Can I fix JSON that contains comments automatically?

There is no automatic fix for comments in standard JSON because removing comments requires understanding which characters are part of a comment versus part of a string value. A // inside a string value is not a comment. A simple regex replacement will corrupt string values that happen to contain //. The correct approach is to use a parser that supports comments — the strip-json-comments npm package understands the full JSON grammar and removes only actual comments, preserving string content. Alternatively, if the file is a VS Code settings.json or tsconfig.json, it is likely JSONC (JSON with Comments) format, which VS Code accepts natively.

Fix Invalid JSON: Common Errors and How to Repair Them

Q: What are the most common JSON syntax errors?

The six most common JSON syntax errors are: (1) Trailing commas — a comma after the last item in an object or array before the closing brace or bracket. (2) Single quotes — JSON requires double quotes for all strings and keys. (3) Unquoted keys — {name: "Alice"} is invalid; keys must be quoted: {"name": "Alice"}. (4) Comments — JSON has no comment syntax; // and /* */ are both invalid. (5) Missing or mismatched brackets — every { needs a } and every [ needs a ]. (6) Invalid escape sequences — backslashes must be escaped as \\ and only a specific set of escape sequences are valid.

Q: How do I find where my JSON is invalid?

The fastest approach is to paste the JSON into a validator tool that highlights the exact error position with a line and column number. In JavaScript, JSON.parse throws a SyntaxError with a position index — use that index to slice the string around the error: raw.slice(Math.max(0, errorPos - 30), errorPos + 60). In the browser DevTools console, paste the JSON as a string literal and call JSON.parse on it — the error message appears immediately. For large files, binary-search by parsing progressively smaller sections to isolate which block contains the error.

Q: What is the difference between JSON and JSON5?

JSON5 is a superset of JSON that adds several features from ECMAScript 5 to make JSON more human-friendly. JSON5 allows: single or double quotes for strings, unquoted keys that are valid JavaScript identifiers, trailing commas in objects and arrays, comments (// and /* */), multi-line strings using backslash line continuation, hexadecimal numbers like 0xFF, and leading/trailing decimal points in numbers like .5 and 5.. Standard JSON allows none of these. JSON5 is used in some config files, but it is not suitable for data interchange where the receiver expects strict JSON. Parse JSON5 with the json5 npm package.

Q: How do I fix unquoted keys in JSON?

To fix unquoted keys, wrap each key in double quotes. The input {name: "Alice", age: 30} becomes {"name": "Alice", "age": 30}. A simple approach is to use a regular expression that matches word-character sequences before a colon: str.replace(/([{,]\s*)([a-zA-Z_][a-zA-Z0-9_]*)\s*:/g, '$1"$2":'), but this is fragile and can corrupt values that contain colons. For reliable conversion, use a JavaScript parser like hjson or json5 which understand unquoted keys natively and can output valid JSON. Generating JSON with JSON.stringify from the start avoids this problem entirely.

Q: What online tools can fix invalid JSON?

Several online tools help fix invalid JSON. A JSON formatter and validator shows the exact error position and reformats the corrected JSON. Jsonic's JSON Formatter identifies syntax errors inline and lets you fix them in the editor. For programmatic fixing, the jsonrepair npm package attempts to automatically repair common issues including trailing commas, single quotes, unquoted keys, and missing brackets. It handles many real-world cases but cannot fix all errors — ambiguous cases like missing closing brackets may be repaired incorrectly. For production data pipelines, always validate JSON at the source rather than relying on repair tools.

Last updated: May 11, 2026

Invalid JSON falls into one of 6 categories: trailing commas, single quotes instead of double quotes, unquoted object keys, mismatched brackets, JavaScript-style comments, or bad escape sequences. The json-repair npm library (v0.12+) fixes the most common errors automatically with 1 function call, and online validators pinpoint the exact line and column of the first syntax error. This guide covers all 6 error types with before-and-after examples, json-repair for automated fixes, Python's JSONDecodeError messages, browser DevTools debugging, and converting JSON5 or JavaScript object literals to valid JSON.

Paste your broken JSON into Jsonic's Validator to see the exact line and character where it breaks before reading on.

