How can I properly extend globalThis?

[haughty edge]

i'm facing an issue where I can't properly extend globalThis. this is basically the setup that I have...

src/globals.d.ts:

export {};

interface Foo {
  x: number;
}

declare global {
  let foo: Foo;
  // uncomment bottom and comment top to fix the error
  // var foo: Foo; 
}

src/index.ts:

globalThis.foo = {
  x: 1
};

console.log(globalThis.foo);

the compile errors on both globalThis.foo accesses. strangely, if i change the let in globals.d.ts to var, it passes. what am i missing?

here's a codesandbox showing the issue: https://codesandbox.io/s/typescript-forked-hm6knw?file=/src/index.ts:0-59

[olive adder]

you shouldbe using var in your declaration files

see https://www.typescriptlang.org/docs/handbook/declaration-files/templates/global-d-ts.html#identifying-a-global-library-from-code

When looking at the code of a global library, you’ll usually see:

• Top-level var statements or function declarations

https://www.typescriptlang.org/docs/handbook/declaration-files/by-example.html#global-variables

let variables are scoped and aren't accessible outiside of the scope (outside of the braces in the declare global {})

it's tries to match the runtime behavior, so that's why

[haughty edge]

mhmm

[olive adder]

there is also a more in-depth explanation here https://www.typescriptlang.org/docs/handbook/variable-declarations.html

(more about general var vs let in JS)

[haughty edge]

okay, thanks

the reason i wasn't sure about using var is because eslint is complaining

i could just shut it up but just wondering if this was the right way of doing it

[olive adder]

right, if not properly configured, ESLint thinks it's a regular TS file

but it's not

[haughty edge]

ah

how do i fix that?

[olive adder]

youcan exclude the .d.ts files

don't remember the right property,but it's possible to do that

anduse an alternative set of rules for typings files