Azure Function Middleware Helper

Here is a simple module to implement middleware patter for Azure Function HTTP handler, similar to what already exists for Express.js. While there are already a few modules similar to this one, creating this module was also a simple exercice I did to better understand the pattern with Azure Fucntion.

How it works

The middleware pattern execute functions sequencialy to check or enhance a request parameter before executing core logic in the function. This purpose is to keep your Azure Functions simple and focused to your core logic. Because middleware are executed at runtime when client request is received, you will want to keep all your reusable context out of scope of middlewares (DB initialization, service discovery...). Middleware is a great use for JWT verification, data structure check or other prerequisites check.

• The Middleware class enhance the context object of the request with a next() function used to stack multiple middlewares.

• If you detect an Error, Middleware expects that you feed the client response before throwing the error. Therefore, you manage data sent back to client.

How to use it

1. Install module

in your Auzre function project, link the module

npm install @matthieugi/azurefunctionmiddleware

Then import it in your code

import { AzureFunctionMiddleware, ContextMiddleware, Middleware } from 'azurefunctionmiddleware';

2. Initiate Middleware class

const middleware = new Middleware();

3. define and stack your middlewares

First, you will need to create your middlewares. You should always call and await context.next() to trigger next element in the stack execution. Here is a simple request logger. ContextMiddleware is a child type of Azure Function "Context" type, enhanced with the next function.

const logger: AzureFunctionMiddleware = async function(context: ContextMiddleware) {
    context.log.info(context.req);
    await context.next();
}

Then stack your middleware. You can chain use function to stack all your middlewares or link to Azure Function

middleware = middleware.use(logger);
middleware = middleware.use(myOtherMiddleware).use(myAzureFunction)

4. Return default handler

Export default handler.

export default middleware.listen();

Other

Please create an issue if you have any questions or comment !