JSON: The Complete Developer Guide (Format, Parse, Validate)

JSON (JavaScript Object Notation) is the universal language of web APIs. Understanding how to work with JSON efficiently can save you hours every week.

JSON Syntax

{
  "name": "John Doe",
  "age": 30,
  "isActive": true,
  "score": 9.5,
  "tags": ["developer", "kotlin", "android"],
  "address": {
    "city": "Dhaka",
    "country": "Bangladesh"
  },
  "phone": null
}

JSON Data Types

• String — "hello" (always double quotes)
• Number — 42, 3.14, -1 (no quotes)
• Boolean — true, false (lowercase)
• Null — null (lowercase)
• Array — [1, 2, 3] or ["a", "b"]
• Object — {"key": "value"}

Parsing JSON in Different Languages

// Kotlin with Gson
data class User(
    val name: String,
    val age: Int,
    val email: String
)

val gson = Gson()

// Parse JSON string to object
val user = gson.fromJson(jsonString, User::class.java)

// Convert object to JSON string
val json = gson.toJson(user)

// Kotlin Serialization (modern approach)
import kotlinx.serialization.*
import kotlinx.serialization.json.*

@Serializable
data class User(val name: String, val age: Int)

val user = Json.decodeFromString(jsonString)
val json = Json.encodeToString(user)

// JavaScript
const json = JSON.stringify({ name: "John", age: 30 });
// → {"name":"John","age":30}

const obj = JSON.parse(json);
// → { name: "John", age: 30 }

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

Common JSON Errors

// ❌ Trailing comma - INVALID
{"name": "John", "age": 30,}

// ❌ Single quotes - INVALID
{name: John}

// ❌ Comments - INVALID (use JSON5 or JSONC for comments)
{
  // This is a comment - NOT valid JSON
  "name": "John"
}

// ✓ Valid JSON
{"name": "John", "age": 30}

Working with Nested JSON

// Access nested values safely
val json = """
{
  "user": {
    "profile": {
      "avatar": "https://example.com/avatar.jpg"
    }
  }
}
"""

val element = JsonParser.parseString(json).asJsonObject
val avatar = element
    ?.getAsJsonObject("user")
    ?.getAsJsonObject("profile")
    ?.get("avatar")
    ?.asString

// With Kotlin Serialization
@Serializable
data class ApiResponse(
    val user: UserData,
    val status: String,
    val timestamp: Long
)

@Serializable
data class UserData(
    val profile: Profile,
    val settings: Settings? = null  // optional field
)