TypeScript aims to be a superset of JavaScript that transpiles to JavaScript. By generating ECMAScript compliant code, TypeScript can introduce new language features while retaining compatibility with existing JavaScript engines. ES3, ES5 and ES6 are currently supported targets.
Optional types are a primary feature. Types allow static checking with the goal of finding errors early and can enhance tooling with features like code refactoring.
TypeScript is an open source and cross platform programming language developed by Microsoft. The source code is available on GitHub.