Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current extension rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• My proposal specifically checks TypeScript syntax, or it proposes a check that requires type information to be accurate.
• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Link to the base rule

https://eslint.org/docs/latest/rules/no-var

Description

Typescript global declarations are more subtle than I ever expected. For a better understanding, I recommend these SO answers:

• Create a global variable in TypeScript
• Using globalThis in Typescript

Here are some key takeaways. If you want to add properties to globalThis:

1. When the typescript file has no import or export keyword (i.e., it's a script, not a module), it is done like so: declare var age: number.
2. When the typescript file does have import or export, you write this instead:

   // an import / export line
   
   declare global {
       var age: number;
   }

If const or let is used instead of var, globalThis.age = 18 would error in both scenarios. So if the intention is to declare globalThis.age, it is necessary to use the var keyword: using const or let would be a mistake. When enabled, the base no-var rule indiscriminately marks all var usage as wrong, but in this case, it is required.

Fail

/*eslint no-var: "error"*/
export {}

var x = "y";
var CONFIG = {};

Pass

export {}

declare global {
    var age: number;
}

Additional Info

No response