How do I detect “shift+enter” and generate a new line in Textarea?

if (evt.keyCode == 13) {
    form.submit();
}

... and you could just change it to

if (evt.keyCode == 13 && !evt.shiftKey) {
    form.submit();
}

reference : https://stackoverflow.com/questions/6014702/how-do-i-detect-shiftenter-and-generate-a-new-line-in-textarea