API Design with Express: The Controller Pattern

In a MERN app, Express acts as the glue. To keep it maintainable, we separate logic into Routes, Controllers, and Services.

The MVC Pattern (Simplified)

• Routes: Define the endpoints (/api/users).
• Controllers: Handle the logic (getting data from the body, calling the service/model).
• Models: The Mongoose definitions.

javascript code
// controllers/userController.js
exports.getAllUsers = catchAsync(async (req, res, next) => {
    const users = await User.find();
    res.status(200).json({
        status: 'success',
        results: users.length,
        data: { users }
    });
});

Global Error Handling

Never use try/catch in every controller. Use a wrapper and a global error handler for clean, professional code.

javascript code
// utils/catchAsync.js
module.exports = fn => {
    return (req, res, next) => {
        fn(req, res, next).catch(next);
    };
};

Input Validation

The industry standard for schema validation in Express is Joi or Zod.

javascript code
const schema = z.object({
  email: z.string().email(),
  password: z.string().min(8),
});

// Validate inside a middleware
const validate = (req, res, next) => {
  schema.parse(req.body);
  next();
};

Statelessness

Your backend should not store sessions in memory. Use JWT (JSON Web Tokens) so that any instance of your server can handle any request.