React Query Selectors, Supercharged

04.08.2025 — ReactJs, React Query, TypeScript, Performance — 6 min read

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Ah, select. One of my favourite features in React Query that you'll hopefully never need. But if you need, it can be a life-saver.

Global State and Subscriptions

As I've described in #18: Inside React Query, React Query consists of one global state - the QueryCache - that holds information about all Queries. Whenever something in a Query changes, we want to inform all QueryObservers (the things that get created with useQuery) about that change.

Now ideally, we'd want to avoid that every component is subscribed to everything. A change in the todos Query shouldn't re-render a component that is only interested in the profile Query. Otherwise, we could've just used a top-level useState and distribute that with React Context throughout our application. Controlling subscriptions, and making them fine-grained, is why state management solutions exist in the first place.

The QueryHash

As you may know, React Query has the above mentioned feature built-in. Of course, useQuery doesn't subscribe to the whole QueryCache. The QueryKey you pass to useQuery gets hashed deterministically into a QueryHash, and useQuery will only get notified about changes to that Query.

In essence, it's like we're pre-filtering down to the Query you're interested in. And mostly, that's enough. Data from one endpoint changes, your component re-renders. Data from another endpoint changes, it doesn't. So what more do we need?

Fine-grained Subscriptions

Sometimes, endpoints return a lot of data, and we aren't interested in all of that. Especially in situations where we have fields that change often, and fields that change rarely, we might want to have subscriptions that are more fine-grained while still storing the full response in the cache. To get there, we can use select.

What is `select`?

select is an option we can pass to useQuery to pick, transform, or otherwise compute a result that our component should subscribe to. It's a way to get derived state that is very similar to how deriving data with selectors works in redux.

Let's say we fetch some product data from an API of ours:

productOptions

1const productOptions = (id: string) => {
2  return queryOptions({
3    queryKey: ['product', id],
4    queryFn: () => fetchProduct(id),
5  })
6}

and we want to create a component that only renders the product title

ProductTitle

1function ProductTitle({ id }: Props) {
2  const productQuery = useSuspenseQuery(productOptions(id))
3
4  return <h1>{productQuery.data.title}</h1>
5}

That will certainly work, and is likely fine for most situations. However, the endpoint could also return information that might change a lot more often than the title, like the number of purchases or comments.

Of course, an occasional re-render of this simple component just because something unrelated changes isn't the end of the world, but let's assume we really want to optimize this.

That's where select can help: Instead of subscribing our component to all product data, we're gonna pick the field (or fields) we're interested in:

select-title

1function ProductTitle({ id }: Props) {
2  const productTitleQuery = useSuspenseQuery({
3    ...productOptions(id),
4    select: (data) => data.title,
5  })
6
7  return <h1>{productTitleQuery.data}</h1>
8}

A component that uses select will only be subscribed to the result of the select function, so it will only re-render if that result changes. In this example, since the title probably doesn't change often, the component will rarely need to re-render, even if other properties of the product data change frequently.

Even better - we can also "pick" multiple properties if we want to, and we don't have to worry about referential stability, because React Query uses structural sharing on the select result. This is different than some other libraries, like zustand, where atomic selectors are preferred.

In essence, this means code like this will also work as you'd expect:

pick-multiple-values

1function Product({ id }: Props) {
2  const productQuery = useSuspenseQuery({
3    ...productOptions(id),
4    select: (data) => ({
5      title: data.title,
6      description: data.description,
7    }),
8  })
9
10  return (
11    <main>
12      <h1>{productQuery.data.title}</h1>
13      <p>{productQuery.data.description}</p>
14    </main>
15  )
16}

If either title or description changes, we get a re-render, otherwise not. That's pretty cool.😎

Typing Select Abstractions

You might not have noticed it in the examples above, but they are all valid TypeScript code. In fact, they are not only valid, but also type-safe and inferred. That means the data field on the object you get back from useQuery will be typed to whatever select returns.

However, this only works if you let type inference do it's magic 🪄, which means we don't want to manually "provide" generic type parameters to useQuery. Please read #6: React Query and TypeScript for more information on that if you haven't done so already.

This is relatively easy to adhere to, but brings one question: How can I write a reusable abstraction in TypeScript that includes select? What type sorcery is needed to e.g. make productOptions that get select as an argument?

how-do-I-type-select?

1const productOptions = (id:string, select) => {
2  return queryOptions({
3    queryKey: ['product', id],
4    queryFn: () => fetchProduct(id),
5    // ⬇️ how do I type select ?
6    select,
7  })
8}

My usual answer is - don't do it! We've built the Query Options API specifically to make it easier to abstract shared options away, while at the same time allowing you to specify additional options directly at the usage site. That's why all the code we've seen so far shows almost no indication that TypeScript is even used. Isn't that great?

