In any development team, there will come a time when you will need to either bug-fix, or enhance a body of code that has already been written in JavaScript. If you are in this situation, then you would want to try and write new areas of code in TypeScript, and integrate them with your existing body of JavaScript. To do so, however, you will need to write your own declaration files for any existing JavaScript that you need to reuse. This may seem like a daunting and time-consuming task, but when you are faced with this situation, just remember to take small steps, and define small sections of code at a time. You will be surprised at how simple it really is.

In this section, let's assume that you need to integrate an existing helper class—one that is reused across many projects, is well tested, and is a development team standard. This...