How Can I Differentiate Between JSON Data and Error Messages in AJAX Responses?

Distinguishing JSON Strings from Error Messages in AJAX Responses

When working with AJAX calls, it's common to receive data from the server that may be either a JSON string containing relevant information or an error message string generated by PHP's mysql_error() function. To handle these responses effectively, it's essential to have a method for identifying whether the received data is a JSON string.

One approach to this problem is to define a function called isJSON that mimics the syntax of the instanceof operator used for array type checking. The isJSON function employs JSON.parse to accomplish this:

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

This function attempts to parse the input string as a JSON object. If the parsing is successful, the function returns true, indicating that the string is a valid JSON string. If the parsing fails due to a syntactic error, the function catches the exception and returns false, indicating that the string is not JSON.

Utilizing this function, we can write code to handle the different scenarios:

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

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