Understanding JSON

What is JSON?

JSON (JavaScript Object Notation) is a lightweight data format used for storing and exchanging information between applications. Originally derived from JavaScript, JSON has become the universal language of web APIs and configuration files across all programming languages.

JSON's popularity stems from its simplicity. Unlike XML or other data formats, JSON is easy for humans to read and write, while also being straightforward for machines to parse and generate. This makes it perfect for everything from API responses to application configuration files.

Basic Structure

JSON organizes data into two fundamental structures: objects and arrays.

JSON Objects

A JSON object is a collection of key-value pairs, wrapped in curly braces {}. Each key is a string (always in double quotes), followed by a colon, then the value:

{
  "name": "John Doe",
  "age": 30,
  "city": "New York",
  "isStudent": false
}

Think of objects as labeled containers. Each label (key) points to a piece of information (value). Keys must be unique within an object, and the order doesn't matter—{"a": 1, "b": 2} is equivalent to {"b": 2, "a": 1}.

JSON Arrays

Arrays store ordered lists of values, wrapped in square brackets []. Unlike objects, arrays don't have keys—values are accessed by their position (index):

[
  "apple",
  "banana",
  "orange"
]

Arrays are perfect for lists where order matters, like a sequence of steps, a list of users, or historical data points.

Data Types in JSON

JSON supports six core data types. Understanding each one is crucial for reading and creating JSON correctly.

1. Strings

Text values enclosed in double quotes. Always use double quotes, never single quotes:

"name": "Alice"
"message": "Hello, world!"

2. Numbers

Integers or decimals without quotes. JSON doesn't distinguish between integers and floats:

"age": 25
"price": 19.99
"temperature": -5

3. Booleans

True or false values (lowercase, no quotes):

"isActive": true
"hasDiscount": false

4. Null

Represents the absence of a value (lowercase, no quotes):

"middleName": null

5. Objects

Nested key-value pairs for complex data structures:

"address": {
  "street": "123 Main St",
  "city": "Boston"
}

6. Arrays

Ordered lists that can contain any data type, including mixed types:

"scores": [95, 87, 92]
"mixedData": ["text", 123, true, null]

Working with Nested Data

JSON's real power comes from nesting—objects can contain other objects and arrays, creating complex data hierarchies:

{
  "user": {
    "id": 101,
    "name": "Sarah Chen",
    "contact": {
      "email": "[email protected]",
      "phone": "+1-555-0123"
    },
    "orders": [
      {
        "orderId": "A001",
        "total": 49.99,
        "items": ["Book", "Pen"]
      },
      {
        "orderId": "A002",
        "total": 29.99,
        "items": ["Notebook"]
      }
    ]
  }
}

To read nested data, follow the path: user.contact.email gives you "[email protected]", while user.orders[0].total gives you 49.99.

{
  "location": "San Francisco",
  "current": {
    "temp": 18,
    "condition": "Cloudy"
  },
  "forecast": [
    {"day": "Monday", "high": 20, "low": 15},
    {"day": "Tuesday", "high": 22, "low": 16}
  ]
}

Common Mistakes to Avoid

Even experienced developers make these JSON errors:

1. Trailing Commas

JSON doesn't allow commas after the last item:

// ❌ Invalid
{"name": "Bob", "age": 30,}

// ✅ Valid
{"name": "Bob", "age": 30}

2. Single Quotes

Always use double quotes for keys and string values:

// ❌ Invalid
{'name': 'Bob'}

// ✅ Valid
{"name": "Bob"}

3. Unquoted Keys

Keys must always be strings in quotes:

// ❌ Invalid
{name: "Bob"}

// ✅ Valid
{"name": "Bob"}

4. Comments

Standard JSON doesn't support comments (though some parsers allow them):

// ❌ Invalid JSON
{
  // This is a comment
  "name": "Bob"
}

Best Practices

Follow these guidelines to create maintainable JSON:

• Use meaningful keys: "userEmail" is clearer than "e"
• Be consistent: Choose either camelCase or snake_case and stick with it
• Keep it flat when possible: Don't nest unnecessarily deep—three levels is usually the maximum before it becomes hard to read
• Use arrays for lists: If data represents multiple items of the same type, use an array
• Validate your JSON: Use tools like JSONLint to catch syntax errors before deployment