what are modules in 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.