JSON Formatter 101: Fix Trailing Commas, Quotes, Encoding & More

Common JSON errors (with examples)

1) Trailing commas

// ❌ Invalid
{
  "name": "Ava",
  "roles": ["admin","editor",],
}
// ✅ Valid
{
  "name": "Ava",
  "roles": ["admin","editor"]
}

2) Single quotes instead of double quotes

// ❌ Invalid
{'name': 'Ava', 'active': true}
// ✅ Valid
{"name": "Ava", "active": true}

3) Unescaped quotes / control characters

// ❌ Invalid
{"note": "He said "hello""}
// ✅ Valid
{"note": "He said \"hello\""}

4) Comments (JSON doesn’t allow them)

// ❌ Invalid
{
  // price in USD
  "price": 10
}
// ✅ Valid (move notes into a field)
{
  "price": 10,
  "_note": "price in USD"
}

5) Duplicate keys (last one wins, unpredictable)

// ❌ Avoid
{"id": 1, "id": 2}
// ✅ Use unique keys
{"id": 2}

6) NaN / Infinity (not in JSON spec)

// ❌ Invalid
{"value": NaN, "limit": Infinity}
// ✅ Valid
{"value": null, "limit": 1e309}

7) Leading zeros in numbers

// ❌ Invalid in many parsers
{"zip": 0123}
// ✅ Quote if it must preserve zeros
{"zip": "0123"}

8) Top-level type surprises

Most parsers accept any JSON value, but some tools expect an object or array at the top. If in doubt, wrap it:

// Safer top-level
{"data": true}

Escaping & encoding

Escape sequences to know

• \" double quote
• \\ backslash
• \n newline, \t tab
• \uXXXX unicode code point (e.g., \u00A9)

Fix encoding issues

• Ensure your file is saved as UTF-8 (no BOM).
• Replace “smart quotes” with straight quotes before parsing.
• Normalize line endings (LF) if you copy from Windows editors.

Numbers & precision (don’t lose money)

• JavaScript safely represents integers up to 9,007,199,254,740,991 (Number.MAX_SAFE_INTEGER).
• For big IDs, account numbers, or precise currency, send as strings (e.g., "amount":"12.34").
• Avoid commas in numbers (1,234)—use 1234 and format later in the UI.

Large files without headaches

• Stream or chunk when possible instead of loading entire files in memory.
• Use NDJSON (newline-delimited JSON) for logs: one JSON object per line.
• Validate early—pretty-print as you build so errors surface near their source.

Safe workflow with the JSON Formatter

1. Paste your payload into JSON Formatter.
2. Click Validate to get the exact line/column of syntax issues.
3. Use Pretty-print for readable indentation and to spot structure mistakes.
4. Toggle minify for production or API requests.
5. Copy result and test in your app or cURL before shipping.

Tip: keep schema examples in version control so your team aligns on shape and types.

Troubleshooting

“Unexpected token” near a bracket: look for a missing comma between properties or a trailing comma before }/].

Works in one tool but not another: the lenient tool might allow comments or single quotes—stick to strict JSON.

Weird characters (�): your file isn’t UTF-8—resave as UTF-8 without BOM.

Large integers changing value: serialize as strings and parse as BigInt/decimal in your app.