The Tool Desk
Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Use useMemo to reuse a calculated value, useCallback to preserve a function’s identity, and memo to let React skip rendering a component when its props are unchanged. None is a correctness guarantee. First make rendering correct, then profile a slow interaction and apply the smallest optimization that addresses the work you find.
How the three memoization tools differ
They operate at different points in a render. useMemo caches a calculation result inside a component; useCallback caches a function definition; and memo wraps a component so React can skip some renders when its props have not changed.
| Tool | What it preserves | Where it acts | What must remain stable | When it may help |
|---|---|---|---|---|
useMemo |
A calculation’s returned value | During a component render | The listed dependencies, compared with Object.is |
A noticeably costly pure calculation with inputs that often stay the same, or a value whose identity matters to a prop or Hook dependency. React useMemo reference |
useCallback |
A function definition’s identity, not the result of calling it | For a callback passed to another component or used as a dependency | The listed dependencies, compared with Object.is |
A function’s identity matters, for example to a memoized child or another Hook’s dependency list. React useCallback reference |
memo |
An opportunity to skip rendering a component | At the child component boundary | The component’s props | The component often receives the same props and its render work is meaningfully expensive. React memo reference |
These are opportunities to avoid work, not promises that work will never happen. React’s documentation makes the same point for all three: rely on them only as performance optimizations.
When to use useMemo
Use useMemo(() => calculate(inputs), [inputs]) when recalculating a pure value is noticeably expensive and its inputs do not change on relevant renders. It can also preserve the identity of an object or other value when that identity matters to a child’s props or a Hook dependency.
#1 Best Overall
- Read Before You Buy — No Video Output: These adapters support charging and USB 2.0 data transfer, but cannot transmit video signals. Except for standard USB webcams (which use USB data only), they are not compatible with HDMI/DisplayPort cables, video-capable USB-C hubs, or docking stations with video output.
- Convert USB-A Ports to USB-C: Designed to connect USB-C earphones, cables, flash drives, card readers, and other USB-C accessories to standard USB-A ports. Plug-and-play with no drivers or software required.
- Aluminum Alloy Housing: Built with a sturdy aluminum alloy shell that aids in heat dissipation and protects against daily wear and scratches. Designed to maintain a stable and secure connection.
- Compact & Travel-Friendly: The ultra-compact design allows the adapter to stay plugged into your device without blocking adjacent ports or adding bulk, reducing wear and tear on your original USB ports.
- 12-Month Warranty: Backed by a 12-month manufacturer warranty for peace of mind. Designed to meet strict quality control standards for reliable everyday performance.
React compares each dependency with Object.is. If all dependencies compare equal, React can reuse the cached result; when one changes, it recalculates. Include every reactive value used by the calculation in the dependency list. The calculation must be pure: it should return the same result for the same inputs and must not rely on being run exactly once.
const visibleItems = useMemo(
() => filterItems(items, filter),
[items, filter]
);This is useful only if avoiding the calculation or keeping the result’s identity stable addresses a real cost. A cache can be discarded, so application behavior must remain correct if the calculation runs again. In development Strict Mode, React may call the calculation twice to expose impure logic; that development check does not affect production.
Rank #2
- 5-in-1 USB-C Hub: Experience comprehensive connectivity featuring a Power Delivery input, two USB-A 2.0 ports, a USB-A 3.0 port, and an HDMI port. (Note: The USB-C power delivery input port is only for connecting an external wall charger to power your laptop and cannot power peripheral devices.)
- 90W Pass-Through Charging: Achieve optimal charging with 90W pass-through power to your laptop, supported by a total input of 100W, with the hub reserving 10W for operational efficiency. (Note: Wall charger not included.)
- Quick Data Transfers: Accelerate your productivity with rapid data transfers using a high-speed 5Gbps USB 3.0 port and two 480Mbps USB 2.0 ports.
- 4K HDMI Display: Enhance your visual experience with a hub capable of delivering 4K resolution at 30Hz in both mirror and extend modes. Please note that this hub is compatible with MacBook (macOS 12 and newer), Windows 10 and 11, ChromeOS, and laptops equipped with DP Alt Mode and Power Delivery. Note: This device is not compatible with Linux.
- What You Get: Anker USB-C Hub (5-in-1, 4K HDMI), welcome guide, 18-month warranty, and our friendly customer service.
When to use useCallback
Use useCallback(fn, [dependencies]) when the identity of a function matters—for example, when passing it to a memo-wrapped child or using it as a Hook dependency. Include every reactive value the function reads.
const handleSelect = useCallback((id) => {
setSelectedId(id);
}, []);useCallback preserves the function definition between renders while its dependencies stay equal. It does not cache the result of calling that function, and it does not prevent JavaScript from creating the inline function expression passed to the Hook on each render. Its benefit is the stable function identity React returns, not avoiding that expression’s creation.
Rank #3
- Sleek 7-in-1 USB-C Hub: Features an HDMI port, two USB-A 3.0 ports, and a USB-C data port, each providing 5Gbps transfer speeds. It also includes a USB-C PD input port for charging up to 100W and dual SD and TF card slots, all in a compact design.
