In my case I was using ClassName.
getComputedStyle( document.getElementsByClassName(this_id)) //error
It will also work without 2nd argument " ".
Here is my complete running code :
function changeFontSize(target) {
var minmax = document.getElementById("minmax");
var computedStyle = window.getComputedStyle
? getComputedStyle(minmax) // Standards
: minmax.currentStyle; // Old IE
var fontSize;
if (computedStyle) { // This will be true on nearly all browsers
fontSize = parseFloat(computedStyle && computedStyle.fontSize);
if (target == "sizePlus") {
if(fontSize<20){
fontSize += 5;
}
} else if (target == "sizeMinus") {
if(fontSize>15){
fontSize -= 5;
}
}
minmax.style.fontSize = fontSize + "px";
}
}
onclick= "changeFontSize(this.id)"