A chai.js developer can be a valuable addition to your team due to their expertise in behavior-driven and test-driven development. They can help ensure your JavaScript applications are robust and bug-free by writing comprehensive tests. Their knowledge of chai.js, a popular BDD/TDD assertion library, allows them to create readable tests, making the codebase easier to understand and maintain. Furthermore, these developers can help improve the overall quality of your software, minimize the cost of fixing bugs, and speed up the development process by quickly identifying and resolving issues.