In this chapter, we have outlined what we need to know in order to write and use our own declaration files. We discussed JavaScript global variables in rendered HTML and how to access them in TypeScript. We then moved on to a small JavaScript helper function and wrote our own declaration file for this JavaScript. We then listed a few module definition rules, highlighting the required JavaScript syntax, and showing what the equivalent TypeScript declaration syntax would be.

In the second section of the chapter, we looked at the strict options of the TypeScript compiler, and compared the code with and without each of these options.

In the next chapter, we will look at how to use existing third-party JavaScript libraries, and how to import existing declaration files for these libraries into your TypeScript projects.