A Mocha.js developer is an asset due to their expertise in a highly versatile testing framework. Mocha.js supports both Behavior Driven Development (BDD) and Test Driven Development (TDD), allowing for flexible and accurate testing. They can create asynchronous tests, ensuring your application performs optimally under various conditions. Their skills in Mocha.js can increase the speed, efficiency, and reliability of your development process, allowing for robust applications. Moreover, they can help foster a culture of test-driven development, enhancing the overall quality of your software.