The answer
It is a legacy behavior. var was designed before ES6 to attach to the global object. let and const were designed to be cleaner: they live in a separate global lexical environment and do not pollute window.
let, const, and the Global Object in JavaScript
Why does var become a property of window but let does not?
More FAQs in let, const, and the Global Object in JavaScript
No. let and const at the top level live in a global lexical environment that is separate from the window object. They are still global (accessible everywhere in the script), but they are not properties of window. var does become a property of window.
No. In ES modules, top-level let, const, and var are module-scoped, not global. They are only accessible outside the module if explicitly exported. This is the cleanest behavior and prevents global pollution.
Use typeof. typeof b !== 'undefined' checks if b exists without throwing, even though b is not a property of window. Do not use 'b' in window, because let globals do not attach to window.
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