Middleware

The Middleware module provides a flexible mechanism for handling requests and responses. It supports both synchronous and asynchronous middleware.

Middleware is a core concept in Akala that allows you to define reusable functions that can be applied to requests or other processes. Middleware functions can perform a variety of tasks such as logging, authentication, validation, and more.

Defining Middleware

Middlewares are generic and will accept any argument that matches their types. Middlewares can be either synchronous or asynchronous.

interface Middleware<T extends unknown[], TSpecialNextParam extends string | void = SpecialNextParam> {
    handle(...context: T): MiddlewareResult<TSpecialNextParam>;
}

interface MiddlewareAsync<T extends unknown[], TSpecialNextParam extends string | void = SpecialNextParam> {
    handle(...context: T): Promise<MiddlewareResult<TSpecialNextParam>>;
}

There is also an ErrorMiddleware interface which mimics the Middleware concept with an additional Error as the first argument.

MiddlewareResult

The MiddlewareResult is a special type used in Akala to handle the result of middleware execution. It is important to understand how to use MiddlewareResult correctly, as it can be counterintuitive for new users.

A middleware function should return a MiddlewareResult to indicate the outcome of its execution. The MiddlewareResult can be one of the following:

  • Error: The error caused during the execution of the middleware. The error will be given to any next error middleware (in the case of a composite middleware), and will then bubble up to all the above middlewares until it reaches the end.
  • undefined: Use this to indicate the middleware could not process the task and continue the middleware chain.
  • 'break': Terminates the current composite middleware. In case of running in nested composite, only the current composite is stopped, letting siblings and parents handle the execution.

Conclusion

Middleware is a powerful feature in Akala that allows you to create reusable functions to handle various tasks in your application. Understanding how to define and use middleware, as well as how to work with MiddlewareResult, is essential for building robust and maintainable applications with Akala.

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