SWR & TypeScript: Fixing Mistyped Fetcher Arguments

Aug 22, 2025 by Lucas 52 views

SWR and TypeScript: A Deep Dive into Mistyped Fetcher Arguments

Hey guys! Today, let's dive deep into a fascinating issue that often pops up when using SWR (Stale-While-Revalidate) with TypeScript: mistyped fetcher arguments. This can be a real head-scratcher, especially when you're expecting TypeScript to catch those type errors but it seems like they're slipping through the cracks. We'll break down the problem, explore the reasons behind it, and, most importantly, figure out how to tackle it effectively.

The Curious Case of Mistyped Fetcher Arguments

So, what exactly are we talking about here? Imagine you're using useSWR to fetch data. You've defined your fetcher function, specifying the types of arguments it should receive. But, to your surprise, TypeScript isn't complaining when you pass the wrong type of arguments. Let's look at the examples provided to get a clearer picture.

// Should complain about wrong fetcher argument type being an array but expecting a string
useSWR("test", ([id]:[string]) => {})

// Should complain about fetcher argument must be an array
useSWR(["test"], (id) => {})

In the first example, we're passing the key "test" and expecting the fetcher function to receive a string id. However, we've mistakenly typed the fetcher argument as an array [string]. TypeScript should ideally flag this as an error, but it doesn't. Similarly, in the second example, we're passing an array ["test"] as the key, which means the fetcher should receive an array as its argument. Instead, we've defined the fetcher to receive a single id, and again, TypeScript remains silent. This can lead to runtime errors and unexpected behavior, which is definitely not what we want.

Why Does This Happen? Unraveling the Mystery

The core question here is: why isn't TypeScript catching these type mismatches? Is it a limitation of TypeScript itself, or is there something about SWR's typings that's causing this? To understand this, we need to delve a bit into how SWR and TypeScript interact.

SWR's useSWR hook is incredibly flexible. It can accept a variety of key types – strings, arrays, or even functions that return keys. This flexibility, while powerful, also introduces complexity in terms of typing. TypeScript needs to infer the correct types for the fetcher arguments based on the key provided. When you pass a simple string key, SWR expects the fetcher to receive that string as an argument. When you pass an array, SWR expects the fetcher to receive an array. However, the way TypeScript infers these types isn't always foolproof, especially when dealing with complex generic types and function overloads.

The issue often lies in the way SWR's type definitions are structured to accommodate this flexibility. The type definitions might be too broad, allowing for a wider range of argument types than intended. This can lead to TypeScript not being able to narrow down the types accurately and, consequently, missing these mistyped argument errors. It's a delicate balancing act between providing a flexible API and ensuring strict type safety.

Digging Deeper: TypeScript's Inference Limitations

To further clarify, let's consider TypeScript's type inference capabilities. TypeScript does an excellent job of inferring types in many scenarios, but it has limitations. When dealing with function overloads and complex generic types, inference can become tricky. In the case of SWR, the useSWR hook is likely defined with multiple function overloads to handle different key types. This means TypeScript has to choose the correct overload based on the arguments provided. If the overloads aren't defined precisely enough, or if there's ambiguity in the types, TypeScript might fall back to a more general type, effectively bypassing the strict type checking we desire.

Furthermore, TypeScript's inference is often based on the information it has available at the point of function invocation. If the type information isn't explicitly provided or cannot be unambiguously inferred, TypeScript might not be able to catch subtle type errors. This is why explicitly typing your fetcher function arguments, as we'll discuss later, can be a crucial step in preventing these issues.

Addressing the Mistyping: Solutions and Best Practices

Okay, so we understand the problem. Now, let's get to the good stuff: how do we fix it? There are several strategies we can employ to ensure our fetcher arguments are correctly typed and that TypeScript catches any mismatches.

1. Explicitly Type Your Fetcher Arguments

This is the most straightforward and effective way to address the issue. By explicitly typing the arguments of your fetcher function, you're providing TypeScript with the necessary information to perform accurate type checking. This eliminates any ambiguity and forces TypeScript to verify that the arguments you're passing to useSWR are compatible with the fetcher's expected types.

Let's revisit our initial examples and see how explicit typing can make a difference:

// Explicitly type the fetcher argument as a string
useSWR<string, any, string>("test", (id: string) => {})

// Explicitly type the fetcher argument as an array of strings
useSWR<string[], any, string[]>(["test"], (id: string[]) => {})

In these corrected examples, we've added type annotations to the fetcher arguments (id: string and id: string[]). We've also added generic type arguments to useSWR to explicitly define the data type, error type, and key type. This tells TypeScript exactly what types to expect, and any mismatch will now be flagged as a type error. The use of the generic types <string, any, string> and <string[], any, string[]> clarifies the data type, error type, and key type respectively, ensuring that TypeScript has all the necessary information for accurate type checking.

2. Leverage TypeScript's Type Inference (When Possible)

While explicit typing is generally recommended, TypeScript's type inference can be quite powerful in certain situations. If your key is simple and the fetcher function is straightforward, TypeScript might be able to infer the correct types automatically. However, it's crucial to be cautious and test your assumptions.

For instance, if you're using a simple string key and your fetcher function directly uses that string, TypeScript might infer the type correctly:

useSWR("test", (id) => {
  // TypeScript might infer 'id' as string
  return fetch(`/api/data/${id}`);
});

In this case, TypeScript might infer that id is a string because it's being used in a string interpolation. However, relying solely on inference can be risky, especially as your codebase grows and becomes more complex. Explicit typing provides a much more robust and maintainable solution.

