Set "!important" style by JavaScript

It is very easy to set style in any DOM element by Javascript like:

var target = document.getElementById('MyDiv');
target .style.marginleft ="100px";

But if you want to add "!important" like this -

target .style.marginleft ="100px !important";
this will not work.

Use this code to get it done.
target.setAttribute('style', 'margin-left:200px !important');