Why is the error method always executed when the jQuery plugin AjaxFileUpload implements ajax file upload?

Today, when I used the jQuery plug-in AjaxFileUpload to upload ajax files, I encountered a problem,

The error is always executed and the succes method cannot be executed. Tracking the source code of ajaxfileupload. js:

If (type = "json") eval ("data =" + data); is executed, an exception is thrown, as a result, status = "error" is always executed when an exception is handled.

Online query, I learned that the eval function is used to execute a piece of js Code, instead of reverse decoding the json string eval ("data =" + data) as I thought ); the value of data is assigned to the data parameter,

However, when I return a simple string, such as "true", to the page, an exception is thrown. Last changedEval ("data = \" "+ data + "\"");The data to be returned is enclosed in double quotation marks as a string and then assigned to data.

Finally succeeded.