cache
cache
lets you cache the result of a data fetch or computation.
const cachedFn = cache(fn);
Reference
cache(fn)
Call cache
outside of any components to create a version of the function with caching.
import {cache} from 'react';
import calculateMetrics from 'lib/metrics';
const getMetrics = cache(calculateMetrics);
function Chart({data}) {
const report = getMetrics(data);
// ...
}
When getMetrics
is first called with data
, getMetrics
will call calculateMetrics(data)
and store the result in cache. If getMetrics
is called again with the same data
, it will return the cached result instead of calling calculateMetrics(data)
again.
Parameters
fn
: The function you want to cache results for.fn
can take any arguments and return any value.
Returns
cache
returns a cached version of fn
with the same type signature. It does not call fn
in the process.
When calling cachedFn
with given arguments, it first checks if a cached result exists in the cache. If a cached result exists, it returns the result. If not, it calls fn
with the arguments, stores the result in the cache, and returns the result. The only time fn
is called is when there is a cache miss.
Caveats
- React will invalidate the cache for all memoized functions for each server request.
- Each call to
cache
creates a new function. This means that callingcache
with the same function multiple times will return different memoized functions that do not share the same cache. cachedFn
will also cache errors. Iffn
throws an error for certain arguments, it will be cached, and the same error is re-thrown whencachedFn
is called with those same arguments.cache
is for use in Server Components only.
Usage
Cache an expensive computation
Use cache
to skip duplicate work.
import {cache} from 'react';
import calculateUserMetrics from 'lib/user';
const getUserMetrics = cache(calculateUserMetrics);
function Profile({user}) {
const metrics = getUserMetrics(user);
// ...
}
function TeamReport({users}) {
for (let user in users) {
const metrics = getUserMetrics(user);
// ...
}
// ...
}
If the same user
object is rendered in both Profile
and TeamReport
, the two components can share work and only call calculateUserMetrics
once for that user
.
Assume Profile
is rendered first. It will call getUserMetrics
, and check if there is a cached result. Since it is the first time getUserMetrics
is called with that user
, there will be a cache miss. getUserMetrics
will then call calculateUserMetrics
with that user
and write the result to cache.
When TeamReport
renders its list of users
and reaches the same user
object, it will call getUserMetrics
and read the result from cache.
Share a snapshot of data
To share a snapshot of data between components, call cache
with a data-fetching function like fetch
. When multiple components make the same data fetch, only one request is made and the data returned is cached and shared across components. All components refer to the same snapshot of data across the server render.
import {cache} from 'react';
import {fetchTemperature} from './api.js';
const getTemperature = cache(async (city) => {
return await fetchTemperature(city);
});
async function AnimatedWeatherCard({city}) {
const temperature = await getTemperature(city);
// ...
}
async function MinimalWeatherCard({city}) {
const temperature = await getTemperature(city);
// ...
}
If AnimatedWeatherCard
and MinimalWeatherCard
both render for the same city, they will receive the same snapshot of data from the memoized function.
If AnimatedWeatherCard
and MinimalWeatherCard
supply different city arguments to getTemperature
, then fetchTemperature
will be called twice and each call site will receive different data.
The city acts as a cache key.
Preload data
By caching a long-running data fetch, you can kick off asynchronous work prior to rendering the component.
const getUser = cache(async (id) => {
return await db.user.query(id);
}
async function Profile({id}) {
const user = await getUser(id);
return (
<section>
<img src={user.profilePic} />
<h2>{user.name}</h2>
</section>
);
}
function Page({id}) {
// ✅ Good: start fetching the user data
getUser(id);
// ... some computational work
return (
<>
<Profile id={id} />
</>
);
}
When rendering Page
, the component calls getUser
but note that it doesn’t use the returned data. This early getUser
call kicks off the asynchronous database query that occurs while Page
is doing other computational work and rendering children.
When rendering Profile
, we call getUser
again. If the initial getUser
call has already returned and cached the user data, when Profile
asks and waits for this data, it can simply read from the cache without requiring another remote procedure call. If the initial data request hasn’t been completed, preloading data in this pattern reduces delay in data-fetching.
গভীরভাবে জানুন
When evaluating an asynchronous function, you will receive a Promise for that work. The promise holds the state of that work (pending, fulfilled, failed) and its eventual settled result.
In this example, the asynchronous function fetchData
returns a promise that is awaiting the fetch
.
async function fetchData() {
return await fetch(`https://...`);
}
const getData = cache(fetchData);
async function MyComponent() {
getData();
// ... some computational work
await getData();
// ...
}
In calling getData
the first time, the promise returned from fetchData
is cached. Subsequent look-ups will then return the same promise.
Notice that the first getData
call does not await
whereas the second does. await
is a JavaScript operator that will wait and return the settled result of the promise. The first getData
call simply initiates the fetch
to cache the promise for the second getData
to look-up.
