Express AsyncLocalStorage Context

Get and set request-scoped context anywhere. It's using a now-stable AsyncLocalStorage. It's heavily inspired by express-http-context, but doesn't suffer from the same memory leak issue that it brought (which was due to cls-hooked issue)

Support

AsyncLocalStorage supports Node.js from 12.17.0 up.

How to use it

Install: npm install --save express-als-context

Use the middleware immediately before the first middleware that needs to have access to the context. You won't have access to the context in any middleware .used before this one.

var express = require('express');
var httpContext = require('express-als-context');

var app = express();
// Use any third party middleware that does not need access to the context here, e.g. 
// app.use(some3rdParty.middleware);
app.use(httpContext.middleware);
// all code from here on has access to the same context for each request

Set values based on the incoming request:

// Example authorization middleware
app.use((req, res, next) => {
  userService.getUser(req.get('Authorization'), (err, result) => {
    if (err) {
      next(err);
    } else {
      httpContext.set('user', result.user)
      next();
    }
  });
});

You can read the values in the code that doesn't have access to the express's req object:

var httpContext = require('express-als-context');

// Somewhere deep in the Todo Service
function createTodoItem(title, content, callback) {
  var user = httpContext.get('user');
  db.insert({ title, content, userId: user.id }, callback);
}