$(this).attr({class: "activeTab"}); is not working because this is undefined

This is my JS code, the problem I have is the one in the title. It seems as though $(this) is returning undefined.:

function load() {
    $(".inactiveTab").attr("onclick", "clickInactiveTab()");
}

function clickInactiveTab() {
    $(this).attr("class", "activeTab");  
}

The html is the following:

<div class="inactiveTab">Tab 1</div>
<div class="inactiveTab">Tab 2</div>