SSR Crash: Function In Router Context With TanStack & React Query

Aug 12, 2025 by Viktoria Ivanova 66 views

Function in Router Context Causes App Crash During SSR Pass

Introduction

Hey guys! Today, we're diving deep into a tricky bug encountered while working with TanStack Router and React Query in a server-side rendering (SSR) environment. Specifically, we'll explore how including a function within a React Query meta property can lead to an application crash during the SSR pass. This issue seems to stem from the interaction between TanStack Router's context and server actions, particularly how the router context is passed to server functions. This article will walk you through the problem, its reproduction steps, the underlying cause, and potential solutions. So, buckle up and let's get started!

Background

Before we dive into the specifics, let's quickly recap the technologies involved. TanStack Router is a powerful routing library for React applications, providing type-safe navigation and a host of other features. React Query, on the other hand, is a data-fetching library that simplifies asynchronous data management in React. Both libraries are incredibly useful, but their interaction in an SSR environment can sometimes lead to unexpected issues. SSR, or server-side rendering, is a technique where the initial rendering of a web application occurs on the server rather than the client's browser. This can improve performance and SEO, but it also introduces complexities, especially when dealing with context and data hydration.

Understanding the Problem

The core issue arises when a function is added to the meta property of a React Query query. In a typical setup, the meta object is used to store additional information related to the query, such as custom configurations or callbacks. However, when this meta object, containing a function, is passed through the router context during server-side rendering, it can cause the application to crash. This is because the query client, which is part of the router's context, gets serialized and passed to server functions. The presence of a function in this process leads to serialization issues, ultimately crashing the app.

Detailed Explanation of the Bug

The bug manifests itself during the server-side rendering (SSR) pass when a function is included within the meta property of a React Query query. This seemingly innocuous action triggers a chain of events that leads to an application crash. To fully grasp the issue, we need to understand the context in which it occurs and the technologies involved.

TanStack Router and React Query Integration

TanStack Router is a robust routing library that simplifies navigation and route management in React applications. It offers features like type-safe routing, dynamic route matching, and integration with other libraries like React Query. React Query, on the other hand, is a powerful data-fetching and caching library that streamlines asynchronous data management in React. It provides hooks for fetching, caching, and updating data, making it an essential tool for modern React applications.

The integration between TanStack Router and React Query is common in applications that require data fetching based on route parameters or user interactions. For instance, you might fetch data based on the current page number or a search query in the URL. This integration often involves sharing context between the router and the query client, allowing queries to be invalidated or refetched when the route changes.

The Role of Server-Side Rendering (SSR)

Server-side rendering (SSR) is a technique where the initial HTML of a web application is rendered on the server before being sent to the client's browser. This approach offers several advantages, including improved initial load time, better SEO, and enhanced accessibility. However, SSR also introduces complexities, particularly when dealing with state management, context, and data hydration.

During SSR, the application's components are rendered on the server, and the resulting HTML is sent to the client. This process often involves fetching data, initializing state, and setting up the application's context. When React Query is used in an SSR environment, it's crucial to ensure that the query client is properly hydrated on the client-side to avoid data mismatches and hydration errors.

The Meta Property and the Crash

The meta property in React Query is a versatile feature that allows you to attach additional information or configuration options to a query. This can include things like custom retry logic, cache invalidation rules, or callbacks. However, the bug in question arises when a function is included in the meta property. When this happens, the function gets serialized as part of the query client's state during SSR. This serialization process is where the problem begins.

The issue stems from the fact that functions cannot be directly serialized into a format suitable for transmission over the network or storage in a database. When the router context, which includes the query client with the function in its meta property, is passed to server functions, the serialization fails, leading to a crash. This is because server functions often operate in an isolated environment where direct access to client-side JavaScript functions is not possible.

Impact of Router Context and Server Actions

The introduction of router context to server actions, as mentioned in the original bug report, exacerbates the problem. Server actions are a feature that allows you to perform server-side operations directly from your React components. When router context is passed to these server actions, it includes the query client, which, as we've seen, contains the problematic function in the meta property. This creates a scenario where the serialization failure is inevitable, causing the application to crash during SSR.

