how to change font size on button click in javascript

1<script type="text/javascript">
2  var fontSizes = [
3    "xx-small",
4    "x-small",
5    "small",
6    "medium",
7    "large",
8    "x-large",
9    "xx-large"
10  ];
11  function getFontSize(id) {
12    return document.getElementById(id).style.fontSize || "medium";
13  }
14  function changeFontSize(id, size = "medium") {
15    document.getElementById(id).style.fontSize = size;
16  }
17  function cycleSize(id) {
18    let i = fontSizes.indexOf(getFontSize(id)) + 1;
19    changeFontSize(id, fontSizes[i >= fontSizes.length ? 0 : i]);
20  }
21</script>
22<p id="p">Click the button below to change my size!</p>
23<button onclick="cycleSize('p')">Change size</button>