Problem
Using the new checkJs feature, it is currently difficult to tell TypeScript about a global variable:

// @ts-check

myGlobal

window.myVar = 'foo';

The only way around this that I know is to create a .d.ts file that defines these:

interface Window {
    myVar: string;
}

declare var myGlobal: string;

JavaScript users should not have to write *.d.ts files.

Possible Approaches
@egamma proposes that may be able to use the global comment from ESlint http://eslint.org/docs/user-guide/configuring : /* global var1, var2 */

I'd also be interested to see if we could put type information in a jsdoc somehow, something like:

/**
 * @global {string} myGlobal 
 */