How to change FontSize By JavaScript?

<span id="span">HOI</span>
<script>
   var span = document.getElementById("span");
   console.log(span);

   span.style.fontSize = "25px";
   span.innerHTML = "String";
</script>

You have two errors in your code:

1. document.getElementById - This retrieves the element with an Id that is "span", you did not specify an id on the span-element.

2. Capitals in Javascript - Also you forgot the capital of Size.

Please never do this in real projects😅:

document.getElementById("span").innerHTML = "String".fontsize(25);

<span id="span"></span>

try this:

var span = document.getElementById("span");
span.style.fontSize = "25px";
span.innerHTML = "String";

JavaScript is case sensitive.

So, if you want to change the font size, you have to go:

span.style.fontSize = "25px";