html input textarea
Check this:
https://codepen.io/DevLorenzo/pen/wvzNPKz
jquery detect change in textarea content
$('#textareaID').bind('input propertychange', function() {
$("#yourBtnID").hide();
if(this.value.length){
$("#yourBtnID").show();
}
});
jquery detect textarea change
$('#myTextAreaID').on('input propertychange paste', function() {
//my Textarea content has changed
});