A Mocha/Chai/Sinon developer builds and maintains JavaScript applications, focusing on testing. They use Mocha, a JavaScript testing framework that runs on Node.js, for asynchronous testing. Chai, a BDD/TDD assertion library, is used for creating readable tests and can be paired with any JavaScript testing framework. Sinon, a standalone test spies, stubs and mocks library, is used for controlling JavaScript functions' behaviors. These tools allow the developer to create robust, reliable applications by ensuring all aspects function as expected through unit tests, integration tests, and end-to-end tests. They also troubleshoot and fix any issues that arise during testing.