UserlandAsyncContext

Userland implementation of https://github.com/nodejs/node/pull/26540

Usage

$ npm install userland-async-context

Then, in your code:

require('userland-async-context');
const { AsyncContext } = require('async_hooks');

Documentation

Class: AsyncContext

This class is used to create asynchronous state between callbacks and promise chains. It allows you to keep variables throughout the lifetime of a web request or any other asynchronous duration. It is similar to thread local storage in other languages.

The following example builds a logger that will always know the current HTTP request and use it to display enhanced logs without needing to explicitly pass the current HTTP request to it.

const { AsyncContext } = require('async_hooks');
const http = require('http');

const kReq = 'CURRENT_REQUEST';
const asyncContext = new AsyncContext();

function log(...args) {
  const store = asyncContext.getStore();
  // Make sure the store exists and it contains a request.
  if (store && store.has(kReq)) {
    const req = store.get(kReq);
    // Prints `GET /items ERR could not do something
    console.log(req.method, req.url, ...args);
  } else {
    console.log(...args);
  }
}

http.createServer((request, response) => {
  asyncContext.run((store) => {
    store.set(kReq, request);
    someAsyncOperation((err, result) => {
      if (err) {
        log('ERR', err.message);
      }
    });
  });
})
.listen(8080);

new AsyncContext()

• Returns: {Object}

Creates a new instance of AsyncContext. Storage is only provided inside a run method call.

asyncContext.run(callback)

• callback {Function}
  • store {Map}

Calling asyncContext.enter(callback) will create a new asynchronous context. This method will call the callback with a Map known as the store as argument.

This store will be persistent through the following asynchronous calls.

asyncContext.exit(callback)

• callback {Function}

Calling asyncContext.exit(callback) will exit the context. Asynchronous operations following the callback cannot access the store.

asyncContext.getStore()

• Returns: {Map}

This methods returns the current store. If this method is called outside of an asynchronous context initialized by calling asyncContext.run will return undefined.