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!'))

More

Extending global namespaces is also covered in the TypeScript Deep Dive.