How Can I Determine if a Server Response is JSON or an Error Message?

Determining the Nature of a Server Response: JSON or Error Message

In the context of AJAX calls, it's often necessary to distinguish between JSON strings containing useful data and error messages from the server. While PHP's mysql_error() function produces error messages, the inability to parse a string as JSON indicates that it's likely an error message.

Solution: Leveraging JSON.parse()

To test if a string is valid JSON, we can utilize JSON.parse(). If the parsing operation succeeds, the string is considered JSON; otherwise, it's an error message.

Example Implementation

The following function, isJson(), implements this test:

function isJson(str) {
  try {
    JSON.parse(str);
  } catch (e) {
    return false;
  }
  return true;
}

Usage

Now, it's easy to test the nature of a server response:

if (isJson(data)) {
  // Process JSON data
} else {
  // Display error message
  alert(data);
}

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