Error Domain=NSCocoaErrorDomain Code=3840 in Swift 4.1

Table of Contents

When working with Swift and parsing JSON data, you might encounter an error like ‘Error Domain=NSCocoaErrorDomain Code=3840’. This error typically occurs when there is an issue with the JSON data being parsed or the way it is being parsed. In this blog post, we’ll explore the common reasons for this error and provide possible solutions with code examples.

Reasons for ‘Error Domain=NSCocoaErrorDomain Code=3840’

  1. Invalid JSON format: The most common reason for this error is that the JSON data being parsed is not valid. This could be due to issues such as missing or extra commas, incorrect data types, or improperly formatted strings.
  2. Incorrect parsing method: Using the wrong method or incorrect data types when parsing JSON data can also lead to this error. It is essential to ensure that the parsing method and data types used align with the structure of the JSON data.

Possible Solutions with Code Examples

Solution 1: Validate and correct the JSON format

Before parsing the JSON data, it is crucial to ensure that it is in a valid format. You can use online JSON validators, such as, to check the JSON data for any formatting issues. If any issues are found, correct them and try parsing the data again.

Solution 2: Use correct parsing methods and data types

Make sure that you are using the appropriate method and data types when parsing the JSON data. Here’s an example of how to parse JSON data correctly using Swift 4.1:

import Foundation

let jsonString = """
    "name": "John Doe",
    "age": 30,
    "isStudent": false

if let jsonData = .utf8) {
    do {
        let json = try JSONSerialization.jsonObject(with: jsonData, options: []) as! [String: Any]

        if let name = json["name"] as? String,
           let age = json["age"] as? Int,
           let isStudent = json["isStudent"] as? Bool {
            print("Name: \(name), Age: \(age), Is Student: \(isStudent)")
        } else {
            print("Error: Invalid data types in JSON")
    } catch {
        print("Error: \(error.localizedDescription)")
} else {
    print("Error: Could not convert JSON string to data")

In this example, we first convert the JSON string to Data and then use JSONSerialization.jsonObject to parse the JSON data. We then cast the parsed JSON data to a dictionary with String keys and Any values. We extract the values from the dictionary using the correct data types and handle any errors that may occur during the process.


The ‘Error Domain=NSCocoaErrorDomain Code=3840’ error in Swift is typically caused by invalid JSON data or incorrect parsing methods and data types. By validating the JSON data format and using the appropriate parsing techniques, you can resolve this error and successfully parse JSON data in your Swift application.

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