Create a global variable in TypeScript

Question

Inside a .d.ts definition file

type MyGlobalFunctionType = (name: string) => void

If you work in the browser, you add members to the browser's window context:

interface Window {
  myGlobalFunction: MyGlobalFunctionType
}

Same idea for NodeJS:

declare module NodeJS {
  interface Global {
    myGlobalFunction: MyGlobalFunctionType
  }
}

Now you declare the root variable (that will actually live on window or global)

declare const myGlobalFunction: MyGlobalFunctionType;

Then in a regular .ts file, but imported as side-effect, you actually implement it:

global/* or window */.myGlobalFunction = function (name: string) {
  console.log("Hey !", name);
};

And finally use it elsewhere in the codebase, with either:

global/* or window */.myGlobalFunction("Kevin");

myGlobalFunction("Kevin");

globalThis is the future.

First, TypeScript files have two kinds of scopes

global scope

If your file hasn't any import or export line, this file would be executed in global scope that all declaration in it are visible outside this file.

So we would create global variables like this:

// xx.d.ts
declare var age: number

// or 
// xx.ts
// with or without declare keyword
var age: number

// other.ts
globalThis.age = 18 // no error

All magic come from var. Replace var with let or const won't work.

module scope

If your file has any import or export line, this file would be executed within its own scope that we need to extend global by declaration-merging.

// xx[.d].ts
declare global {
  var age: number;
}

// other.ts
globalThis.age = 18 // no error

You can see more about module in official docs

This is how I have fixed it:

Steps:

1. Declared a global namespace, for e.g. custom.d.ts as below :

declare global {
    namespace NodeJS {
        interface Global {
            Config: {}
        }
    }
}
export default global;

2. Map the above created a file into "tsconfig.json" as below:

"typeRoots": ["src/types/custom.d.ts" ]

3. Get the above created global variable in any of the files as below:

console.log(global.config)

Note:

1. typescript version: "3.0.1".

2. In my case, the requirement was to set the global variable before boots up the application and the variable should access throughout the dependent objects so that we can get the required config properties.

Hope this helps!

Thank you

I found a way that works if I use JavaScript combined with TypeScript.

logging.d.ts:

declare var log: log4javascript.Logger;

log-declaration.js:

log = null;

initalize-app.ts

import './log-declaration.js';

// Call stuff to actually setup log.  
// Similar to this:
log = functionToSetupLog();

This puts it in the global scope and TypeScript knows about it. So I can use it in all my files.

NOTE: I think this only works because I have the allowJs TypeScript option set to true.

If someone posts an pure TypeScript solution, I will accept that.

im using only this

import {globalVar} from "./globals";
declare let window:any;
window.globalVar = globalVar;

I spent couple hours to figure out proper way to do it. In my case I'm trying to define global "log" variable so the steps were:

1) configure your tsconfig.json to include your defined types (src/types folder, node_modules - is up to you):

...other stuff...
"paths": {
  "*": ["node_modules/*", "src/types/*"]
}

2) create file src/types/global.d.ts with following content (no imports! - this is important), feel free to change any to match your needs + use window interface instead of NodeJS if you are working with browser:

/**
 * IMPORTANT - do not use imports in this file!
 * It will break global definition.
 */
declare namespace NodeJS {
    export interface Global {
        log: any;
    }
}

declare var log: any;

3) now you can finally use/implement log where its needed:

// in one file
global.log = someCoolLogger();
// in another file
log.info('hello world');
// or if its a variable
global.log = 'INFO'