What is the difference between JSONEncoder and JSONSerialization in Swift?

JSONEncoder is the modern Swift approach — it works with Codable types and is type-safe. JSONSerialization is the older Objective-C-era API — it works with Any (Dictionary, Array) but requires manual type casting. For new code, always use JSONEncoder with Codable structs.

Convert Swift to JSON Mock data, encoder snippets or patterns.

Q: How do I convert a Swift struct to JSON?

Use JSONEncoder: let encoder = JSONEncoder(); let data = try encoder.encode(myStruct); let jsonString = String(data: data, encoding: .utf8). For pretty output: encoder.outputFormatting = .prettyPrinted. Select the JSONEncoder Snippets mode for ready-to-run code for every scenario.

Q: How do I send a Swift struct as JSON in a URLSession POST request?

Encode the struct with JSONEncoder, set the request body and Content-Type header: var request = URLRequest(url: url); request.httpMethod = 'POST'; request.httpBody = try JSONEncoder().encode(myStruct); request.setValue('application/json', forHTTPHeaderField: 'Content-Type'). Select the URLRequest mode for the complete pattern.

Swift to JSON — 3 modes 100% client-side

Swift struct input

Instances

Formatting

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JSONEncoder vs JSONSerialization

Two Swift JSON encoding APIs — use JSONEncoder for all new code.

	JSONEncoder	JSONSerialization
Input	Any `Encodable` type	`Any` (Dictionary, Array)
Type safety	Compile-time — type errors caught by Swift	Runtime — type casting required
Snake case	`.keyEncodingStrategy = .convertToSnakeCase`	Manual key mapping
Date format	`.dateEncodingStrategy = .iso8601`	Manual `ISO8601DateFormatter`
Use for	All new Swift code with Codable models	Legacy ObjC codebases only

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The mock JSON parser reads Swift struct property declarations — no Swift compiler, no Xcode. The snippets and patterns are generated from your selection — correct JSONEncoder API with encoding options, date strategies and key encoding strategies.

JSONEncoder options

outputFormatting, keyEncodingStrategy and dateEncodingStrategy — all three encoder options covered in the snippets.

URLSession async/await

Modern iOS 15+ pattern using async/await — not the closure-based URLSession.dataTask pattern from older tutorials.

Alamofire support

Alamofire's JSONParameterEncoder — the idiomatic way to send Codable structs as POST bodies in Alamofire 5+.

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Frequently asked questions

How do I convert a Swift struct to JSON?

Use JSONEncoder: let data = try JSONEncoder().encode(myStruct); let jsonString = String(data: data, encoding: .utf8). The struct must conform to Encodable or Codable. For pretty output: let encoder = JSONEncoder(); encoder.outputFormatting = .prettyPrinted; let data = try encoder.encode(myStruct).

How do I send a Swift Codable struct as a POST body?

Encode with JSONEncoder and set the request body: var request = URLRequest(url: url); request.httpMethod = "POST"; request.httpBody = try JSONEncoder().encode(myStruct); request.setValue("application/json", forHTTPHeaderField: "Content-Type"). Select the URLRequest scenario in the Snippets mode for the complete pattern with async/await.

How do I encode a Swift struct with snake_case JSON keys?

Set the keyEncodingStrategy on JSONEncoder: let encoder = JSONEncoder(); encoder.keyEncodingStrategy = .convertToSnakeCase; let data = try encoder.encode(myStruct). This automatically converts camelCase Swift property names to snake_case JSON keys — firstName becomes first_name, isActive becomes is_active.

Convert Swift to JSONMock data, encodersnippets or patterns.

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Convert Swift to JSON
Mock data, encoder
snippets or patterns.

Swift parsed in your
browser. No Xcode needed.