A Knex.js developer designs, develops, and manages SQL queries and migrations for Node.js applications. They utilize the Knex.js library to build database schemas, handle database connections, seed databases with test data, and perform CRUD operations. They also write complex queries using Knex’s query builder, ensuring efficient data retrieval and manipulation. Additionally, they optimize database performance, identify and fix bugs, and ensure application security. They often work closely with other developers to integrate databases with user-facing elements. Moreover, they contribute to the overall software development life cycle, from conception to deployment, maintaining code quality and documentation.