But TkDodo, what if I really want it? Okay, fine, if you must do this, it's a bit more involved, especially if you want select to be optional:

select-abstraction

1const productOptions = <TData = ProductData>(
2  id: string,
3  select?: (data: ProductData) => TData
4) => {
5  return queryOptions({
6    queryKey: ['product', id],
7    queryFn: () => fetchProduct(id),
8    select,
9  })
10}

The trick here is to add a type parameter TData that defaults to whatever your queryFn returns, and then define select as a function from that type towards TData. That way, if you don't provide select, your data will be ProductsData, but if you do provide the transformation function, you will get back whatever this function returns. Here's a TypeScript Playground if you're curious.

Supercharging `select` with Memoization

Since we're already optimizing, why not take it one step further and assume that our function we run in select is really expensive. Like, maybe we're looping through a large product list and calculate the average rating for each product based on thousands of user reviews, filtering out invalid entries, and sorting the top-rated items — all in one pass. We'll call it expensiveSuperTransformation from now on. If we write our code normally with select, here's what we get:

expensiveSuperTransformation

1function ProductList({ filters }: Props) {
2  const productsQuery = useSuspenseQuery({
3    ...productListOptions(filters),
4    select: (data) => expensiveSuperTransformation(data),
5  })
6
7  return (
8    <ul>
9      {productsQuery.data.map((product) => (
10        <li>{product.summary}</li>
11      ))}
12    </ul>
13  )
14}

This should work fine, but there's one catch: Our expensiveSuperTransformation will now run on every render - not matter what caused it. To understand why, we have to know that React Query will re-run the select function in two cases:

When data changes.
This should be obvious, because if we get new data, we might get a new result from our transformation, so we have to run it. This is fine.
When the select function itself changes.
React Query tracks the referential identity of the select function as a additional performance measure. If it gets "the same function" passed in, it knows that it can only produce the same result, so it can skip running it again (Unless you rely on shared mutable state inside it, so just don't do that 😅).

Inline functions though are always newly created, on each render, so that optimization doesn't apply. This is actually a good feature, because it means we can close over additional props and don't have to worry about stale results:

minRating

1function ProductList({ filters, minRating }: Props) {
2  const productsQuery = useSuspenseQuery({
3    ...productListOptions(filters),
4    select: (data) => expensiveSuperTransformation(data, minRating),
5  })
6
7  return (
8    <ul>
9      {productsQuery.data.map((product) => (
10        <li>{product.summary}</li>
11      ))}
12    </ul>
13  )
14}

Now, if minRating changes, we're quite happy that the select function re-runs to give us the newest computation. So how can we tell React Query that it's fine to skip the computation in the first example, but not in the second?

Stabilizing `select`

The key is to pass a stable reference to select, and React has a good, built-in way to achieve that: useCallback.

select-useCallback

1function ProductList({ filters, minRating }: Props) {
2  const productsQuery = useSuspenseQuery({
3    ...productListOptions(filters),
4    select: React.useCallback(
5      (data) => expensiveSuperTransformation(data, minRating),
6      [minRating]
7    ),
8  })
9
10  return (
11    <ul>
12      {productsQuery.data.map((product) => (
13        <li>{product.summary}</li>
14      ))}
15    </ul>
16  )
17}

Now, we'll get a stable reference passed to select unless minRating changes, and that's exactly what we want. And if we don't have any dependencies, we can even just move the function outside of our component to make it stable - no useCallback with empty dependency array necessary:

stable-select

1const select = (data: Array<Product>) =>
2  expensiveSuperTransformation(data)
3
4function ProductList({ filters }: Props) {
5  const productsQuery = useSuspenseQuery({
6    ...productListOptions(filters),
7    select,
8  })
9
10  return (
11    <ul>
12      {productsQuery.data.map((product) => (
13        <li>{product.summary}</li>
14      ))}
15    </ul>
16  )
17}

The Final Boss

This works great so far, but wait, there's more. What happens if we render the same component multiple times now. How often do you think select would run?

Well, it runs once per component. Specifically, once per QueryObserver, because that's where the result of select is cached. Each call to useQuery creates a QueryObserver, so it has to run select at least once for each of those.

That's a bummer because it means our expensiveSuperTransformation might still run more than once per data, and we really don't want that. What can we do?

We add more memoization, of course!

More Memoization

We really just want to memoize our expensiveSuperTransformation by its inputs, but this has to happen outside of React Query, because React Query caches this per observer. So, we can bring in a library like fast memoize that'll do just that:

fast-memoize

1import memoize from 'fast-memoize'
2
3const select = memoize((data: Array<Product>) =>
4  expensiveSuperTransformation(data)
5)
6
7function ProductList({ filters }: Props) {
8  const productsQuery = useSuspenseQuery({
9    ...productListOptions(filters),
10    select,
11  })
12
13  return (
14    <ul>
15      {productsQuery.data.map((product) => (
16        <li>{product.summary}</li>
17      ))}
18    </ul>
19  )
20}

Let's say we render our ProductList three times now. What happens is that select will run all three times (once per QueryObserver, there's no way around that), but the expensiveSuperTransformation will only run once because it will hit the cache of fast-memoize twice, as it runs with the same data. And if data changes, same thing: three runs of select, but only one run of the expensiveSuperTransformation.

That is as optimized as we can be.

That's it for today. Feel free to reach out to me on bluesky if you have any questions, or just leave a comment below. ⬇️

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