Don't use `var` in js code

[sobolevn]

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

Linked PRs

• gh-136938: Do not use var in js, use let and const #136939

[hoodmane]

Emscripten's code style exclusively uses var. I think because it minifies better (@sbc100)? I can't say I love this though, since I am a big fan of the way scopes work in JS. For instance, the upstream fd_read system call we're modifying uses var in the same place:
https://github.com/emscripten-core/emscripten/blob/main/src/lib/libwasi.js#L333

For code that is closely coupled to Emscripten, it's simpler to keep var so that there are fewer things to change if we have to copy new code from Emscripten or if we want to upstream these changes. But I am not strongly opinionated.

For comparison, Pyodide currently has 666 instances of let, 493 instances of const, and only 30 instances of var.

[hoodmane]

Anyways, as Python programmers, we're at least used to scopes working like this.

[sbc100]

Indeed, on the emscripten side we (sadly) have been slowed in our switch away from var because let / const don't minify as a well (since unlike, var they cannot all be combined into a single declaration per function). And also because is some non-zero cost to using let / const due to the VM having to track the "dead zone" of each variable (I've never measured this myself, but I'm told it exists).

[brettcannon]

Dropping os-wasi as the JS stuff is entirely Emscipten-specific.

[freakboy3742]

While I agree that var is (broadly speaking) confusing and best avoided, given the implications for minification and synchronisation with Emscripten upstream, avoiding var entirely is more of an aspirational goal than something we can apply absolutely.

[StanFromIreland]

Seeing as the PR was closed, is there anything left to do here?