TypeScript is a superset of JavaScript that adds optional type annotations, improving development experience and tooling. It can be added to an existing JavaScript project by installing the TypeScript compiler and modifying the `tsconfig.json` file. The compiler can also be used to create a new project from scratch. To get started with TypeScript, install it locally in your project using `npm install typescript --save`, and set up your IDE to use Visual Studio Code or similar tools. Create a `tsconfig.json` file that defines the compiler options, such as target version and module system. Use definition files to get type information for existing libraries, and install them using `npm install @types/express @types/node @types/body-parser --save`. Write your first TypeScript file by creating a new class with public methods and annotating their parameters with types. You can also use generics to support polymorphism. Finally, you can compile your TypeScript code to ES5 JavaScript using the `outDir` option in the `tsconfig.json` file, allowing you to run your project on older versions of Node.js.