Writing middlewares using Middy for AWS Lambda

[Middy](https://github.com/middyjs/middy) is the stylish Node.js middleware engine for AWS Lambda. It allows us to focus on the strict business logic of your Lambda and attach common modular elements such as authentication, authorization, validation etc. To install middy via NPM ``` npm install --save @middy/core ``` Middy supports multiple middlewares, we can create a new folder called ``middlewares`` and define the middlewares there. Let's create ``middlewares/index.js``. In this file, it includes other middlewares. You can treat it as an entry point of all middlewares. ```js const middy = require('@middy/core') const middleware1 = require("./middleware1"); const middleware2 = require("./middleware2"); const middleware3 = require("./middleware3"); const middlewares = [ middleware1(), middleware2(), middleware3() ]; module.exports = { middlewares }; ``` For example, if you wanna update your handler header for each Lambda function, you can create a middleware like ```js const NEW_RESPONSE_HEADER = { "Access-Control-Allow-Origin": "*", "Access-Control-Allow-Credentials": true, "Access-Control-Allow-Methods": "POST,GET", "Access-Control-Allow-Headers": "*", "Strict-Transport-Security": "max-age= 63072000", "X-Content-Type-Options": "nosniff", "X-Frame-Options": "DENY", "X-XSS-Protection": "1; mode=block", "Referrer-Policy": "same-origin", }; const updateHandlerHeaders = (handler) => { if (!handler.response) { handler.response = {}; } handler.response.headers = { ...RESPONSE_SECURITY_HEADER, ...handler.response.headers, }; }; const headers = () => ({ before: async (handler) => updateHandlerHeaders(handler), after: async (handler) => updateHandlerHeaders(handler), onError: async (handler) => updateHandlerHeaders(handler), }); module.exports = headers; ``` In your ``handle.js``, import core and your middlewares ```js const middy = require('@middy/core') const { middlewares }= require("../middleware"); ``` Then define your business logic ```js const func = (event, context, callback) => { const { data } = event.body // business logic goes here return callback(null, { result: 'success', message: 'hello world'}) } const handler = middy(func).use(middlewares) ``` Middlewares have two phases - ``before`` and ``after``. Therefore, the order does matter. In the previous example, there are three middlewares, the expected order of execution is ``` middleware1 (before) middleware2 (before) middleware3 (before) handler middleware3 (after) middleware2 (after) middleware1 (after) ``` By using Middy, we can address some common concerns like setting CORS headers and keep our business logic clean. For more details, please check out [here](https://github.com/middyjs/middy).