Just by one line-
var select_button_text = $('#SelectQButton option:selected')
.toArray().map(item => item.text);
Output: ["text1", "text2"]
var select_button_text = $('#SelectQButton option:selected')
.toArray().map(item => item.value);
Output: ["value1", "value2"]
If you use .join()
var select_button_text = $('#SelectQButton option:selected')
.toArray().map(item => item.text).join();
Output: text1,text2,text3