A JasmineJS developer writes and maintains unit tests in JavaScript using the Jasmine framework. They design and implement test cases for JavaScript code to ensure its functionality and stability. They work closely with other developers to understand the codebase and create appropriate tests, ensuring codes meet quality standards. By using Jasmine's features like spies, stubs, and mocks, they simulate the behavior of functions or objects. They also debug and fix issues identified during testing. Their work is crucial in Agile environments, supporting continuous integration and delivery by automating testing tasks and ensuring code reliability before deployment.