Что такое модули в TypeScript

Starting with ECMAScript 2015, JavaScript has a concept of modules. 
TypeScript shares this concept.

Modules are executed within their own scope,
not in the global scope; this means that variables, functions, classes, etc. 
declared in a module are not visible outside the module unless
they are explicitly exported using one of the export forms. 
Conversely, to consume a variable, function, class, interface, etc. exported from a
different module, it has to be imported using one of the import forms.
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