It's the same question than: How do I extend the typings of Express.Application to give typings for app.locals but the suggested solution doesn't work for me.

I have server.ts :

const express = require('express');



type Message = {
  author: string,
  message: string
}

declare global {
  namespace Express {
    interface Locals {
      chat: Array
    }
  }
}

/** Setup Express server */
const app = express();
app.use(express.json());


let sampleMessage: Message = {
  author: 11
}

app.locals.resetChat = () => {
  app.locals.chat = [{author: 11, message: 'Welcome to the chat!'}];
};
app.locals.resetChat();

export {};

I purposefully wrote 2 mistakes, in order to see if type checking works. When I run tsc server.ts, it only catches the error at let sampleMessage = ... but not the affectation to app.locals.chat = .... Here's the output:

server.ts:24:3 - error TS2322: Type 'number' is not assignable to type 'string'.

24   author: 11
     ~~~~~~

  server.ts:6:3
    6   author: string,
        ~~~~~~
    The expected type comes from property 'author' which is declared here on type 'Message'


Found 1 error in server.ts:24

What is wrong. I don't understand how the declare global and namespace Express work…