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You could do:

css:

.div {
  width: 300px;
  height: 200px;
  position: absolute;
  top: 50px;
}

.modified {
  top: 100px;
}

javascript:

const div = document.getElementsByTagName('DIV')[0];
div.className = 'div';

const handleClick = () => {
    const className = div.className;
    div.className = className === 'div modified' ? 'div' : 'div modified';
};

div.onclick = handleClick;

Here I made a working example so you can experiment with it: jsfiddle.net/camkida/mffkmwzb