express-async-context

Zero-dependency context-provision for express-application based on the AsyncLocalStorage. [](https://github.com/DScheglov/ express-async-context/actions/workflows/run-tests.yml)

• Installation
• Usage
• Motivation
• API Reference

Installation

npm install express-async-context

Usage

Live Demo on Sandbox

import express from 'express';
import createContext from 'express-async-context';

const Context = createContext(req => ({
  traceId: req.headers['x-request-id'] ?? Math.random().toFixed(20).slice(2),
}));

const app = express();

app.use(Context.provider);

app.get('/trace-id', Context.consumer(
  (req, res) => ({ traceId }) => res.json({ traceId }),
));

app.listen(8080, () => {
  console.log('Server is listening on port: 8080');
  console.log('Follow: http://localhost:8080/trace-id');
});

curl -H "X-Request-Id: 58895124899023443277" http://localhost:8080/trace-id

Motivation

The express-async-context library is designed to aproach context provision to the chain of request handling in the express-application without mutation of the request or/and response.

Under the hood library uses AsyncLocalStorage and is based on the thunk-idiom that means calculation postponed until it will be provided with the context.

The main benifit of context we can get when we use IoC-container as a context. To make such injection safe the static type-safe containers required, as instance: true-di.

See Live DI Demo on Sandbox

API Reference

• function createContext
• type ContextFactory<T>
• interface ContextManager<T>
• type HandlerThunk<T>
• type ErrorHandlerThunk<T>
• type Thunk<T, R = void>
• type RunFn<T>

function createContext

<T>(contextFactory: ContextFactory<T>): ContextManager<T>;

Accepts contextFactory function and creates a ContextManager.

type ContextFactory<T>

<T>(req: express.Request, res: express.Response) => T;

The type describes function that accepts express.Request, express.Response and returns context data of any type T.

interface ContextManager<T>

interface ContextManager<T> {
  provider: (req: express.Request, res: express.Response, next: express.NextFunction) => void;
  consumer: {
    (handler: express.RequestHandler | HandlerThunk<T>): express.RequestHandler;
    (handler: express.ErrorRequestHandler | ErrorHandlerThunk<T>): express.ErrorRequestHandler;
  }

The interface contains two members:

• provider - is an usual express middleware that creates context data for each request using contextFactory and "binds" this data to the request

• consumer - is a decorator for HandlerThunk<T> and ErrorHandlerThunk that converts them to usual express.RequestHandler and express.ErrorRequestHandler.

type HandlerThunk<T>

(req: express.Request, res: express.Response, next: express.NextFunction) =>
  (context: T, run: RunFn<T>) => void;

The curried request handler that requires two-times application.

HandlerThunk could be considered as an express.RequestHandler that returns a postponed handling of the request -- the Thunk

type ErrorHandlerThunk<T>

(err: any, req: express.Request, res: express.Response, next: express.NextFunction) => 
  (context: T, run: RunFn<T>) => void;

The curried handler of error trhown during the request processing.

ErrorHandlerThunk could be considered as an express.ErrorRequestHandler that returns a postponed handling of the error -- the Thunk

type Thunk<T, R = void>

(context: T, run: RunFn<T>) => R

The postponed calculation, including handler of the request or an error. The correspondent function receives context data and the run-function, that runs any other Thunk.

type RunFn<T>

<R>(fn: Thunk<T, R>) => R

Runs and injects the context data and itself to the postponed calculation that accepts as a single argument.

RunFn returns the result of execution of its argument-function.