Back in the day when JavaScript ES5 was the active version, writing JavaScript code that adhered to encapsulation and modularity was not a trivial task. Things such as prototypes, namespaces, and self-executing functions could certainly improve your code; unfortunately, many JavaScript developers did not invest the necessary effort to improve the manageability of their code using those constructs.
Consequently, new technologies surfaced to help with this matter; a popular example relevant at that time was CoffeeScript. Then, JavaScript ES6 was released. ES6, with its great added features, such as modules, classes, and more specific variable scoping, basically overturned CoffeeScript and similar alternatives at that time.
Evidently, JavaScript is still missing a key feature, and that is type information. JavaScript's dynamic type system is a strong feature at times; unfortunately, the manageability and reusability of existing code suffers due to this fact. For that key purpose, every large-scale project should examine the use of tools that compensate the situation; those leading the field now are TypeScript and Flow.
TypeScript, an open source scripting language, is developed and maintained by Microsoft. Its killer feature is bringing static typing to JavaScript-based systems and allowing you to annotate the code with type information. Additionally, it supports augmenting existing JavaScript code with external type information, similar to the header files pattern, as it's commonly known in C++.
TypeScript is a superset of JavaScript, that meaning it has seamless integration with JavaScript, and makes JavaScript developers feel right at home. Being a JavaScript developer makes you a TypeScript developer as well since JavaScript code is actually valid TypeScript. Importantly, this makes the gradual upgrade of existing JavaScript code fairly easy.
Another example of how great TypeScript is is the fact that Angular, a framework built by Google, adopts it extensively, and is actually implemented in TypeScript internally.