express-context-store

Store data related to current request without polluting request object

Install

yarn add express-context-store
# or
npm install express-context-store

Usage

const express = require('express');
const context = require('express-context-store');

const app = express();
app.use(context());

app.use((req, res, next) => {
  req.context.set('key', 'value');
  next();
});

app.get('/', (req, res, next) => {
  const value = req.context.get('key');
  res.send(value);
});

express-context-store uses Map under the hood with different API for .get and .set, so you can actually use Map method to req.context:

app.get('/', (req, res, next) => {
  // you can get the size
  console.log(req.context.size);
  // or check whether key is exists
  console.log(req.context.has('key'));
  // or get iterable for all entries
  console.log(req.context.entries());
});

Keep in mind that the value stored inside the map is actually an object that contains the value that you provide in .set method. This is why we provide .toObject method to get the actual value from the map. See serialization below

Rewrite context values

By default, context values are meant to be read only. Any attempt to override its value will throw error, for example:

app.use((req, res, next) => {
  req.context.set('a', 'b');
  req.context.set('a', 'c'); // throw error
});

This default behavior is implemented to prevent surprise when reading context key. If you want to change its behavior, you need to pass third option argument in .set method called writable:

app.use((req, res, next) => {
  req.context.set('a', 'b', { writable: true });
  // you need to pass writable: true again if you want to modify it later
  req.context.set('a', 'c', { writable: true });
});

Serialization

You can convert context store to plain javascript object by calling .toObject method. This is useful where you want to log current request context using JSON logger.

app.use((req, res, next) => {
  const context = req.context.toObject();
  JSONLogger.log(context);
});

Custom property name

If for some reason you can't use req.context, you can change the default property name by passing option object when creating the middleware:

const context = require('express-context-store');

// use req.data instead of req.context
app.use(context({ property: 'data' }));

app.use((req, res, next) => {
  req.data.set('key', 'value');
  next();
});

app.get('/', (req, res, next) => {
  const value = req.data.get('key');
  res.send(value);
});

License

MIT