How to calculate content height by using pure javascript

Question

I have used this code for defining "mainContent div height" but it alerts undefined!

window.onload = function () {
    alert(mainContent.height);
}


function viewPort() {
    var pageWrapper = document.getElementById('pageWrapper').offsetHeight;
    pageFooter = document.getElementById('pageFooter').offsetHeight;
    pageHeader = document.getElementById('pageHeader').offsetHeight;
    differenceInHeight = pageWrapper - (pageHeader + pageFooter);
    mainContent = document.getElementById('mainContent').style.height = differenceInHeight + 'px';
};