Validate JSON now

1. Trailing commas

The most common mistake when hand-writing or editing JSON. JavaScript object literals allow trailing commas; JSON does not.

// ❌ Invalid
{
  "name": "Alice",
  "role": "admin",
}

// ✅ Valid
{
  "name": "Alice",
  "role": "admin"
}

The same rule applies inside arrays: ["a", "b",] is invalid. Remove the last comma before the closing bracket.

2. Single quotes instead of double quotes

JSON requires double quotes for both string values and object keys. Single quotes are valid in JavaScript but not in JSON.

// ❌ Invalid
{'name': 'Alice'}

// ✅ Valid
{"name": "Alice"}

If your payload came from a JavaScript console.log or Python's str() output, this is the most likely cause.

3. Unquoted keys

JSON object keys must be strings in double quotes. Bare identifiers are valid in JavaScript object literals but not in JSON.

// ❌ Invalid
{name: "Alice", age: 30}

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

4. Comments

JSON has no comment syntax. If you are editing a config file that uses // ... or /* ... */, the format may be JSONC (JSON with Comments) rather than strict JSON. Strip all comments before parsing.

// ❌ Invalid JSON
{
  // user info
  "name": "Alice" /* admin user */
}

// ✅ Valid JSON
{
  "name": "Alice"
}

5. Missing or mismatched brackets

Every { needs a }, and every [ needs a ]. Truncated API responses and copy-paste errors are common causes.

// ❌ Invalid (missing closing brace)
{
  "user": {
    "name": "Alice"
  }

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

A JSON formatter with tree view makes mismatched brackets obvious because the nesting stops rendering correctly at the broken level.

6. Bad escape sequences

Inside a JSON string, backslashes must be escaped as \\. The only valid escape sequences are \", \\, \/, \n, \r, \t, \b, \f, and \uXXXX. Anything else is invalid.

// ❌ Invalid (unescaped backslash in Windows path)
{"path": "C:
ew	est"}

// ✅ Valid
{"path": "C:\new\test"}

Always generate JSON with JSON.stringify() rather than building strings manually to avoid this class of error entirely. See the JSON.stringify tutorial for safe patterns.

7. Undefined and NaN values

JSON does not support undefined, NaN, or Infinity. These are JavaScript-only values. JSON.stringify silently drops undefined keys and converts NaN and Infinity to null.

// ❌ Not valid JSON
{"value": undefined}
{"ratio": NaN}
{"limit": Infinity}

// ✅ Valid representations
{"ratio": null}
{"limit": null}

Quick reference: common errors

Error	Cause	Fix
`Unexpected token ','`	Trailing comma	Remove last comma before `}` or `]`
`Unexpected token "'"`	Single quotes	Replace with double quotes
`Unexpected token identifier`	Unquoted key	Wrap key in double quotes
`Unexpected end of JSON`	Missing bracket	Add the missing `}` or `]`
`Bad escape character`	Unescaped backslash	Replace `\` with `\\`

Fix your JSON in one step

Paste the broken payload into the JSON Formatter & Validator. It highlights the exact error position and reformats once you fix it.

Open JSON Validator

Frequently asked questions

What are the most common JSON syntax errors?

The top six: trailing commas, single quotes instead of double quotes, unquoted keys, comments, missing or mismatched brackets, and invalid escape sequences like an unescaped backslash.

How do I find where my JSON is invalid?

Paste it into a validator tool for a precise line and column number. In JavaScript, the SyntaxError from JSON.parse includes a position index you can use to slice the string around the error.

Can I fix JSON with comments automatically?

Use the strip-json-comments npm package — it removes only real comments and preserves // inside string values. A simple regex replacement is not safe.

What is the difference between JSON and JSON5?

JSON5 is a superset that allows single quotes, unquoted keys, trailing commas, and comments. It is useful for config files but not suitable for data interchange where strict JSON is required.

How do I fix unquoted keys in JSON?

Wrap each key in double quotes. Use the hjson or json5 npm package for reliable conversion rather than a regex, which can corrupt values containing colons.

What online tools can fix invalid JSON?

Jsonic's JSON Formatter identifies syntax errors inline and lets you fix them in the editor. The jsonrepair npm package automatically repairs trailing commas, single quotes, and unquoted keys for programmatic use.