Don't use var in js code #136938
Description
While reviewing recent js code changes related to WASM / Emscripten I've noticed that there are lots of existing and new var declarations.
Good reference: https://javascript.plainenglish.io/why-you-should-never-use-var-again-seriously-447183bd8d62
We should change them to be let and const.
Related: https://github.com/python/cpython/pull/136837/files