TDZ error: Cannot access before initialization when bundled with Vite manualChunks

[seunghan91]

Bug Report

Summary

When using bits-ui v2.x with Vite 5 and the manualChunks option to group svelte + bits-ui into a single vendor chunk, a Temporal Dead Zone (TDZ) error occurs at runtime:

Uncaught ReferenceError: Cannot access 'xu' before initialization
    at vendor-svelte-XXXXXXXX.js:10:2286

Environment

• bits-ui: v2.x (Svelte 5 compatible)
• svelte: 5.x
• vite: 5.x
• @sveltejs/vite-plugin-svelte: latest
• Framework: Rails 8 + Inertia.js + Svelte 5 (single-server deploy)

Reproduction

vite.config.ts:

build: {
  rollupOptions: {
    output: {
      manualChunks(id) {
        if (id.includes('node_modules')) {
          // ❌ This causes the TDZ error
          if (id.includes('svelte') || id.includes('bits-ui') || id.includes('lucide-svelte'))
            return 'vendor-svelte'
          return 'vendor'
        }
      },
    },
  },
},

Root Cause

bits-ui v2.x (Svelte 5 version) has circular imports between its internal modules. When Rollup is forced to put all these modules into a single chunk via manualChunks, it must linearize the circular dependency graph. The chosen initialization order results in a const binding being accessed before it is initialized (TDZ).

The same error occurs even with these mitigations:

• externalLiveBindings: false — no effect on the TDZ
• hoistTransitiveImports: false — changes chunk content but doesn't fix the circular dep ordering
• optimizeDeps.include: ['bits-ui'] — only affects dev mode, not production builds

Fix

Remove svelte, bits-ui, and related packages from manualChunks and let Rollup handle their chunking automatically:

build: {
  rollupOptions: {
    output: {
      externalLiveBindings: false,
      hoistTransitiveImports: false,
      manualChunks(id) {
        if (id.includes('node_modules')) {
          // ✅ Do NOT force svelte/bits-ui into a manual chunk
          if (id.includes('@inertiajs')) return 'vendor-inertia'
          if (id.includes('date-fns')) return 'vendor-date'
          // ... other non-circular vendors
        }
      },
    },
  },
},

When Rollup auto-chunks the svelte ecosystem, it properly handles the circular dependency ordering and the TDZ error disappears.

Notes

This may be worth documenting in the bits-ui migration guide or FAQ for Vite users. The error is cryptic (minified variable name + no stack trace pointing to user code) and took several deploy iterations to diagnose.