How to get 2 side-by-side div to automatically maintain the same level of content

Recently encountered a demand in the work:

There are 2 sides of the div,2 a div can not limit height. Left Div for navigation, right div for content. How can I make the left Div block automatically get the same height as the right div block? Just into the company a good period of tangled up at that time technology good garbage passing (now also very rubbish)

Please look at the code

function $ (ID) {     return document.getElementById (ID)} function getheight () {     if ($ ("left"). offsetheight>=$ ( "Right"). offsetheight) {        $ (' right '). style.height=$ ("left"). offsetheight + "px";    }    else{        $ ("left"). style.height=$ ("right"). offsetheight + "px";    }    } Window.onload = function () {    getheight ();}

How to get 2 side-by-side div to automatically maintain the same level of content