如何使用Javascript在文本区域中获取键入字母的位置？

Question

I am creating a dropdown while typing in a textarea. How can I get the position of the typed letter given a keyup event?

Answer 1

var oldValue = '';
var keyup = function() {
    var value = document.getElementById('myTextArea').value;
    for(var i=0; i<value.length; i++) {
        if(i >= oldValue.length) {
            // letter typed at end
            oldValue = value;
            return; // not really necessary since we should be at the end of the loop anyway
        } else if(value.charAt(i) !== oldValue.charAt(i)) {
            // letter typed at i
            oldValue = value;
            return; // no need to keep searching
        }
    }
    // no new letters typed
}

Answer 2

Greg's answer seems to work. But if you want a more simpler way to get it, you can access the selectionStart property of the textarea.

For example

var myTextArea = $("#mytextarea");

myTextArea.keyup(function () {

   console.log("The last typed character is at: ", myTextArea.get(0).selectionStart - 1);

});

http://jsfiddle.net/wasjdhtu/