jQuery $.ajax dataType Parameter Explained

The dataType parameter in jQuery's $.ajax() method specifies the type of data you expect to receive back from the server. By defining this setting, you instruct jQuery on how to parse the raw HTTP response before passing it to your success callback handlers. This guide explains how the parameter works, its available options, and the common pitfalls associated with its use.

How dataType Works

When a web server responds to an AJAX request, it typically sends data as a raw text string alongside an HTTP Content-Type header. The dataType setting pre-processes this incoming string into an appropriate JavaScript format.

For instance, if the server returns a string of JSON data and you set dataType: "json", jQuery will automatically run JSON.parse() on the response, delivering a fully accessible JavaScript object directly to your done() or success() callback.

Supported Values

jQuery supports several predefined options for dataType:

  • "json": Parses the response as a JSON string and returns a native JavaScript object or array.
  • "xml": Parses the response into an XML DOM document, allowing you to traverse it using standard jQuery traversal methods.
  • "html": Returns the response as plain text. Any <script> tags embedded in the HTML will execute when the markup is inserted into the DOM.
  • "text": Delivers the raw, unmodified text string returned by the server.
  • "script": Evaluates the response as JavaScript code and executes it immediately, returning the response as plain text.
  • "jsonp": Loads a JSON block using the JSONP pattern, enabling cross-domain requests by appending a callback query string.

Intelligent Guessing (Default Behavior)

If you do not specify a dataType, jQuery attempts to infer it automatically by inspecting the server's Content-Type response header:

  • An application/xml or text/xml header results in XML.
  • An application/json header results in JSON.
  • A text/javascript or application/javascript header results in script execution.
  • Any other MIME type defaults to plain text or HTML.

While automatic detection is convenient, explicitly defining dataType is considered best practice. It prevents unexpected parsing errors caused by misconfigured server headers.

Strict Parsing and Error Handling

Specifying dataType introduces strict validation. If you set dataType: "json" but the server returns invalid JSON, empty content, or an HTML error page, the parsing step will fail. When this occurs, jQuery triggers the fail() or error callback with a parsererror, even if the server returned an HTTP 200 OK status code.

dataType vs. contentType

These two settings are frequently confused:

  • contentType: The format of the data you are sending to the server in the request body (e.g., application/json; charset=UTF-8).
  • dataType: The format of the data you expect to receive from the server in the response.