supports specific data types. Here are common mistakes:

// ❌ INVALID JSON - These won't work
{
  "undefined": undefined,           // undefined is not valid JSON
  "function": function() { return 1; }, // functions not allowed
  "date": new Date(),                 // Date objects not valid
  "infinity": Infinity,              // Infinity not allowed
  "nan": NaN,                        // NaN not allowed
  "comment": /* comment */           // comments not allowed
}

// ✅ VALID JSON - Correct alternatives
{
  "undefined": null,              // Use null instead
  "result": 1,                   // Use the return value
  "date": "2025-01-14T10:30:00Z", // Use ISO date string
  "infinity": 1.7976931348623157e+308, // Use max number
  "nan": null,                   // Use null for missing values
  "description": "This is a comment"    // Use properties for notes
}

Invisible characters and encoding issues can break JSON parsing:

Common Hidden Characters:

• BOM (Byte Order Mark): Invisible character at start of file
• Smart Quotes: Curly quotes from word processors
• Non-breaking Spaces: Look like spaces but aren't
• Zero-width Characters: Completely invisible but break parsing

// ❌ These look correct but contain hidden characters
{
  "name": "John",  // Smart quotes (curly quotes)
  "age": 30        // Non-breaking space instead of regular space
}

// ✅ Clean JSON with proper characters
{
  "name": "John",  // Straight quotes
  "age": 30         // Regular space
}

Error: "Unexpected token '}' in JSON at position X"

Cause: Trailing comma before closing brace

Solution: Remove the comma after the last property

Error: "Unexpected token ''' in JSON at position X"

Cause: Single quotes instead of double quotes

Solution: Replace all single quotes with double quotes

Error: "Unexpected token 'u' in JSON at position X"

Cause: undefined value in JSON

Solution: Replace undefined with null or remove the property

Cause: Incomplete JSON - missing closing brackets, braces, or quotes

Common scenarios:

• Truncated JSON during copy/paste
• Network timeout cutting off JSON response
• Missing closing bracket at end of file
• Unclosed string (missing closing quote)

Solution: Check that all opening brackets have matching closing brackets

Cause: Invalid character in JSON (often hidden/invisible characters)

Solution:

1. Copy JSON to plain text editor
2. Enable "Show All Characters" mode
3. Look for unusual characters or spacing
4. Re-type the problematic section

Cause: Invalid number format

Examples of invalid numbers:

// ❌ Invalid number formats
{
  "hex": 0xFF,          // Hex not allowed
  "octal": 0755,        // Octal not allowed  
  "infinity": Infinity,  // Infinity not allowed
  "nan": NaN,           // NaN not allowed
  "leading": 01.5       // Leading zeros not allowed
}

// ✅ Valid number formats
{
  "hex": 255,          // Use decimal
  "octal": 493,        // Use decimal equivalent
  "infinity": null,    // Use null or very large number
  "nan": null,         // Use null for missing values
  "decimal": 1.5       // No leading zeros
}

When JSON is syntactically correct but fails schema validation:

Common Schema Issues:

• Missing required fields: Check that all required properties are present
• Wrong data types: Verify string vs number vs boolean types
• Invalid enum values: Ensure values match allowed options exactly
• Format violations: Check email, date, URI formats
• Range violations: Verify min/max length, value constraints

// Schema validation debugging
function validateWithDetailedErrors(data, schema) {
  const ajv = new Ajv({ allErrors: true });
  const validate = ajv.compile(schema);
  const valid = validate(data);
  
  if (!valid) {
    console.log('Validation errors:');
    validate.errors.forEach(error => {
      console.log(`- ${error.instancePath}: ${error.message}`);
      console.log(`  Data: ${JSON.stringify(error.data)}`);
      console.log(`  Schema: ${JSON.stringify(error.schema)}`);
    });
  }
  
  return valid;
}