hideSpinner.html

<script>
    function showSpinner() {
        $spinner = document.getElementsByClassName("slds-spinner_container")[0];
        $spinner.style.display = "block";
    }
    function hideSpinner() {
        $spinner.style.display = "none";
    }
    
    ...
    
    function postData() {
        fetch("http://example.com/myData.json", {
            method: "POST",
            headers: {
                ...
            },
            body: ({
                ...
            })
        }).then(function(res) {
            hideSpinner(), window.alert("Success")
        }).catch(function(err) {
            hideSpinner(), window.alert("Error")
        })
    }
</script>