brad20.html

<script>
function calcSum(){
    let n = document.getElementById("n").value;
    let i = 1, sum = 0;
    while (i <= n) sum += i++;
    document.getElementById("result").innerHTML = sum;
}
</script>
1 + 2 + ... + 
<input type="number" id="n" />
<input type="button" value="=" onclick="calcSum()" />
<span id="result"></span>