JavaScript "Are you Sure?" dialog box script?

I have a button I created for deleting a template-generated page using the following script: 

this.deletePages(this.pageNum, this.pageNum) ;

I would like to add a popup warning, so that when the user clicks the button they are warned first before the page is deleted. e.g., "Delete current page? This action is not reversible."

OK proceeds, Cancel cancels the action.

I feel this is very possible, but a bit outside my Java knowledge.