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Complete Guide to JSON

Master JSON syntax, validation, and best practices

12 min read • Updated January 2025

What is JSON?

JSON (JavaScript Object Notation) is a lightweight, text-based data interchange format that's easy for humans to read and write, and easy for machines to parse and generate. Despite its name, JSON is language-independent and is used across virtually every programming language and platform.

Created by Douglas Crockford in the early 2000s, JSON has become the de facto standard for data exchange on the web. It replaced XML in many applications due to its simplicity, smaller size, and natural compatibility with JavaScript.

Key Characteristics:

Text-based and human-readable
Language-independent with parsers available in all major languages
Lightweight compared to XML (typically 30-50% smaller)
Built on two universal data structures: objects (key-value pairs) and arrays (ordered lists)
Specification defined in RFC 8259 and ECMA-404

JSON Syntax Rules

JSON has strict syntax rules that must be followed for valid parsing:

1. Data is in name/value pairs

"name": "John"

2. Data is separated by commas

{
  "firstName": "John",
  "lastName": "Doe",
  "age": 30
}

Important: No trailing comma after the last item!

3. Curly braces hold objects

{
  "user": {
    "name": "John",
    "age": 30
  }
}

4. Square brackets hold arrays

{
  "users": ["John", "Jane", "Bob"]
}

5. Strings must use double quotes

// Correct
"name": "John"

// WRONG - single quotes not allowed
'name': 'John'

6. Property names must be quoted

// Correct
"age": 30

// WRONG - unquoted property names not allowed
age: 30

JSON Data Types

JSON supports six data types:

String

Text enclosed in double quotes. Must escape special characters with backslash.

"name": "John Doe"
"message": "Hello, \"world\"!"
"path": "C:\\Users\\Documents"

Number

Integer or floating-point. No distinction between types.

"age": 30
"price": 19.99
"temperature": -5.5
"scientific": 1.5e10

Boolean

True or false (lowercase only).

"isActive": true
"hasAccount": false

Null

Represents absence of value (lowercase only).

"middleName": null
"deletedAt": null

Object

Unordered collection of key-value pairs enclosed in curly braces.

"address": {
  "street": "123 Main St",
  "city": "New York",
  "zipCode": "10001"
}

Array

Ordered list of values enclosed in square brackets. Values can be any type.

"tags": ["javascript", "json", "web"]
"scores": [95, 87, 92]
"mixed": ["text", 123, true, null]

Validating JSON

JSON validation ensures your data conforms to the JSON specification. Invalid JSON will cause parsing errors in applications.

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Validation checks for:

Proper use of quotes (double quotes only)
Balanced brackets and braces
No trailing commas
Properly escaped special characters
Valid data types
Correct comma placement between elements

JavaScript Validation Example:

try {
  const data = JSON.parse(jsonString);
  console.log("Valid JSON!", data);
} catch (error) {
  console.error("Invalid JSON:", error.message);
}

Common JSON Errors

Understanding common mistakes helps you write valid JSON quickly:

❌ Trailing Commas

{
  "name": "John",
  "age": 30,  // ← WRONG: comma after last item
}

❌ Single Quotes

{
  'name': 'John'  // ← WRONG: must use double quotes
}

❌ Unquoted Keys

{
  name: "John"  // ← WRONG: keys must be quoted
}

❌ Comments

{
  "name": "John", // This is a comment ← WRONG: no comments allowed
  "age": 30
}

❌ Undefined Value

{
  "value": undefined  // ← WRONG: use null instead
}

Parsing JSON in JavaScript

JavaScript provides native methods for working with JSON:

JSON.parse() - Convert JSON string to JavaScript object

const jsonString = '{"name":"John","age":30}';
const obj = JSON.parse(jsonString);
console.log(obj.name); // "John"
console.log(obj.age);  // 30

JSON.stringify() - Convert JavaScript object to JSON string

const obj = { name: "John", age: 30 };
const jsonString = JSON.stringify(obj);
console.log(jsonString); // '{"name":"John","age":30}'

// With formatting (indentation)
const formatted = JSON.stringify(obj, null, 2);

Error Handling

function safeJSONParse(jsonString) {
  try {
    return { success: true, data: JSON.parse(jsonString) };
  } catch (error) {
    return { success: false, error: error.message };
  }
}

Best Practices

✓
Use consistent indentation - 2 or 4 spaces for readability
✓
Validate before deployment - Use validators to catch errors early
✓
Keep nesting shallow - Deeply nested structures are hard to read and maintain
✓
Use meaningful key names - Descriptive names improve clarity
✓
Minify for production - Remove whitespace to reduce file size
✓
Consider schema validation - Use JSON Schema for complex data structures
✓
Handle parsing errors gracefully - Always use try-catch when parsing

Real-World Use Cases

1. RESTful APIs

JSON is the standard format for API requests and responses. Almost every modern web API uses JSON for data exchange.

2. Configuration Files

package.json, tsconfig.json, .eslintrc.json - many tools use JSON for configuration due to its simplicity and wide support.

3. Data Storage

NoSQL databases like MongoDB store data in JSON-like formats (BSON). JSON is perfect for document-oriented databases.

4. Web Storage

LocalStorage and SessionStorage APIs work with strings - JSON is perfect for storing complex objects in the browser.

5. Data Exchange

JSON is the universal format for importing/exporting data between applications, perfect for data migration and integration.

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