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JSON

JSON (JavaScript Object Notation) Breakdown

JSON is a lightweight data format used for data exchange between systems. It's text-based, human-readable, and follows a key-value structure similar to dictionaries in Python or objects in JavaScript.

1. JSON Data Structure

JSON consists of:

  1. Objects { key: value } (like dictionaries)
  2. Arrays [value1, value2] (like lists)
  3. Primitive Values (strings, numbers, booleans, null)

Example JSON

{
  "name": "Alice",
  "age": 25,
  "isStudent": false,
  "hobbies": ["reading", "gaming"],
  "address": {
    "city": "New York",
    "zip": "10001"
  }
}

2. JSON Data Types

JSON supports the following six data types:

Type Example Notes
String "hello" Enclosed in double quotes
Number 25, 3.14 Integers & floating points
Boolean true, false No quotes needed
Null null Represents an empty value
Object { "key": "value" } Collection of key-value pairs
Array [1, 2, "text"] Ordered list of values

3. JSON Objects ({})

A JSON object is a collection of key-value pairs.

Example

{
  "id": 101,
  "name": "Bob",
  "email": "bob@example.com"
}
  • Keys must be strings (double-quoted).
  • Values can be any valid JSON type.

4. JSON Arrays ([])

JSON arrays are ordered collections of values.

Example

{
  "fruits": ["apple", "banana", "cherry"]
}
  • Arrays can contain mixed types, though it’s uncommon:
{
  "mixed": [42, "hello", true, null]
}

5. Nesting Objects & Arrays

JSON supports nesting, meaning objects can contain arrays and vice versa.

Example

{
  "user": {
    "id": 1,
    "name": "John",
    "contacts": ["john@email.com", "123-456-7890"]
  }
}

6. JSON Syntax Rules

  • Use double quotes (") for keys and strings.
  • No trailing commas (unlike JavaScript objects).
  • Keys must be strings.
  • Booleans and null are lowercase (true, false, null).

✅ Valid JSON:

{ "name": "Alice", "age": 30 }

❌ Invalid JSON (trailing comma):

{ "name": "Alice", "age": 30, }

7. JSON vs JavaScript Objects

Feature JSON Example JavaScript Object Example
Key Quotes { "name": "Alice" } { name: "Alice" } (No quotes)
Property Access jsonObj["name"] obj.name or obj["name"]
Allowed Values Strings, numbers, arrays, objects Functions, dates, regex

8. Converting JSON in Different Languages

JavaScript

// Convert JSON string to object
let obj = JSON.parse('{"name": "Alice"}');

// Convert object to JSON string
let jsonStr = JSON.stringify(obj);

Python

import json

# Convert JSON string to dictionary
data = json.loads('{"name": "Alice"}')

# Convert dictionary to JSON string
json_str = json.dumps(data, indent=2)

9. Common JSON Operations

Reading JSON from a File

Python

with open("data.json", "r") as file:
    data = json.load(file)

JavaScript (Node.js)

const fs = require('fs');
let data = JSON.parse(fs.readFileSync("data.json", "utf-8"));

10. Common Mistakes in JSON

Mistake Example Fix
Single quotes {'key': 'value'} Use double quotes (")
Trailing comma { "name": "Alice", } Remove comma
Unquoted keys { name: "Alice" } Use "name"
Functions { "sayHello": function() { return "Hi"; } } Functions are not allowed

11. JSON in APIs

JSON is widely used in REST APIs for data exchange.

Example API Response (GET Request)

{
  "status": "success",
  "data": {
    "id": 1,
    "name": "Alice"
  }
}

Clients (browsers, mobile apps) parse this response to extract and display data.

12. JSON Schema (Validation)

To ensure JSON is structured correctly, JSON Schema is used.

Example JSON Schema

{
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "integer" }
  },
  "required": ["name", "age"]
}

13. JSON Tools

  • Online Formatters: jsonlint.com

  • CLI Pretty Print (Linux/Mac):

    cat data.json | jq .

  • PowerShell Pretty Print:

    Get-Content data.json | ConvertFrom-Json | ConvertTo-Json -Depth 10

Key Takeaways

✅ JSON is simple and widely used.
✅ Always use double quotes for keys and strings.
✅ Objects ({}) store key-value pairs.
✅ Arrays ([]) store ordered values.
✅ JSON is not the same as JavaScript objects (no functions, strict syntax).

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