Detect entered character with JavaScript

Use keypress instead of keydown. While keydown relates to every press of a key, keypress relates to the translated characters, so for example a can be different to a while the shift key is pressed, composed characters work, dead-keys work, and other differences in keyboard mappings are handled.

How about checking if @ was entered as the last character in the field value?

$("body").on("keyup", "textarea", function(e) {
    if (this.value.indexOf("@") == this.value.length - 1) {
        console.log("Starting autocomplete");
    }
});​

DEMO: http://jsfiddle.net/FKhPW/2/