TypeScript is a superset of JavaScript that adds static typing and classes to address the problem of writing large applications in JavaScript. It allows for intelligent IDE features like autocompletion and refactoring by adding types that are removed during compilation, ultimately producing plain JavaScript code. TypeScript aims to make large scale JavaScript development easier with types and tooling while still targeting the JavaScript that browsers understand.

1. TypeScript

3.  TypeScript is about addressing this problem
 A problem that we hear increasingly more and more from
customers and internal users that writing really large apps in
JavaScript is hard.
 And JavaScript was really never intended for writing large apps
 JavaScript has no concept for structuring large code bases. No
classes, not yet at least, no modules, no interfaces, and there's no
static typing at all.
 It's a purely dynamically typed language.

4.  The intelligent IDEs that we as programmers rely on every day , at
the core of most of it is static typing, or the ability to reason about
the code in intelligent ways
 so the IDE can deliver invaluable things like statement completion,
go to definition, find all references, refactorings that are known to
be safe.
 That just gets a super, super hard in big dynamic code bases.

6.  And that really, in a nutshell, is whatTypeScript is.
 It's a language for large scale JavaScript app development.

8.  In a more technical sense, it is a statically typed super set of
JavaScript that compiles to plain JavaScript.
 SoTypeScript itself is written inTypeScript and therefore compiles
to plain JavaScript.

12.  Basically, it starts with JavaScript.
 So it's important to understand thatTypeScript is not a new
language or different language.
 It is at the core of it JavaScript.
 But it's JavaScript with type annotations and the ability to declare
types.

14.  So what it adds is types, including generics and all sorts of other
fun stuff.

16.  And then, powered by that is excellent tooling, tooling that does
things that you just couldn't do with purely a dynamic
programming language.

18.  But when you compile, it all goes away, and you're just left with
JavaScript.
 In fact, you're left with what I would argue is idiomatic JavaScript
that looks like a human wrote it.
 The code that you get is the code that you wrote, minus the types.

19. reference
 https://www.youtube.com/watch?v=Ut694dsIa8w
 https://www.youtube.com/watch?v=K68NVae-PBs