Eclipse Theia is a modular framework for building cloud and desktop IDEs. It is implemented in TypeScript and leverages state of the art technology as the Monaco editor and the language server protocol (LSP) which also powering VS Code. Theia has been designed in open-source by TypeFox and Ericsson through 2017.
The LSP is a protocol between editors or IDEs and language servers providing language features like diagnostics, auto-completion, find references etc. Over last year it has got wide adoption among different tools and languages including Theia and Xtext.