Shallot

A middleware framework for serverless functions written in TypeScript or JavaScript.

Inspired by Express.js and Middy.js

Benefits over middy

• First class support for TypeScript
• Simpler codebase
• Mildly faster execution

Drawbacks from middy

• Only supports promise based handlers/middleware with async/await

  **This is an intentional decision to allow for the simplified codebase and faster execution

• Less robust third party middleware options

Installation

Add shallot to your project

npm install --save @shallot/aws

TypeScript support is out of the box so you do not need an additional types module.

Usage

REST API Wrapper Example:

import ShallotAWS from '@shallot/aws';

import { HTTPJSONBodyParser } from '@shallot/http-json-body-parser';
import { HTTPCors } from '@shallot/http-cors';
import { HTTPErrorHandler } from '@shallot/http-error-handler';

const _handler = async (event, context) => {
  // Your handler code here
};

export const handler = ShallotAWS(handler)
  .use(HTTPJSONBodyParser())
  .use(HTTPCors())
  .use(HTTPErrorHandler());

How it Works

**Artwork credit: Middy.js

Middlewares are applied as layers that can execute before and after. This loosely can be analogized to an onion (and Serverless starts with an S) hence the name Shallot.

The middlewares can modify the request context, response, or handle exceptions thrown during runtime to take care of common tasks. Canonical use cases include establishing database connections, parsing request bodies from json, and authenticating users.

Official Middlewares

http-json-body-parser

Shallot middleware that parses and replaces the JSON body of HTTP request bodies. Requires the Content-Type header to be properly set.

import { HTTPJSONBodyParser } from '@shallot/http-json-body-parser';

http-cors

Shallot middleware that handles the setting of response CORS headers according to the MSDN spec. https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS

import { HTTPCors } from '@shallot/http-cors';

http-error-handler

Shallot middleware that catches "HTTP Errors" thrown by the http-errors npm module and returns the corresponding status code in the response.

import { HTTPErrorHandler } from '@shallot/http-error-handler';

Creating a Custom Middleware

A middleware is an object that defines any of a before, after, onError, or finally block.

• before: executes before the handler
• after: executes after the handler
• onError: executes in case of exception
• finally: executes after either the after or onError stage

**If an error is thrown at any point, execution of all other middlewares/the handler is terminated and the onError middlewares + finally middlewares execute instead.

const middleware = {
  before: async (request) => {
    console.log(request);
  },
};

ShallotAWS(handler).use(middleware);

You may also want to pass config options to the middleware which you can do by exporting your middleware as a function instead of an object.

const middlewareWithOptions = (config) => ({
  before: async (request) => {
    if (config.shouldLog) {
      console.log(request);
    }
  },
});

const config = {
  shouldLog: true,
};

ShallotAWS(handler).use(middlewareWithOptions(config));

The request object

The request object passed to each middleware at runtime has the following properties.

{
  event, // The lambda event object
  context, // The lambda runtime context object
  response, // The user-defined response
  error, // Error object set before calling onError middlewares
  __handledError, // If true, allows skipping of remaining onError middlewares
}