What Makes JSON Invalid? The 7 Most Common JSON Syntax Errors

JSON is strict. Patterns that work fine in JavaScript object literals silently break JSON parsers. These are the seven errors that account for the vast majority of "Unexpected token" and "JSON parse error" messages.

1. Trailing comma

The most common JSON error. JavaScript allows trailing commas; JSON does not.

// ❌ Invalid JSON
{
  "name": "Alice",
  "age": 30,   ← trailing comma
}

// ✅ Valid JSON
{
  "name": "Alice",
  "age": 30
}

2. Single-quoted strings

JSON requires double quotes. Single quotes are a JavaScript convenience, not part of the JSON spec.

// ❌ Invalid
{ 'name': 'Alice' }

// ✅ Valid
{ "name": "Alice" }

3. Unquoted keys

Object keys must be quoted strings in JSON. Unquoted keys are valid in JavaScript ({ name: "Alice" }) but invalid in JSON.

// ❌ Invalid JSON
{ name: "Alice" }

// ✅ Valid JSON
{ "name": "Alice" }

4. Comments

JSON has no comment syntax. Neither // single-line nor /* block */ comments are valid. If you need comments in config files, consider JSONC (JSON with Comments, used by VS Code), YAML, or TOML instead.

5. NaN, Infinity, and undefined

These are valid JavaScript values but not valid JSON. JSON.stringify() silently converts them:

• NaN → null
• Infinity → null
• undefined → key is omitted entirely

This means round-tripping through JSON can silently lose or change data if your objects contain these values.

6. Unescaped control characters in strings

Literal newlines, tabs, and other control characters (U+0000 to U+001F) inside string values must be escaped. A raw newline in a string value is invalid.

// ❌ Invalid — literal newline in string
{ "message": "line one
line two" }

// ✅ Valid — escaped newline
{ "message": "line one\nline two" }

7. Numbers: hex, leading zeros, bare decimal point

JSON numbers must be decimal only. Hexadecimal literals, octal literals, and numbers with a leading decimal point are not valid.

// ❌ Invalid JSON numbers
{ "a": 0xFF, "b": 077, "c": .5, "d": 1. }

// ✅ Valid
{ "a": 255, "b": 63, "c": 0.5, "d": 1.0 }

Quick fix

Paste your broken JSON into the validator below — it highlights the exact line and column of the error and explains what went wrong.