Adding class to existing html id using purely javascript -

so have div id no class so...

<div id="leftgrapharea" style="float:left; width:100%;height:90%;border:0px solid blue;">             left graphing area         </div> 

what add classes id using javascript. know can add classes div id in html typing in can't because have code creates graphs , there no real limit how many graphs can made. basically, codes in javascript result in same html code below.

<div class="graph" draggable="true"><header>a</header></div> <div class="graph" draggable="true"><header>b</header></div> <div class="graph" draggable="true"><header>c</header></div> 

i'd appreciate if me this. in advance.

you can use classlist

document.getelementbyid('leftgrapharea').classlist.add('graph'); 

or classname

document.getelementbyid('leftgrapharea').classname += ' graph';