This chapter covered some of the most useful advanced features of the Dart language. The Dart language was designed with the modules in mind. Modularity in Dart is realized through packages, libraries, and classes. The code compiled in JavaScript loses all the library information.

Functions are first-class citizens because they support all the operations generally available to other types. A function that can be referenced with an access to the variables in its lexical scope is called a closure.

Programs are designed to use objects as instances of classes that interact with each other to organize functionality. The Dart language is dynamically typed, so we can write programs with or without the type annotations in our code.

In the next chapter, we will talk about generics, errors and exceptions, and annotations and reflection.