JSON Cheat Sheet

Basic Syntax

{
  "string": "value",
  "number": 42,
  "float": 3.14,
  "boolean": true,
  "null": null,
  "array": [1, 2, 3],
  "object": {"nested": true}
}

• ✅ Double quotes for strings
• ✅ No trailing commas
• ✅ No comments allowed
• ✅ Keys must be strings

Data Types

• "string" - Text in double quotes
• 123 - Number (int or float)
• true/false - Boolean
• null - Null value
• [] - Array
• {} - Object

// Example
{
  "name": "John",      // string
  "age": 30,           // number
  "active": true,      // boolean
  "spouse": null,      // null
  "hobbies": [],       // array
  "address": {}        // object
}

Arrays

// Simple array
[1, 2, 3, 4, 5]

// Mixed types
["text", 123, true, null]

// Array of objects
[
  {"id": 1, "name": "John"},
  {"id": 2, "name": "Jane"}
]

// Nested arrays
[[1, 2], [3, 4]]

Objects

// Simple object
{"key": "value"}

// Nested object
{
  "user": {
    "name": "John",
    "address": {
      "city": "NYC"
    }
  }
}

// Object with array
{
  "users": [
    {"id": 1},
    {"id": 2}
  ]
}

JavaScript Methods

// Parse JSON string
const obj = JSON.parse(jsonString);

// Convert to JSON string
const str = JSON.stringify(obj);

// Pretty print (2-space indent)
JSON.stringify(obj, null, 2);

// With replacer function
JSON.stringify(obj, (k, v) => 
  k === 'password' ? undefined : v
);

// Parse with reviver
JSON.parse(str, (k, v) => 
  k === 'date' ? new Date(v) : v
);

Common Errors

• ❌ Single quotes: 'value'
• ✅ Double quotes: "value"
• ❌ Trailing comma: {"a": 1,}
• ✅ No trailing comma: {"a": 1}
• ❌ Unquoted keys: {key: "value"}
• ✅ Quoted keys: {"key": "value"}
• ❌ Comments: // comment
• ✅ No comments allowed in JSON

Escape Sequences

• \" - Double quote
• \\ - Backslash
• \/ - Forward slash
• \n - Newline
• \r - Carriage return
• \t - Tab
• \b - Backspace
• \f - Form feed
• \uXXXX - Unicode

"He said \"Hello\""
"Path: C:\\Users\\Name"
"Line 1\nLine 2"

Best Practices

• ✅ Use consistent indentation
• ✅ Validate before sending
• ✅ Use schema validation
• ✅ Minify for production
• ✅ Enable gzip compression
• ✅ Handle parse errors
• ✅ Never use eval() to parse
• ✅ Sanitize user input

Common HTTP Status Codes

• 200 OK - Success
• 201 Created - Resource created
• 204 No Content - Success, no body
• 400 Bad Request - Invalid JSON
• 401 Unauthorized - Auth required
• 404 Not Found - Not found
• 422 Unprocessable - Validation fail
• 500 Server Error - Server issue

HTTP Headers

// Request
Content-Type: application/json
Accept: application/json

// Response
Content-Type: application/json
Cache-Control: max-age=3600

// Fetch example
fetch(url, {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  body: JSON.stringify(data)
})

JSONPath Syntax

• $ - Root element
• @ - Current element
• . - Child element
• .. - Recursive descent
• * - Wildcard
• [] - Array subscript
• [,] - Union operator
• [?()] - Filter expression

$.store.book[0].title
$.store.book[*].author
$..author
$.store.book[?(@.price < 10)]

Performance Tips

• Minify JSON in production
• Enable gzip/brotli compression
• Use pagination for large datasets
• Cache parsed JSON objects
• Stream large files
• Use Web Workers for parsing
• Consider binary formats (MessagePack)
• Limit nesting depth