Vite ignores resolve.mainFields, if ssr is false

[sheremet-va]

Describe the bug

When transforming files without ssr flag Vite resolves package path to module entry point in package.json, even if resolve.mainFields is empty.

This doesn't happen, if we transform files with ssr: true. Not sure if this is a bug, or this options is intended to work differently between web and ssr.

Reproduction

https://stackblitz.com/edit/node-gslx3n?file=vite.config.ts,package.json,Comp.vue,vite-repro.mjs

System Info

Any

Used Package Manager

npm

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to https://github.com/vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[sapphi-red]

This part runs without regarding resolve.mainFields when ssr is false.

vite/packages/vite/src/node/plugins/resolve.ts

Lines 740 to 783 in 86a55d3

	// if exports resolved to .mjs, still resolve other fields.
	// This is because .mjs files can technically import .cjs files which would
	// make them invalid for pure ESM environments - so if other module/browser
	// fields are present, prioritize those instead.
	if (targetWeb && (!entryPoint || entryPoint.endsWith('.mjs'))) {
	// check browser field
	// https://github.com/defunctzombie/package-browser-field-spec
	const browserEntry =
	typeof data.browser === 'string'
	? data.browser
	: isObject(data.browser) && data.browser['.']
	if (browserEntry) {
	// check if the package also has a "module" field.
	if (
	!options.isRequire &&
	typeof data.module === 'string' &&
	data.module !== browserEntry
	) {
	// if both are present, we may have a problem: some package points both
	// to ESM, with "module" targeting Node.js, while some packages points
	// "module" to browser ESM and "browser" to UMD.
	// the heuristics here is to actually read the browser entry when
	// possible and check for hints of UMD. If it is UMD, prefer "module"
	// instead; Otherwise, assume it's ESM and use it.
	const resolvedBrowserEntry = tryFsResolve(
	path.join(dir, browserEntry),
	options
	)
	if (resolvedBrowserEntry) {
	const content = fs.readFileSync(resolvedBrowserEntry, 'utf-8')
	if (
	(/typeof exports\s*==/.test(content) &&
	/typeof module\s*==/.test(content)) ||
	/module\.exports\s*=/.test(content)
	) {
	// likely UMD or CJS(!!! e.g. firebase 7.x), prefer module
	entryPoint = data.module
	}
	}
	} else {
	entryPoint = browserEntry
	}
	}
	}

This seems to be the reason.

[sheremet-va]

I've also noticed it still uses browser main field, even if I remove if from mainFields, when ssr is true: https://github.com/gdonoso94/vitest-prisma-integration-test