Create a global variable in TypeScript

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

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");