Steps to Reproduce the Bug

To reproduce this bug, follow these steps:

Clone the Repository: Start by cloning the provided GitHub repository, which contains a minimal reproduction of the issue.
```
git clone https://github.com/arackaf/tanstack-start-middleware-blog-post
cd tanstack-start-middleware-blog-post
```
Checkout the Correct Branch: Make sure to checkout the special/bug-repro branch, as this branch contains the specific code that triggers the bug.
```
git checkout special/bug-repro
```
Install Dependencies: Install the necessary dependencies using npm.
```
npm install
```
Run the Development Server: Start the development server.
```
npm run dev
```
Navigate to the Application: Open your browser and go to http://localhost:3000/app.
Click the Link: Click on the link provided on the page, which will take you to http://localhost:3000/app/epics?page=1.
Initial Success: The page should load correctly on the first attempt.
Refresh the Page: Refresh the page, and you should observe the application crashing. This crash is the manifestation of the bug we're investigating.
Inspect the Code: Examine the code in src/queries/util.ts, particularly line 23. This is where the function is added to the meta property of the React Query query, triggering the crash during SSR.

By following these steps, you can reliably reproduce the bug and observe the application crashing during the server-side rendering pass.

Code Snippet

The problematic code snippet is located in src/queries/util.ts:

// Example code causing the crash
useQuery({
  queryKey: ['example'],
  queryFn: () => Promise.resolve('data'),
  meta: {
    // Adding ANY function to meta causes the crash
    fn: () => {},
  },
});

Expected Behavior vs. Actual Behavior

Expected Behavior

The expected behavior is that the application should render correctly both on the client-side and during server-side rendering (SSR). Adding a function to the meta property of a React Query query should not cause the application to crash. The application should load the data, display the content, and allow navigation between different routes without any issues.

Actual Behavior

In reality, the application crashes during the SSR pass when a function is added to the meta property of a React Query query. This crash occurs when the page is refreshed or accessed directly, triggering the server-side rendering process. The presence of a function in the meta property leads to a serialization error, preventing the application from rendering correctly.

Specifically, the following scenario unfolds:

The application loads initially without any issues.
The user navigates to a route that triggers the data fetching logic using React Query.
The query is executed, and the data is fetched successfully.
The page renders correctly on the client-side.
However, when the page is refreshed or accessed directly, the server-side rendering process is initiated.
During SSR, the application attempts to serialize the query client's state, which includes the meta property with the function.
The serialization process fails because functions cannot be directly serialized.
This failure causes the application to crash, resulting in an error message or a blank page.

The discrepancy between the expected and actual behavior highlights the bug's severity. It prevents the application from functioning correctly in an SSR environment, which can negatively impact performance, SEO, and user experience.

Root Cause Analysis

The root cause of this bug lies in the interaction between TanStack Router, React Query, and the way server-side rendering handles context and data serialization. Specifically, the issue arises from the following factors:

Function in Meta Property

The primary trigger for the bug is the inclusion of a function within the meta property of a React Query query. The meta property is designed to store additional information or configuration options related to the query. While it can be useful for various purposes, adding a function to this property introduces a serialization challenge.

Serialization Issues

Functions, unlike primitive data types or plain JavaScript objects, cannot be directly serialized into a format suitable for storage or transmission over a network. Serialization is the process of converting a data structure or object into a format that can be easily stored or transmitted and later reconstructed. Common serialization formats include JSON (JavaScript Object Notation), which is widely used in web applications.

When the application attempts to serialize the query client's state during server-side rendering (SSR), the presence of a function in the meta property causes the serialization process to fail. This failure is because the serialization algorithm cannot handle functions, leading to an error.

Router Context and Server Actions

The context in which this bug manifests is crucial to understanding its root cause. TanStack Router provides a context that allows components to access router-related information, such as the current route, parameters, and navigation methods. This context is often shared with other libraries, including React Query, to enable seamless integration.

