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SOLVED: JSON Decoding Error Using A Generic

Forums > Swift

I've come across a couple similar posts related to JSON decoding errors, but I think my problem is different enough to ask it:


struct UserSLPForgotPasswordRequest: Codable {
    var email: String

struct UserSLPForgotPassword: Codable {
    let email: String

API Service Session/Encoding

URLSession.shared.dataTask(with: request) { (data, response, error) in

            if data != nil { print(data!) }

            if response != nil { print(response!) }

            if error != nil { print(response!) }

            guard let httpResponse = response as? HTTPURLResponse, (200...299).contains(httpResponse.statusCode) else {
                completion(.failure(.custom(errorMessage: "Non-200 Response")))
//                print(response!)
            guard let data = data, error == nil else {
                completion(.failure(.custom(errorMessage: "No Data")))
            self.decode(data: data, completion: completion)


Generic Decoder

    private func decode<T: Decodable>(data: Data, completion: @escaping (Result<T, AuthenticationError>) -> Void) {
        do {
            let jsonDecoded = try JSONDecoder().decode(T.self, from: data)
        } catch (let error) {

Somewhere inside the decoder function I recieve this error:

dataCorrupted(Swift.DecodingError.Context(codingPath: [], debugDescription: "The given data was not valid JSON.", underlyingError: Optional(Error Domain=NSCocoaErrorDomain Code=3840 "Invalid value around line 1, column 0." UserInfo={NSDebugDescription=Invalid value around line 1, column 0., NSJSONSerializationErrorIndex=0})))

My back-end is working fine and sending me a 200 response, but I'm just not sure where the decoder is having its problems with the JSON.

I have only used the decoder with GET and POST requests that have multiple fields involved and this particular endpoint only receives a single field and technically isn't returning anything, so maybe it's something to do with that? IDK...


Please post a sample of the JSON response. There is something in there the decoder doesn't like, right at the very beginning.


Hey @roosterboy, you're right. The decoder is the problem, in that it's completely unneccessary :-)

I figured it out a bit later and the problem is I wasn't familiar with using Void() in place of the Decoders work for this particular endpoint; since it doesn't return any JSON, only a server response code.

So instead of

self.decode(data: data, completion: completion)

I just used


And instead of being


I'm now



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