Fetch API css-loader

I'm using fetch API to access a JSON-object.

My questions is how can I implement a CSS loader before the actual data is displayed, and how do I know that the data has been displayed.

This is what I've tried:

var startSevenDaysInterval = (new Date).getTime();
var endSevenDaysInterval = startSevenDaysInterval - 604800000;
function fetchData2() {
    fetch('https://vannovervakning.com/api/v1/measurements/3/' + endSevenDaysInterval + '/' + startSevenDaysInterval + '/')
        .then(function (response) {
            if(response === 404){
                document.getElementById("loader").style.display = 'none';
            }
            return response.json();
        })
        .then(function (data) {
            document.getElementById("loader").style.display = 'none';
            printTemperature(data);
        });
}
function printTemperature(data) {
    var html = "<h5>";
    html += data.data["TEMPERATURE"][0].value;
    html += "</h5>";
    document.getElementById('temp1').innerHTML = html;
}
setInterval(function () {
    fetchData2();
}, 1000);

My loader looks like this:

.loader {
border: 4px solid #f3f3f3; /* Light grey */
border-top: 4px solid #e0e0e0;
border-radius: 30px;
width: 30px;
height: 30px;
animation: spin 1s linear infinite;
}

@keyframes spin {
    0% { transform: rotate(0deg); }
    100% { transform: rotate(360deg); }
}

And my HTML looks like this:

<div class="tempVal">
    <div class="loader"></div>
    <div id="temp1"></div>
</div>

You are already doing that, but using getElementById for a class (it's for IDs only). When you're not sure about these things make sure you have actually got the element you want e.g. using a debugger statement or console.log(document.getElementById("loader")).

Get a class with querySelector which can handle all css selectors:

document.querySelector('.loader').style.display = 'none'

Note if you have multiple classes with that name it will get the first one. You don't need to repeat it in both .then's.