jQuery .ajax数据未定义

I have a $.ajax request, and when i send response data with success:function(data), I get the data but for some reason I can't use it in If Statement:

    $.ajax({
      type: "POST",
      url: "control.php",
      data: {"txtNumFattura" : $("#txtNumFattura").val(),
             "txtDataFattura": $("#txtDataFattura").val()},

      success: function(data) {
        console.log(data);
        if (data == 'OK') {
          console.log("Chiave non ancora utilizzata");
          $("#submitMyForm").removeAttr("disabled");
        } 
        else if(data == 'KO') {
          $("#mySubmitForm").attr("disabled","disabled");
          console.log("Chiave Utilizzata");
        };
      }
    });

For instance console.log gives me "OK" or "KO" but it seems like it doesn't read it in the if Statement.

There might be extra whitespace after the OK or KO. Use

console.log("*" + data + "*");

If there is you could use replace() to remove these. For example,

data = data.replace(/\s/g, "");

\s is a whitespace character and 'g' means globally.

Try

if(data.toLowerCase().indexOf('ok') != -1){
  // ok was found
} else if (data.toLowerCase().indexOf('ko') != -1) {
  // ko was found
} else {
  // neither was found
}

If you are sure that it gives you OK or KO, you can try this:

if ($.trim(data) === 'OK') {
  console.log("Chiave non ancora utilizzata");
  $("#submitMyForm").removeAttr("disabled");
} else if($.trim(data) === 'KO') {
  $("#mySubmitForm").attr("disabled","disabled");
  console.log("Chiave Utilizzata");
};