How do i add content in an div with javascript?

How do i change the content of an div on an website that has an id with javascript?

<div class="chat-area" id="chat-area">[Changeable]</div>

I want to know how i add content where it says "[Changeable]" with javascript so i can run the command through the console. I'd like if you keep your explainage simple, because im still very new to html/css/javascript ;)

via Chebli Mohamed