Notes for using jquery Ajax

I encountered a problem when I was doing ajax yesterday,

$. Ajax ({url: '<{$ weburl}>/main. php? Action = getDepart & cid = '+ cid, type: 'post', dataType: 'json', success: function (data) {// data = eval ('+ data +'); if (data. s = 1) invalid parameter ('{}.html('{}}('{}'}.html (data. departs);} else {alert ('faculty acquisition failed ');}}});

I have not paid special attention to this issue before. If dataType: 'json' is not set at the beginning, the returned data cannot be parsed and data is processed as a string instead of a json object, I used eval for conversion, and the data returned by the server contains a lot of html tags and cannot be correctly read. As a result, data reports an error. I remember that dataType was not used in the past, it is strange that I don't know if it is related to the jquery version. It seems that it is better to explicitly set dataType later.