Understanding TypeScript Declaration Files

The declaration files use the declare and module keywords to define objects and namespaces. We can use interfaces in the same way that we do within TypeScript in order to define custom types for variables. Declaration files allow us to use the same syntax that we would in TypeScript to describe types. These types can be used everywhere types are used in normal TypeScript, including function overloading, type unions, classes, and optional properties.

Let’s take a quick look at these techniques with a few simple code samples to illustrate this feature further.

Function overloading

Declaration files allow for function overloads, where the same function can be declared with different arguments, as follows: