Just grabbing the style.fontSize
of an element may not work. If the font-size
is defined by a stylesheet, this will report ""
(empty string).
You should use window.getComputedStyle.
var el = document.getElementById('foo');
var style = window.getComputedStyle(el, null).getPropertyValue('font-size');
var fontSize = parseFloat(style);
// now you have a proper float for the font size (yes, it can be a float, not just an integer)
el.style.fontSize = (fontSize + 1) + 'px';