Because, you're executing the script before document is completely loaded, the element #popup is not found.
- Use DOMContentLoaded
Use DOMContentLoaded to check if the DOM is completely loaded.
<script type="text/javascript">
document.addEventListener("DOMContentLoaded", function(event) {
console.log("DOM fully loaded and parsed");
if (document.getElementById("popup")) {
window.alert("hi");
}
});
</script>
- Using jQuery
ready
Using jQuery, you can use ready method to check if DOM is ready.
$(document).ready(function() {
if ($("#popup").length) {
window.alert("hi");
}
});
- Moving
script to the end of body
You can move your script to the end of body.
<!DOCTYPE html>
<html>
<head>
</head>
<body>
<h1 id="popup">dfdfs</h1>
// Move it here
<script type="text/javascript">
if (document.getElementById("popup")) {
window.alert("hi");
}
</script>
</body>
</html>