Extend Express Request object using Typescript

You want to create a custom definition, and use a feature in Typescript called Declaration Merging. This is commonly used, e.g. in method-override.

Create a file custom.d.ts and make sure to include it in your tsconfig.json's files-section if any. The contents can look as follows:

declare namespace Express {   export interface Request {      tenant?: string   }}

This will allow you to, at any point in your code, use something like this:

router.use((req, res, next) => {    req.tenant = 'tenant-X'    next()})router.get('/whichTenant', (req, res) => {    res.status(200).send('This is your tenant: '+req.tenant)})

As suggested by the comments in the index.d.ts, you simply declare to the global Express namespace any new members. Example:

declare global {  namespace Express {    interface Request {      context: Context    }  }}

Full Example:

import * as express from 'express';export class Context {  constructor(public someContextVariable) {  }  log(message: string) {    console.log(this.someContextVariable, { message });  }}declare global {  namespace Express {    interface Request {      context: Context    }  }}const app = express();app.use((req, res, next) => {  req.context = new Context(req.url);  next();});app.use((req, res, next) => {  req.context.log('about to return')  res.send('hello world world');});app.listen(3000, () => console.log('Example app listening on port 3000!'))

Extending global namespaces is covered more at my GitBook.

For newer versions of express, you need to augment the express-serve-static-core module.

This is needed because now the Express object comes from there: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/8fb0e959c2c7529b5fa4793a44b41b797ae671b9/types/express/index.d.ts#L19

Basically, use the following:

declare module 'express-serve-static-core' {  interface Request {    myField?: string  }  interface Response {    myField?: string  }}