Always declare your variables in JavaScript!

One mistake many people who are new to JavaScript often make is forgetting to declare their variables before using them. It is a mistake I often see when browsing through questions on StackOverflow. Instead of continuing to write very similar comments pointing out the problems with not declaring variables and how to avoid making this mistake, I decided to write this post so I can link to it in a more terse comment and give a more in depth explanation of the problem than I could in a SO comment.