If by the second call the promise is still pending, then await
will pause for the result. The optimization is that while we wait on the fetch
, React can continue with computational work, thus reducing the wait time for the second call.
If the promise is already settled, either to an error or the fulfilled result, await
will return that value immediately. In both outcomes, there is a performance benefit.
গভীরভাবে জানুন
All mentioned APIs offer memoization but the difference is what they’re intended to memoize, who can access the cache, and when their cache is invalidated.
useMemo
In general, you should use useMemo
for caching a expensive computation in a Client Component across renders. As an example, to memoize a transformation of data within a component.
'use client';
function WeatherReport({record}) {
const avgTemp = useMemo(() => calculateAvg(record)), record);
// ...
}
function App() {
const record = getRecord();
return (
<>
<WeatherReport record={record} />
<WeatherReport record={record} />
</>
);
}
In this example, App
renders two WeatherReport
s with the same record. Even though both components do the same work, they cannot share work. useMemo
’s cache is only local to the component.
However, useMemo
does ensure that if App
re-renders and the record
object doesn’t change, each component instance would skip work and use the memoized value of avgTemp
. useMemo
will only cache the last computation of avgTemp
with the given dependencies.
cache
In general, you should use cache
in Server Components to memoize work that can be shared across components.
const cachedFetchReport = cache(fetchReport);
function WeatherReport({city}) {
const report = cachedFetchReport(city);
// ...
}
function App() {
const city = "Los Angeles";
return (
<>
<WeatherReport city={city} />
<WeatherReport city={city} />
</>
);
}
Re-writing the previous example to use cache
, in this case the second instance of WeatherReport
will be able to skip duplicate work and read from the same cache as the first WeatherReport
. Another difference from the previous example is that cache
is also recommended for memoizing data fetches, unlike useMemo
which should only be used for computations.
At this time, cache
should only be used in Server Components and the cache will be invalidated across server requests.
memo
You should use memo
to prevent a component re-rendering if its props are unchanged.
'use client';
function WeatherReport({record}) {
const avgTemp = calculateAvg(record);
// ...
}
const MemoWeatherReport = memo(WeatherReport);
function App() {
const record = getRecord();
return (
<>
<MemoWeatherReport record={record} />
<MemoWeatherReport record={record} />
</>
);
}
In this example, both MemoWeatherReport
components will call calculateAvg
when first rendered. However, if App
re-renders, with no changes to record
, none of the props have changed and MemoWeatherReport
will not re-render.
Compared to useMemo
, memo
memoizes the component render based on props vs. specific computations. Similar to useMemo
, the memoized component only caches the last render with the last prop values. Once the props change, the cache invalidates and the component re-renders.
Troubleshooting
My memoized function still runs even though I’ve called it with the same arguments
See prior mentioned pitfalls
- Calling different memoized functions will read from different caches.
- Calling a memoized function outside of a component will not use the cache.
If none of the above apply, it may be a problem with how React checks if something exists in cache.
If your arguments are not primitives (ex. objects, functions, arrays), ensure you’re passing the same object reference.
When calling a memoized function, React will look up the input arguments to see if a result is already cached. React will use shallow equality of the arguments to determine if there is a cache hit.
import {cache} from 'react';
const calculateNorm = cache((vector) => {
// ...
});
function MapMarker(props) {
// 🚩 Wrong: props is an object that changes every render.
const length = calculateNorm(props);
// ...
}
function App() {
return (
<>
<MapMarker x={10} y={10} z={10} />
<MapMarker x={10} y={10} z={10} />
</>
);
}
In this case the two MapMarker
s look like they’re doing the same work and calling calculateNorm
with the same value of {x: 10, y: 10, z:10}
. Even though the objects contain the same values, they are not the same object reference as each component creates its own props
object.
React will call Object.is
on the input to verify if there is a cache hit.
import {cache} from 'react';
const calculateNorm = cache((x, y, z) => {
// ...
});
function MapMarker(props) {
// ✅ Good: Pass primitives to memoized function
const length = calculateNorm(props.x, props.y, props.z);
// ...
}
function App() {
return (
<>
<MapMarker x={10} y={10} z={10} />
<MapMarker x={10} y={10} z={10} />
</>
);
}
One way to address this could be to pass the vector dimensions to calculateNorm
. This works because the dimensions themselves are primitives.
Another solution may be to pass the vector object itself as a prop to the component. We’ll need to pass the same object to both component instances.
import {cache} from 'react';
const calculateNorm = cache((vector) => {
// ...
});
function MapMarker(props) {
// ✅ Good: Pass the same `vector` object
const length = calculateNorm(props.vector);
// ...
}
function App() {
const vector = [10, 10, 10];
return (
<>
<MapMarker vector={vector} />
<MapMarker vector={vector} />
</>
);
}