Common JSON Syntax Errors and How to Fix Them

Almost every "invalid JSON" error traces back to one of a small set of recurring mistakes - most of them stemming from JSON's similarity to (but stricter rules than) JavaScript object literals. Here's each one, with a before/after example.

1. Trailing commas

// Invalid
{"a": 1, "b": 2,}

// Valid
{"a": 1, "b": 2}

JavaScript object literals tolerate a trailing comma; JSON does not. This is the single most common cause of invalid JSON, often introduced when a field is deleted and the preceding comma is left behind.

2. Single quotes

// Invalid
{'name': 'Alice'}

// Valid
{"name": "Alice"}

JSON requires double quotes, full stop - for both keys and string values.

3. Unquoted keys

// Invalid
{name: "Alice"}

// Valid
{"name": "Alice"}

4. Missing commas between items

// Invalid
{"a": 1 "b": 2}

// Valid
{"a": 1, "b": 2}

5. Mismatched braces or brackets

// Invalid
{"items": [1, 2, 3}

// Valid
{"items": [1, 2, 3]}

An array must close with ], and an object with } - mixing them up is easy to miss in deeply nested structures.

6. Comments

// Invalid
{
  "name": "Alice" // the user's name
}

// Valid - move the note elsewhere, JSON has no comment syntax
{
  "name": "Alice"
}

Standard JSON has no comment syntax at all - not // and not /* */. If you need comments, you're likely working with JSON5 or JSONC, which are supersets - see JSON vs JSON5: What's the Difference? for that distinction.

7. Invalid escape sequences in strings

// Invalid - backslash isn't escaped
{"path": "C:\Users\alice"}

// Valid
{"path": "C:\\Users\\alice"}

A literal backslash inside a JSON string must be escaped as \\ - a single unescaped backslash followed by an unrecognized character is invalid.

8. Numbers with a leading zero or plus sign

// Invalid
{"code": 007}
{"count": +5}

// Valid
{"code": "007"}
{"count": 5}

JSON numbers can't have leading zeros (other than the number 0 itself) or an explicit plus sign. If you need to preserve a leading zero, like a US ZIP code, store it as a string.

9. Smart/curly quotes

// Invalid - curly quotes from a word processor
{“name”: “Alice”}

// Valid - straight double quotes
{"name": "Alice"}

This one is easy to miss visually - curly quotes look almost identical to straight quotes at a glance, but a JSON parser treats them as completely different, invalid characters. They usually sneak in when JSON is copy-pasted out of a word processor or a rich-text field.

Quick reference table

Summary

Nearly every JSON syntax error comes from one of these nine patterns - most of them differences from JavaScript's more permissive object literal syntax. Once you recognize the pattern, the fix is almost always a one-character change.