How to Format JSON: Pretty-Print and Validate

“API responses come back as one unreadable line” or “my config JSON keeps throwing errors” - these are moments when JSON formatting comes to the rescue.

In this article, we explain how to format JSON: the difference between pretty-printing and minifying, common syntax errors and how to fix them, and how to validate JSON in your browser.

What is JSON?

JSON (JavaScript Object Notation) is a standard text format for data. It powers web APIs and application config files everywhere, standardized in RFC 8259.

{"name":"Taro","age":30,"address":{"city":"Tokyo","zip":"100-0001"}}

The line above is valid JSON, but hard to read. Pretty-printing makes it look like this:

{
  "name": "Taro",
  "age": 30,
  "address": {
    "city": "Tokyo",
    "zip": "100-0001"
  }
}

Common JSON errors and fixes

JSON is strict, and tiny mistakes break parsing. The most common errors:

Error	Example	Fix
Trailing comma	[1, 2, 3,]	No comma after the last item
Single quotes	{'name': 'Taro'}	JSON strings use double quotes
Comments	// note	JSON has no comments (remove them)
Unquoted keys	{name: "Taro"}	Keys must be double-quoted
Unquoted strings	{"age": 30 years}	Strings must be double-quoted

Example: finding the error

What is wrong with this JSON?

{
  "name": "Taro",
  "age": 30,
  "hobbies": ["reading", "walking",],
}

Two problems:

1. A trailing comma in the array ("walking",)
2. A trailing comma after the object (},)

Correct version:

{
  "name": "Taro",
  "age": 30,
  "hobbies": ["reading", "walking"]
}

How to format JSON

With the Tools Hub JSON formatter, paste your JSON and format, minify or validate instantly.

JSON data types you will meet

JSON supports a small set of data types. Knowing them makes errors much easier to spot:

Type	Example	Notes
String	"name": "Taro"	Must be in double quotes
Number	"age": 30	No quotes, no thousand separators
Boolean	"paid": true	true or false only
Array	"tags": ["a", "b"]	Comma-separated list
Object	"address": { "city": "Tokyo" }	Nested key-value pairs
Null	"note": null	A deliberate empty value

A common beginner mistake is wrapping numbers in quotes ("age": "30"). It parses fine, but the value becomes a string instead of a number - which can break calculations downstream. Validation catches the syntax errors, but only you can decide whether the types match your needs.

Formatting tips

• Japanese text: UTF-8 JSON formats fine as-is. If you see garbled characters, align the file encoding to UTF-8
• Line endings: Windows (CRLF) and macOS (LF) differ. Formatters usually handle this automatically
• Large JSON: Processing depends on browser memory, but typical API responses are no problem

Summary

• JSON is the standardized data format defined in RFC 8259
• Pretty-print for reading, minify for size - the data stays the same
• Trailing commas, single quotes and comments are the main error sources
• JSON supports strings, numbers, booleans, arrays, objects and null
• Validation pinpoints where errors are
• The Tools Hub formatter works entirely in your browser - nothing is uploaded

Next time you need to format JSON, use the steps above or the Tools Hub JSON formatter.

FAQ

How do I pretty-print JSON nicely?

Paste it into the JSON formatter and press “Format”. It is displayed with clean indentation.

How do I find JSON errors?

Use the “Validate” feature - it reports where and what the problem is. Trailing commas and single quotes are the usual culprits.

What is the difference between minify and format?

Format adds indentation for readability; minify strips whitespace to shrink the data. Neither changes the data itself.

Can I format JSON containing Japanese text?

Yes. UTF-8 Japanese text formats without issue. If it appears garbled, check the encoding of your input.