How can I dynamically increase font size with Javascript and why is my current function not working?

Question

I'm trying to increase the font size of my page using a javascript function but it's not working. Is there a syntax problem with my code or is it not possible to do what I'm trying to do?

Javascript:

function changeFontSize(fontvar) {
var div = document.getElementById("webchat_history");
var currentFont = div.style.fontSize.value;

div.style.fontSize = currentFont + fontvar+ "px";
}

HTML:
<span onClick="changeFontSize(10);" style="font-size:16px;">Aa</span>

I want to increase it by x amount (fontvar) rather than specifying a specific font size because my font sizes are set in an external stylesheet. When/if I need to modify the stylesheets, I'd rather not have to update the Javascript too.

Answer 1

This script should work:

function changeFontSize(fontvar) {
    var div = document.getElementById("webchat_history");
    var currentFont = div.style.fontSize.replace("px", "");

    div.style.fontSize = parseInt(currentFont) + parseInt(fontvar) + "px";
}

Answer 2

I found this amazing script: https://github.com/simplefocus/FlowType.JS

Demo: http://simplefocus.com/flowtype/demo.html

Answer 3

After additional debugging, I realized the "id" I was using, webchat_history , was not actually an ID. It was the class for ID history . I'm going to create additional classes and use document.getElementById('history').className = "webchat_history_medium"; to change font sizes.