In addition, the introduction of router context to server actions plays a significant role in triggering the bug. Server actions are functions that run on the server and can be invoked from client-side components. When router context is passed to server actions, it includes the query client, which contains the meta property with the function. This creates a scenario where the serialization failure becomes inevitable.

Query Client and State Hydration

The query client in React Query is responsible for managing the state of queries, including cached data, loading states, and error information. During SSR, the query client's state needs to be serialized and then hydrated on the client-side to ensure consistency between the server-rendered HTML and the client-side application.

When a function is included in the meta property, the serialization of the query client's state fails, preventing proper hydration on the client-side. This can lead to various issues, including data mismatches, hydration errors, and application crashes.

Downstream Effects

The bug described in the original report is believed to be a downstream effect of adding router context to server actions. This means that the primary cause of the bug is the interaction between these two features, rather than a direct issue within React Query or TanStack Router themselves. The combination of these factors creates a scenario where the serialization of functions in the meta property becomes problematic.

Potential Solutions and Workarounds

To address this bug, several potential solutions and workarounds can be considered. These approaches aim to either prevent the function from being serialized or handle the serialization process more gracefully.

Avoid Storing Functions in Meta

The most straightforward solution is to avoid storing functions directly in the meta property of React Query queries. Instead, consider alternative approaches for managing the logic or callbacks associated with the query.

For example, you can store a string identifier or a key in the meta property and then use a separate mapping or lookup table to retrieve the corresponding function when needed. This approach ensures that only serializable data is stored in the meta property, preventing the serialization error during SSR.

Serialize Functions Manually

If you must store functions in the meta property, you can attempt to serialize them manually before passing them through the router context. This involves converting the function into a serializable representation, such as a string, and then deserializing it back into a function on the client-side.

However, this approach can be complex and may not be suitable for all types of functions. It requires careful consideration of the function's scope, dependencies, and behavior to ensure that it can be correctly serialized and deserialized.

Use a Custom Serialization Method

Another approach is to use a custom serialization method that can handle functions more gracefully. This involves implementing a custom serialization algorithm that can identify and process functions differently from other data types.

For example, you can create a custom JSON serializer that replaces functions with a placeholder value during serialization and then restores them during deserialization. This approach requires more advanced knowledge of serialization techniques but can provide a more flexible solution.

Separate Concerns

A more architectural solution is to separate the concerns of data fetching and routing more clearly. This involves decoupling the React Query queries from the router context as much as possible.

For example, you can fetch data in a separate module or service that is not directly tied to the router. This allows you to manage the query client and its state independently, avoiding the issue of serializing functions through the router context.

Conditional Meta Property

One way to prevent functions from being serialized during SSR is to conditionally set the meta property based on the environment. You can check if the code is running on the server and exclude the function from the meta property if it is.

// Example of conditional meta property
const isServer = typeof window === 'undefined';

useQuery({
  queryKey: ['example'],
  queryFn: () => Promise.resolve('data'),
  meta: {
    // Only include the function in meta on the client-side
    ...(isServer ? {} : { fn: () => {} }),
  },
});

Conclusion

In conclusion, the bug described in this article highlights the complexities of integrating different libraries in a server-side rendering environment. The interaction between TanStack Router, React Query, and server actions can lead to unexpected issues, such as the serialization error caused by including a function in the meta property of a React Query query.

To mitigate this bug, it's crucial to understand the root cause and consider various solutions and workarounds. Avoiding the storage of functions in the meta property, using custom serialization methods, or separating concerns between data fetching and routing can help prevent the application from crashing during SSR.

By carefully managing context, data serialization, and the interaction between different libraries, developers can build robust and performant React applications that function correctly in both client-side and server-side rendering environments. Understanding these nuances is key to creating a smooth user experience and maintaining the integrity of the application.

Key Takeaways

Adding functions to React Query's meta property can cause SSR crashes.
This is often due to serialization issues when passing router context to server actions.
Solutions include avoiding functions in meta, custom serialization, or conditional property setting.
Understanding the interplay between libraries is crucial for robust SSR applications.