JSON Formatter: Prettify, Validate, and Lint JSON Like a Pro

JSON formatting is one of those things you do ten times a day but rarely think about. Here's everything you need to know.

JSON.stringify: The Basics

const data = { name: "Alice", roles: ["admin", "editor"] };

// Minified (default)
JSON.stringify(data)
// '{"name":"Alice","roles":["admin","editor"]}'

// Pretty-printed (2-space indent)
JSON.stringify(data, null, 2)
// {
//   "name": "Alice",
//   "roles": [
//     "admin",
//     "editor"
//   ]
// }

// Tab indent
JSON.stringify(data, null, '\t')

The Replacer Function (Secret Power)

// Filter sensitive fields
JSON.stringify(data, (key, value) => {
  if (key === 'password' || key === 'token') return undefined;
  return value;
}, 2);

// Or pick specific keys
JSON.stringify(data, ['name', 'email'], 2);

Command Line: jq

jq is the JSON Swiss Army knife:

# Pretty-print
cat data.json | jq '.'

# Minify
cat data.json | jq -c '.'

# Sort keys
cat data.json | jq -S '.'

# Extract fields
cat data.json | jq '.users[] | {name, email}'

Python

import json

# Pretty-print
json.dumps(data, indent=2, ensure_ascii=False)

# Sort keys (great for diffs)
json.dumps(data, indent=2, sort_keys=True)

# Minify
json.dumps(data, separators=(',', ':'))

# From command line
python3 -m json.tool input.json

JSON Validation Checklist

Common errors that break JSON:

• Trailing commas: {"a": 1,} — INVALID
• Single quotes: {'a': 1} — INVALID
• Comments: {"a": 1 // note} — INVALID
• Unquoted keys: {a: 1} — INVALID
• NaN/Infinity: not valid JSON values

JSON5 and JSONC

If you need comments and trailing commas, use JSON5:

// json5 - JavaScript
import JSON5 from 'json5';
const data = JSON5.parse(`{
  // This is a comment
  name: 'Alice',
  age: 30,
}`);

VS Code uses JSONC (JSON with Comments) for tsconfig.json and settings.json.

CI/CD Integration

# GitHub Actions - validate all JSON files
- name: Lint JSON
  run: |
    find . -name "*.json" -not -path "*/node_modules/*" \
      -exec sh -c 'python3 -m json.tool "$1" > /dev/null || echo "Invalid: $1"' _ {} \;

Large File Performance

Parser	Speed (1GB JSON)	Memory
simdjson	~2s	Low
jq --stream	~10s	Constant
Python ijson	~30s	Constant
JSON.parse()	~5s	Very high

For files over 100MB, use streaming parsers.

Try It Online

Format, validate, minify, and explore JSON with DevToolBox's JSON Formatter — syntax highlighting, error detection, tree view, copy with one click.

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