How to Change a Custom HTML Attribute by Screen Width using Javascript [duplicate]

I’m trying to change the data-smooth-scroll-offset attribute value for the following div if the screen width is larger than 959px:

This div:

<div id="Table-Of-Contents" data-smooth-scroll="1" data-smooth-scroll-offset="20"></div>

The JS:

function adjustWidth(){

    var actual_width = window.innerWidth;

    if(actual_width > 959) {
        document.getElementById("Table-Of-Contents").data-smooth-scroll-offset="111";
    }
}

It appears that data-smooth-scroll-offset is a custom html attribute. Any ideas how to change its value?