A Jasmine.js developer designs and implements testing frameworks for JavaScript applications. They use Jasmine.js, a behavior-driven development framework, to create robust, well-structured unit tests for JavaScript code. Their tasks include writing test cases, debugging code, maintaining test suites, and ensuring the functionality, reliability, and performance of software applications. They work closely with other developers to integrate testing solutions into the development process. Moreover, they continuously monitor test results to identify and resolve issues, contributing to the overall quality of the software. They also adapt to changes in project requirements and update tests accordingly.