How to create writable global variable in node.js typescript

Question

I have declared a global variable

import * as io from "socket.io";

declare let SocketServer: io.Server

and tried to write in another file but this variable is read-only.

So how to make it writable?

Update

// global.d.ts 
import * as io from "socket.io";

declare global {
  let server_socket: io.Server
  let user_socket: io.Socket
}

// server.ts
import * as io from "socket.io";

console.log(server_socket)

"and tried to write in another file"... can you elaborate? declare only tells TypeScript that the variable exists somewhere, it's not actually creating the variable for you, which is why you get the crash below. If you want to instantiate SocketServer, you'd still need to do new io.SocketServer(...). Or assign io.SocketServer to your SocketServer — pushkin
– pushkin, Commented Dec 23, 2018 at 17:28

Karol Majewski · Accepted Answer · 2018-12-23 18:41:01Z

2

In TypeScript, declare blocks are used to describe your global variables. In other words, they are a way of telling TypeScript what it can expect in the global namespace — but it's your responsibility as a developer to make sure they are actually there.

The way you can create (and describe) global variables depends on the platform.

The browser (DOM)

import * as io from 'socket.io';

window.SocketServer = io.default();

declare global {
  interface Window {
    SocketServer: io.Server;
  }
}

Requires @types/socket.io. Is consumed by accessing window.SocketServer.

Node.js

import * as io from 'socket.io';

declare global {
  namespace NodeJS {
    interface Global {
      SocketServer: io.Server
    }
  }
}

global.SocketServer = io.default();

Requires @types/socket.io and @types/node. Is consumed by accessing global.SocketServer.

Universal

You can also describe a variable that is already a part of your environment and is accessible to both the client and the server.

An example of such a variable would be process — it's a part of Node.js environment, but build tools like Webpack can expose its contents to the client.

import * as io from 'socket.io';

declare global {
  var SocketServer: io.Server;
}

edited Dec 23, 2018 at 18:41

answered Dec 22, 2018 at 19:34

Karol Majewski

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3 Comments

Karol Majewski Over a year ago

You still need to import import * as io from "socket.io" in order to use it.

Karol Majewski Over a year ago

I thought you had already created your global variable and were looking only for a way to tell TypeScript about it. I updated my answer to cover how both can be done.

WasiF Over a year ago

Actually I am developing a MEAN app. Angular7 & REST Api depends on same configuration files created by Angular. Getting error like Property 'SocketServer' does not exist on type 'Global'.

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