Express HTTP Context

Modern request-scoped storage support for Express.js, based on native Node.js Asynchronous Local Storage. It's a great place to store user state, claims from a JWT, request/correlation IDs, and any other request-scoped data.

This package is a drop-in replacement and has been inspired by the work done in express-http-context, but with ESM support, no dependencies, and only supporting modern versions of Node.js (+v14).

The implementation is derived from the start made in express-http-context2. We diverged to support our own implementation because the bug highlighted in artberri/express-http-context2#1 was ultimately ignored and the repository apparently abandoned by its maintainer.

Installation

npm install @dalet-oss/express-http-context
# or
yarn add @dalet-oss/express-http-context
# or
pnpm add @dalet-oss/express-http-context

Requirements

@dalet-oss/express-http-context is a middleware intended for express, so although it is not explicitly declared as a dependency or peer dependency, it requires express to work, as well as @types/express if you are using Typescript. The reason the dependency is not explicitly declared is that it could also eventually be used with fastify or other Node.js HTTP servers.

Configuration

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.

const express = require('express')
const httpContext = require('@dalet-oss/express-http-context')

const app = express()
// Use any third party middleware that does not need to access the context here, e.g.
// app.use(some3rdParty.middleware);
app.use(httpContext.middleware)
// All code from this point on will have access to the per-request context

Note that some popular middlewares (such as body-parser, express-jwt) can cause the context to be lost. To work around such problems, it is recommended that you use any third-party middleware that does NOT require the context BEFORE using this middleware.

Usage

Examples of setting values:

const httpContext = require('@dalet-oss/express-http-context')
const { nanoid } = require('nanoid') // This is just an example, nanoid is not included in this lib

app.use((req, res, next) => {
	// Get the user ID from wherever and save it for later use...
	httpContext.set('userId', userId)
	// Create a request ID to be able to trace/correlate everything that happens within the same request
	httpContext.set('requestId', nanoid())
})

Get them from anywhere in your code:

var httpContext = require('@dalet-oss/express-http-context')

function logError(error) {
	const userId = httpContext.get('userId')
	const requestId = httpContext.get('requestId')
	console.error(error, { userId, requestId })
}

API

middleware

It is an Express.js middleware that is responsible for initializing the independent context for each request. The get and set calls will operate on a set of keys/values unique to those contexts.

Example

import { middleware } from '@dalet-oss/express-http-context'

app.use(middleware)

set

Adds a value to the request context by key. If the key already exists, its value will be overwritten. No value will persist if the context has not yet been initialized.

Parameters

• key a string key to store the variable by
• value any value to store under the key for the later lookup.

Example

import { set } from '@dalet-oss/express-http-context'

set('user', { id: 'overwrittenUser', email: '[email protected]' })

get

Gets a value from the request context by key. Will return undefined if the context has not yet been initialized for this request or if a value is not found for the specified key.

Parameters

• key a string key to retrieve the stored value for

Example

import { get } from '@dalet-oss/express-http-context'

const user = get('user')

License

MIT License. See LICENSE.

Contributors

• Oliver Lockwood (@oliverlockwood)
• Alberto Varela Sánchez (@artberri)

Interesting in contributing? Take a look at the Contributing Guidlines