What is JSON, and how do you parse and serialize it in mobile applications?

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JSON, which stands for JavaScript Object Notation, is the standard data interchange format used by most REST APIs. Understanding how to parse JSON responses into usable objects and serialize objects back to JSON for requests is essential for mobile development.

JSON represents data using a simple text format with two primary structures: objects (key-value pairs enclosed in curly braces) and arrays (ordered lists enclosed in square brackets). Values can be strings, numbers, booleans, null, objects, or arrays. A typical API response might look like an object with an id number, a name string, an email string, and a roles array containing strings.

In React Native and JavaScript, JSON handling is straightforward because JSON is native to the language. JSON.parse converts a JSON string into a JavaScript object, and JSON.stringify converts a JavaScript object back to a JSON string. When using fetch, calling response.json() automatically parses the response body. With TypeScript, you define interfaces or types that describe the shape of the JSON data, giving you autocompletion and type checking. However, TypeScript types are compile-time only and do not validate the actual runtime data.

For runtime validation in React Native, libraries like Zod let you define schemas that both describe the expected shape and validate incoming data. This catches issues where the API returns unexpected data before it causes errors deeper in your application. A Zod schema can parse the JSON and throw a descriptive error if the data does not match the expected structure.

In Flutter, JSON handling requires more explicit work because Dart is statically typed. The basic approach uses dart:convert to call jsonDecode, which returns a dynamic Map. You then manually access fields with bracket notation. This works but lacks type safety. The better approach is creating model classes with fromJson factory constructors and toJson methods. You define a class like User with typed fields and write fromJson to extract values from the map and toJson to convert back.

For large projects, code generation with json_serializable and build_runner automates the creation of fromJson and toJson methods. You annotate your class with JsonSerializable, define the fields, and run the build runner to generate the serialization code. This eliminates manual parsing errors and keeps serialization consistent.

In native iOS with Swift, Codable protocol provides powerful JSON handling. You define a struct conforming to Codable, and JSONDecoder automatically maps JSON keys to struct properties. It handles nested objects, arrays, custom date formats, and key name mapping through CodingKeys. In native Android with Kotlin, Gson, Moshi, or kotlinx.serialization provide similar functionality with annotation-based or reflection-based JSON mapping.

Best practices include always defining typed models rather than working with raw dictionaries, validating incoming data before using it, handling missing or null fields gracefully with optional types, and keeping serialization logic centralized in model files rather than scattered across the codebase.