[Runner VM Bug] behavior of {}.constructor === Object does not match node behavior

[monoclex]

Minimal Repro

import { VMScriptRunner } from "@miniflare/runner-vm";

console.log("node     :", {}.constructor === Object);

const runner = new VMScriptRunner();
const results = await runner.run(globalThis, {
  code: `console.log("miniflare:", {}.constructor === Object);`,
});

Miniflare's current, incorrect output

node     : true
miniflare: false

Ideal, expected output

node     : true
miniflare: true

Rationale for fixing bug

I came across this bug while attempting to use the React Material UI library, @mui/material. After copying their code and debugging cryptic errors, it led me to this crucial difference between a typical node environment and miniflare that prevented the code from running correctly.

This specific behavior is used in deepmerge of Material UI, which is use to merge default theme values together. Specifially, this check can be found here: https://github.com/mui-org/material-ui/blob/a9903917f919092f80d84075f39fb51d51f241f2/packages/mui-utils/src/deepmerge.ts#L1-L3. The bug from a user standpoint (when using Material UI) looks as such:

TypeError: Cannot read properties of undefined (reading 'fontWeightBold')
    at styles (/tmp/functionsWorker.js:23779:41)
    at styles (/tmp/functionsWorker.js:23720:29)
    at GlobalStyles.globalStyles (/tmp/functionsWorker.js:18528:26)
    at handleInterpolation (/tmp/functionsWorker.js:15228:22)
    at serializeStyles2 (/tmp/functionsWorker.js:15471:20)
    at /tmp/functionsWorker.js:15772:24
    at Object.render (/tmp/functionsWorker.js:15561:16)
    at ReactDOMServerRenderer2.render (/tmp/functionsWorker.js:10150:50)
    at ReactDOMServerRenderer2.read2 [as read] (/tmp/functionsWorker.js:9990:37)
    at renderToString2 (/tmp/functionsWorker.js:10571:35)

Software versions

> ./node_modules/.bin/wrangler2 --version
0.0.0-485e0dc
> node --version
v17.3.0

"@miniflare/runner-vm": {
  "version": "2.0.0-rc.3",

[mrbbot]

Hey! 👋 Quite a few other people have faced this issue:

• [v2] Comparison var o = {}; o.constructor === Object; wrongly evaluates to false #109
• new object - literal vs. constructor notation #137
• Object Prototypes Incorrect workers-sdk#91

As explained here #109 (comment), this behaviour was originally added to make Rust development easier. However, I'm beginning to think making this the default was a bad idea... 😅

While we can still making breaking changes before the full v2 release, I'll probably switch this over to requiring something like a --proxy-primitive flag, and leave Object untouched by default. This still has the problem that prototype/constructor/instanceof checks for Objects created outside the sandbox (e.g. KV/cached responses) would fail, but I think that's less likely to be an issue, since the return types are always known in those cases.

[mrbbot]

Hey! 👋 Miniflare 2.0.0-rc.5 has just been released, which implements this fix. You can find the full changelog here. Should be in wrangler soon.