var activities = document.getElementById("fieldname");
activities.addEventListener("change", function() {
value = activities.value;
value = value.replace(/\s/g, "");
activities.value = value;
});
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>JS Bin</title>
</head>
<body>
<input type="text" id="fieldname">
</body>
</html>