A Mongoose.js developer designs, implements, and manages web applications using Mongoose.js, a MongoDB object modeling tool. Their primary tasks include creating database schemas to represent business processes, integrating MongoDB with Node.js, and troubleshooting application errors. They utilize Mongoose.js for easier database access and manipulation, ensuring data consistency and integrity. They work closely with other developers to optimize application performance, maintain code quality, and enhance user experience. Additionally, they are responsible for testing their code, fixing bugs, and updating systems as necessary. They also often collaborate with other teams to understand requirements and provide technical solutions.