On many occasions we might need to access some HTML DOM element inside a component from page using JavaScript. Example scenarios can be page fetching value of text field inside a component using JavaScript.

Though best is using native visualforce tags and server side communication as described here : http://wiki.developerforce.com/index.php/Controller_Component_Communication. As going client side requires careful usage of DOM element ids. So any minor change to html structure can easily break your code.

Read More: https://abhinav.fyi/visualforce-page-to-component-javascript-client-side-communication/