<!DOCTYPE HTML>
<html>
<head><meta charset="windows-1251" /> </head>
<body>
<textarea></textarea>
<button style="display: none">push</button>
<script>
button=document.querySelector("button")
textarea=document.querySelector("textarea")
textarea.oninput=function(){
if(this.value==="") return button.style.display="none"
button.style.display="block"
}
</script>
</body>
</html>