<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>untitled</title>
</head>
<body id="body">
<input type="button" value="Нравится" onclick="like()">
<script>
function like() {
document.body.children[0].value = "Нравится +1"
}
</script>
</body>
</html>
<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>untitled</title>
</head>
<body id="body">
<input type="button" value="Нравится" onclick="like()">
<script>
function like() {
var like_btn = document.body.children[0]
if ("Нравится".length == like_btn.value.length)
like_btn.value = "Нравится +1"
else
{
var num = like_btn.value.substring("Нравится +".length, like_btn.length)
like_btn.value = "Нравится +" + ++num
}
}
</script>
</body>
</html>