Common JSON Errors and How to Fix Them Instantly

You paste JSON into your parser and get: "Unexpected token at position 142" — and now you're squinting at a wall of text looking for a single missing comma. Sound familiar?

90% of JSON errors are the same 5 mistakes, repeated endlessly. Here's how to spot and fix each one in seconds.

Error #1: Trailing Comma

The #1 JSON mistake. JSON does not allow a comma after the last item in an array or object.

// ❌ WRONG — trailing comma after "editor"
{
  "roles": ["admin", "editor",]
}

// ✅ FIX — remove the trailing comma
{
  "roles": ["admin", "editor"]
}

Why this happens: JavaScript allows trailing commas, so developers writing config files in JS style accidentally bring this habit to JSON. ESLint even has a comma-dangle rule that encourages trailing commas in JS — bad habit for JSON.

Error #2: Unquoted Keys

All JSON object keys must be double-quoted strings.

// ❌ WRONG — unquoted keys
{
  name: "Alice",
  age: 30
}

// ✅ FIX — quote all keys
{
  "name": "Alice",
  "age": 30
}

Why this happens: JavaScript object literals don't require quoted keys. Developers write JavaScript-style objects and treat them as JSON. They're not the same.

Error #3: Single Quotes Instead of Double Quotes

JSON requires double quotes for strings. Single quotes are not valid JSON.

// ❌ WRONG — single quotes
{
  'name': 'Alice',
  'message': 'Hello, world!'
}

// ✅ FIX — double quotes everywhere
{
  "name": "Alice",
  "message": "Hello, world!"
}

Error #4: Missing or Extra Brackets

Unbalanced curly braces {} or square brackets [].

// ❌ WRONG — missing closing brace
{
  "users": [
    {
      "name": "Alice"
    // ← missing } and ]
}

// ✅ FIX — close every opening bracket
{
  "users": [
    {
      "name": "Alice"
    }
  ]
}

Quick fix tip: In VS Code, click on a bracket — the matching bracket highlights. If nothing highlights, you have an unbalanced bracket somewhere.

Error #5: Comments in JSON

JSON does not support comments. No //, no /* */.

// ❌ WRONG — comments
{
  // User object
  "name": "Alice",
  /* Active status */
  "active": true
}

// ✅ FIX — remove comments, or use a "_comment" field
{
  "_comment": "User object",
  "name": "Alice",
  "_comment_active": "Active status",
  "active": true
}

JSON5 and JSONC: Some config file formats (like VS Code's settings.json, tsconfig.json) support comments because they use JSONC (JSON with Comments). But standard JSON parsers will reject it.

Bonus: Non-JSON Data Types

// ❌ WRONG — these are not valid JSON types
{
  "date": new Date(),       // not valid
  "value": undefined,       // not valid
  "fn": function() {},      // not valid
  "num": NaN,               // not valid
  "inf": Infinity           // not valid
}

// ✅ FIX — use strings or null
{
  "date": "2026-06-12T00:00:00Z",
  "value": null,
  "fn": null,
  "num": null,
  "inf": null
}

Valid JSON types: String, Number, Boolean, null, Array, Object. That's it. No Date, undefined, functions, NaN, or Infinity.

The Fastest Way to Fix JSON Errors

1. Paste your JSON into the DevTools JSON validator.
2. The tool instantly shows the exact error position (line + character).
3. Check the line above the error — in 90% of cases, the problem is a missing comma, extra comma, or unquoted key on the previous line.