Extending TypeScript Global object in node.js

I have a node.js app that attaches some config information to the global object:

global.myConfig = {
    a: 1,
    b: 2
}

The TypeScript compiler doesn't like this because the Global type has no object named myConfig:

> TS2339: Property 'myConfig' does not exist on type 'Global'.

I don't want to do this:

global['myConfig'] = { ... }

How do I either extend the Global type to contain myConfig or just tell TypeScript to shut up and trust me? I'd prefer the first one.

I don't want to change the declarations inside node.d.ts. I saw this SO post and tried this:

declare module NodeJS  {
    interface Global {
        myConfig: any
    }
}

as a way to extend the existing Global interface, but it doesn't seem to have any effect.

> I saw this SO post and tried this:

You probably have something like vendor.d.ts:

// some import 
// AND/OR some export

declare module NodeJS  {
    interface Global {
        spotConfig: any
    }
}

Your file needs to be clean of any root level import or exports. That would turn the file into a module and disconnect it from the global type declaration namespace.

More : https://basarat.gitbooks.io/typescript/content/docs/project/modules.html

To avoid Typescript claim something like this:

>TS2339: Property 'myConfig' does not exist on type 'Global'.

I suggest to define custom types. I do it under src/types/custom.d.ts file in my project:

declare global {
  namespace NodeJS {
    interface Global {
        myConfig: {
          a: number;
          b: number;
        }
    }
  }
}

Then I ensure these are considered by Typescript in tsconfig.json file:

{
  ...
  "files": [
    ...
    "src/types/custom.d.ts"
  ]
}

Now you're safe to use your custom property:

console.log(global.myConfig.a);

As of node@16 the NodeJS.Global interface has been removed in favor of globalThis.

You can declare new global variable in a module file as:

declare global {
  var NEW_GLOBAL: string;
}

And in a non-module file (no top-level import/export) as:

declare var NEW_GLOBAL: string;

Important note: variable must be declared as var. let or const variables doesn't show up on globalThis.

Putting the following file into our project's root directory worked.

global.d.ts

declare namespace NodeJS {
  export interface Global {
    myConfig: any
  }
}

We're using "@types/node": "^7.0.18" and TypeScript Version 2.3.4. Our tsconfig.json file looks like this:

{
    "compilerOptions": {
        "module": "commonjs",
        "target": "es6"  
    },
    "exclude": [
        "node_modules"
    ]
}

Use 'namespace' instead of 'module' to declare custom TypeScript

If you use any of the above answers and are using a newer version of Typescript you'll get a nag about using "module". You should consider namespace instead.

In order to satisfy the requirement here you'll actually need more than extending the Global interface. You'll also need to create a constant with the type as well if you want it to be accessible directly from the "globalThis" context.

NOTE: while the OP asked about an object literal the process is the same as you see here below. Rather than the "Debug" type being a function you would simply define the interface as need, then change "debug:" to myConfig or whatever you wish.

// typically I'll store the below in something like "typings.d.ts"
// this is because, at least typically, these overrides tend to
// be minimal in nature. You could break them up and Typescript
// will pick them up if you wish.

// Augmentations for the global scope can only be directly nested 
// in external modules or ambient module declarations.
export {}

declare global {
  // Definition or type for the function.
  type Debug = (label: string) => (message: any, ...args: any[]) => void

  // If defining an object you might do something like this
  // interface IConfig { a: number, b: number }

  // Extend the Global interface for the NodeJS namespace.
  namespace NodeJS {
    interface Global {
      // Reference our above type, 
      // this allows global.debug to be used anywhere in our code.
      debug: Debug
    }
  }
  
  // This allows us to simply call debug('some_label')('some debug message')
  // from anywhere in our code.
  const debug: Debug
}

How the above might be used

For completeness in this example all we did was define a global so we can log a simple debug message. Here's how we'd bind the method to our global context.

global.debug = (label: string) => (message: any, ...args: any[]) => console.log(message, ...args)

We can also call our global debug method directly:

debug('express')(`${req.method}: ${req.url}`)

The only thing that works for me is this:

// lib/my.d.ts

import Global = NodeJS.Global;
export interface CDTGlobal extends Global {
  cdtProjectRoot: string
}

and then using it in other files like so

import {CDTGlobal} from "../lib/my.d.ts";
declare const global: CDTGlobal;

const cwd = global.cdtProjectRoot; // works

What worked for me is:

declare global {
  module NodeJS {
    interface Global {
      myConfig: any;
    }
  }
}

global.myConfig = 'it works!';

Only downside is when using it you will have to turn off the ESLint rule @typescript-eslint/no-namespace.

For completeness here is my tsconfig.json:

{
  "compilerOptions": {
    "declaration": true,
    "emitDecoratorMetadata": true,
    "esModuleInterop": true,
    "experimentalDecorators": true,
    "forceConsistentCasingInFileNames": true,
    "jsx": "react",
    "lib": ["dom", "es2017"],
    "module": "commonjs",
    "moduleResolution": "node",
    "noEmitOnError": true,
    "noImplicitReturns": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "outDir": "dist",
    "removeComments": true,
    "resolveJsonModule": true,
    "rootDir": "src",
    "sourceMap": true,
    "strict": true,
    "target": "es6"
  },
  "exclude": ["dist", "node_modules"]
}

> Copy my answer of another post:

globalThis is the future.

// Way 1
var abc: number
globalThis.abc = 200 // no error

// Way2
declare var age: number
globalThis.age = 18 // no error

I can get both type check and code intelligence.

declare namespace NodeJS {
  interface Global {
    property: string
  }
}

But interface Global is point to global.GLOBAL.

You can get correct type check, is because:

declare var global: NodeJS.Global & typeof globalThis;

But you can not get better code intelligence by useing global.property, unless use global.GLOBAL.property.

So, you need to define global var global and extend interface Global both:

// myglobal.d.ts
declare namespace NodeJS {
  interface Global {
    property: string
  }
}
declare var global: NodeJS.Global & typeof globalThis

And now you can get property intelligence when you type global.