Fix HMR failure with js error on load, take 2

[mischnic]

↪️ Pull Request

See example.

I'm not sure if my changes have some side effect?
Might have a negative effect on scope-hoisting build sizes.

Fixes #879, previous attempt: #898

💻 Examples

On first load:

throw new Error("STOP");

console.log("test");

Removing the Error line doesn't hmr successfully.

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[devongovett]

This seems dangerous. Why are we overwriting the global variable here? Won't that already happen when the function returns?

[mischnic]

Because returning doesn't work if an error is thrown and parcelRequire is then undefined for subsequent HMR updates

parcel/packages/core/parcel-bundler/src/builtins/prelude.js

Lines 114 to 121 in 75a71be

	parcelRequire = newRequire;
	if(error){
	// throw error from earlier, _after updating parcelRequire_
	throw error;
	}
	return newRequire;

And this overwrites a global variable as well:

parcel/packages/core/parcel-bundler/src/builtins/prelude.js

Line 10 in 75a71be

parcelRequire = (function (modules, cache, entry, globalName) {