3. Review and Refine SWR's Type Definitions (If Necessary)

In some cases, the issue might stem from the SWR type definitions themselves. While SWR's typings are generally well-maintained, there might be edge cases or specific scenarios where they could be improved. If you encounter a situation where you believe the type definitions are too broad or not accurately reflecting the expected types, consider contributing to the SWR project by submitting a pull request with improved typings.

This is an advanced approach and requires a deep understanding of TypeScript's type system and SWR's internal workings. However, contributing to open-source projects like SWR is a great way to give back to the community and help improve the overall developer experience.

4. Use a Linter and Code Formatter

A linter like ESLint and a code formatter like Prettier can help you maintain consistent code style and catch potential errors, including type-related issues. Configuring your linter with TypeScript-specific rules can help you enforce best practices and prevent common mistakes.

For example, you can use ESLint with the @typescript-eslint/eslint-plugin to enforce rules like no-explicit-any or explicit-function-return-type. These rules can help you catch situations where you're not explicitly typing your function arguments or return types, which can contribute to mistyped fetcher arguments.

Real-World Scenarios and Examples

Let's look at some real-world scenarios where these mistyped fetcher arguments can cause problems and how we can address them with explicit typing.

Scenario 1: Fetching User Data by ID

Imagine you're building a user profile page and need to fetch user data based on a user ID. You might use useSWR like this:

function UserProfile({ userId }: { userId: string }) {
  const { data, error } = useSWR(["/api/users", userId], (key, id) =>
    fetch(`/api/users/${id}`).then((res) => res.json())
  );

  if (error) return <div>Failed to load user</div>;
  if (!data) return <div>Loading...</div>;

  return (
    <div>
      <h1>{data.name}</h1>
      <p>{data.email}</p>
    </div>
  );
}

In this example, we're passing an array ["/api/users", userId] as the key. We expect the fetcher function to receive this array as its argument. However, we've defined the fetcher to receive two separate arguments: key and id. This is a common mistake that can lead to runtime errors.

To fix this, we need to explicitly type the fetcher argument as an array:

function UserProfile({ userId }: { userId: string }) {
  const { data, error } = useSWR<{
    name: string;
    email: string;
  },
  any,
  [string, string]>(["/api/users", userId], ([key, id]) =>
    fetch(`/api/users/${id}`).then((res) => res.json())
  );

  if (error) return <div>Failed to load user</div>;
  if (!data) return <div>Loading...</div>;

  return (
    <div>
      <h1>{data.name}</h1>
      <p>{data.email}</p>
    </div>
  );
}

Here, we've explicitly typed the fetcher argument as ([key, id]). We've also used the generic types in useSWR to specify the data type ({ name: string; email: string; }), the error type (any), and the key type ([string, string]). This ensures that TypeScript correctly infers the types and catches any mismatches.

Scenario 2: Fetching Data with Multiple Parameters

Another common scenario is fetching data with multiple parameters. For example, you might want to fetch a list of products based on category and price range. You could use useSWR like this:

function ProductList({ category, minPrice, maxPrice }: {
  category: string;
  minPrice: number;
  maxPrice: number;
}) {
  const { data, error } = useSWR(
    ["/api/products", category, minPrice, maxPrice],
    (key, cat, min, max) =>
      fetch(`/api/products?category=${cat}&minPrice=${min}&maxPrice=${max}`).then((res) =>
        res.json()
      )
  );

  if (error) return <div>Failed to load products</div>;
  if (!data) return <div>Loading...</div>;

  return (
    <ul>
      {data.map((product) => (
        <li key={product.id}>{product.name}</li>
      ))}
    </ul>
  );
}

In this case, we're passing an array ["/api/products", category, minPrice, maxPrice] as the key. We expect the fetcher function to receive this array. However, we've defined the fetcher to receive four separate arguments: key, cat, min, and max. This is another example of a mistyped fetcher argument.

To fix this, we need to explicitly type the fetcher argument as an array:

function ProductList({
  category,
  minPrice,
  maxPrice,
}: {
  category: string;
  minPrice: number;
  maxPrice: number;
}) {
  const { data, error } = useSWR<{
    id: string;
    name: string;
  }[],
  any,
  [string, string, number, number]>(
    ["/api/products", category, minPrice, maxPrice],
    ([key, cat, min, max]) =>
      fetch(`/api/products?category=${cat}&minPrice=${min}&maxPrice=${max}`).then((res) =>
        res.json()
      )
  );

  if (error) return <div>Failed to load products</div>;
  if (!data) return <div>Loading...</div>;

  return (
    <ul>
      {data.map((product) => (
        <li key={product.id}>{product.name}</li>
      ))}
    </ul>
  );
}

Here, we've explicitly typed the fetcher argument as ([key, cat, min, max]). We've also used the generic types in useSWR to specify the data type ({ id: string; name: string; }[]), the error type (any), and the key type ([string, string, number, number]). This ensures that TypeScript correctly infers the types and catches any mismatches.

Conclusion: Mastering SWR and TypeScript

Mistyped fetcher arguments can be a tricky issue when working with SWR and TypeScript. However, by understanding the underlying causes and employing the solutions we've discussed, you can ensure your code is type-safe and prevent unexpected runtime errors. Explicitly typing your fetcher arguments is the most effective strategy, but leveraging TypeScript's type inference when appropriate and using linters and code formatters can also help. Remember, a little extra attention to detail in your type definitions can save you a lot of headaches down the road.

By mastering the interplay between SWR and TypeScript, you'll be well-equipped to build robust, scalable, and maintainable applications. So, keep those type definitions sharp, and happy coding, guys!