CoffeeScript is used for application development, component libraries, scripts, quick prototypes, and tests. We generally believe that CoffeeScript is more productive to read and write than JavaScript. Therefore, we use it for large code bases that change quickly.

JavaScript is used for open source projects, shared utility libraries, and infrastructure systems. We intend for these projects to become more stable, so we prefer making the code more accessible to a wider network of users and contributors. Direct JavaScript development also gives us more direct control and insight into critical systems.