Wrong Type Imports Are Breaking Your Full-Stack React App

You're running browser tests in your Next.js app with Vitest, and suddenly you're hit with a cascade of Vite pre-transform errors. The error message points to a seemingly innocent import statement, but your tests are failing. If you're using next-auth with TypeScript, there's a good chance you've encountered this exact problem.

The issue stems from a subtle but critical difference in how TypeScript handles type imports, and how bundlers like Vite interpret them. Understanding this difference will save you hours of debugging.

The Problem

When running browser tests with Vitest, you might see errors like this:

[vite] (client) Pre-transform error: No known conditions for "./adapters" specifier in "next-auth" package
  Plugin: vite:import-analysis
  File: /path/to/src/server/db/schema.ts:25:36
  20 |  import {} from "next-auth/adapters";
     |                  ^

The error repeats multiple times, and while your tests might still pass, these warnings clutter your output and indicate a deeper bundling issue.

The Culprit

The problem is in how you're importing types from next-auth/adapters. You might have written:

import { type AdapterAccount } from "next-auth/adapters";

This looks correct at first glance. You're using the type keyword, so TypeScript knows it's a type import. However, this syntax has a different meaning to bundlers like Vite.

The Solution

Change your import to use the import type syntax instead:

import type { AdapterAccount } from "next-auth/adapters";

This single change eliminates the Vite pre-transform errors and ensures your bundler handles the import correctly.

Understanding the Difference

There are two ways to import types in TypeScript, and they behave differently:

import type { ... } - Type-Only Import

import type { AdapterAccount } from "next-auth/adapters";

This is a type-only import statement. The entire import is marked as type-only, which means:

• TypeScript removes it completely at compile time
• The module isn't evaluated or analyzed by the bundler
• Vite can skip analyzing the module entirely during pre-transform
• No runtime code is generated

import { type ... } - Value Import with Type-Only Named Import

import { type AdapterAccount } from "next-auth/adapters";

This is a value import statement with a type-only named import. Even though the specific import is marked as type-only:

• The import statement itself is still a value import
• The bundler may still analyze the module
• Vite attempts to resolve the module during pre-transform
• This can cause issues with packages like next-auth that have complex export conditions

Why This Matters for Vite and Bundlers

Modern bundlers like Vite perform static analysis on your code before transformation. When they encounter an import statement, they need to understand:

1. Is this a type import that can be ignored?
2. Does this module need to be bundled?
3. What are the export conditions for this package?

With import type { ... }, the bundler receives a clear signal: "This entire import is type-only, skip it." With import { type ... }, the bundler sees a value import and may attempt to analyze the module, even if the specific named export is type-only.

For packages like next-auth that use conditional exports (like "./adapters"), this analysis can fail when the bundler can't determine the correct export condition for the build target.

Best Practices

When importing types in TypeScript, especially in projects using Vite, Webpack, or other modern bundlers:

1. Prefer import type for type-only imports - Use import type { ... } when you're only importing types
2. Use import { type ... } sparingly - Only use this syntax when mixing type and value imports from the same module
3. Be consistent - Choose one pattern and stick with it across your codebase

Conclusion

The difference between import type { ... } and import { type ... } might seem minor, but it has real implications for how bundlers process your code. When working with Next.js, React, and packages like next-auth, using the correct type import syntax prevents bundler errors and keeps your test output clean.

If you're seeing Vite pre-transform errors related to type imports, check your import statements and convert them to use import type syntax. Your bundler—and your future self—will thank you.

Next Steps: Review your codebase for type imports and standardize on import type syntax. This small change will improve your build performance and eliminate unnecessary bundler warnings.