- Flawless 4K@60Hz Video with HDMI: Delivers exceptional clarity and smoothness with its 4K@60Hz HDMI port, making it ideal for high-definition presentations and entertainment. (Note: Only the HDMI port supports video projection; the USB-C port is for data transfer only.)
- Double Up on Efficiency: The two USB-A 3.0 ports and a USB-C port support a fast 5Gbps data rate, significantly boosting your transfer speeds and improving productivity.
- Fast and Reliable 85W Charging: Offers high-capacity, speedy charging for laptops up to 85W, so you spend less time tethered to an outlet and more time being productive.
- What You Get: Anker USB-C Hub (7-in-1), welcome guide, 18-month warranty, and our friendly customer service.
The equivalent idea can be expressed with useMemo(() => fn, dependencies), but useCallback is the clearer API for preserving a function.
When to use memo
Wrap a component in memo when it often receives the same props and rendering it is costly enough to matter. React can then skip rendering it when props are unchanged.
Rank #4
- Dual Converters, Infinite Potential：Includes 2× USB C male to USB A female adapters and 2× USB A male to USB C female adapters. Perfect for a wide range of uses—tablets with Bluetooth keyboards, expand USB ports on macbook, and more. Two different converters for all your daily needs
- Next-Level 10Gbps & 3A Charging: No more slow 480Mbps, this usb to usb c adapter has a transfer speed of up to 10Gbps, allowing you to do more transferring in less time. This usb adapter fits both USB A and USB C charger, supporting up to 3A fast charging
- Upgraded Exquisite Craftsmanship: With an aluminum alloy housing and metal connector, the usbc to usb adapter is extremely durable and sturdy. Rigorously tested to withstand more than 10,000 times of plugging and unplugging, ensuring long-lasting performance
- Broad Compatible: The usb c to usb adapter widely supports all USB C/ USB A devices like laptops, tablets, cellphones, car chargers, and phone chargers. Such as compatible with MacBook Pro/Air 2023/2022, Thunderbolt 4/3 Devices,Apple MagSafe Watch 9/8/7/SE/Ultra, iPad Pro 2022/2021, Samsung Galaxy S23/S20/S10, and iPhone 17/16/15 Pro. Plug and play
- Please Note: To reach 10Gbps speed, keep the cable under 3.3 ft. For USB A Male to USB C adapters, try flipping the USB C connector. USB C Male to USB A adapters support bidirectional 10Gbps transfer within 3.3 ft
const ResultsList = memo(function ResultsList({ items, onSelect }) {
return items.map(item => (
<button key={item.id} onClick={() => onSelect(item.id)}>
{item.name}
</button>
));
});Memoization is most effective when props are stable. Primitive props are usually straightforward; a newly created object, array, or function is a new prop identity and can make the memo ineffective. Stabilize such a value only when measurement shows the child’s render is worth avoiding, or move its creation to a place where it does not need to be recreated.
memo does not guarantee that a component will never render. It is an optimization React may use when the props are unchanged, not a way to control correctness or component lifecycle.
Best Value
- 5-in-1 Connectivity: Equipped with a 4K HDMI port, a 5 Gbps USB-C data port, two 5 Gbps USB-A ports, and a USB C 100W PD-IN port. Note: The USB C 100W PD-IN port supports only charging and does not support data transfer devices such as headphones or speakers.
- Powerful Pass-Through Charging: Supports up to 85W pass-through charging so you can power up your laptop while you use the hub. Note: Pass-through charging requires a charger (not included). Note: To achieve full power for iPad, we recommend using a 45W wall charger.
- Transfer Files in Seconds: Move files to and from your laptop at speeds of up to 5 Gbps via the USB-C and USB-A data ports. Note: The USB C 5Gbps Data port does not support video output.
- HD Display: Connect to the HDMI port to stream or mirror content to an external monitor in resolutions of up to 4K@30Hz. Note: The USB-C ports do not support video output.
- What You Get: Anker 332 USB-C Hub (5-in-1), welcome guide, our worry-free 18-month warranty, and friendly customer service.
How to decide what to optimize
- Make the component correct without caches. Keep render logic pure and ensure behavior does not depend on a calculation, callback, or child being skipped.
- Look for avoidable work before adding memoization. Prefer keeping state local rather than lifting it unnecessarily, avoid Effects that trigger redundant state updates, and simplify dependency structure where possible.
- Profile the slow interaction. Identify whether the delay comes from a calculation, repeated child rendering, or another source. The React references do not set a numeric threshold for “expensive”; measure your own application.
- Apply the narrowest relevant tool. Use
useMemofor a costly value calculation or identity-sensitive value,useCallbackfor a function identity that matters, andmemofor an expensive child receiving unchanged props. - Measure again. Confirm that the change improves the interaction and has not added needless complexity.
Hooks such as useMemo and useCallback must be called at the top level of a component or custom Hook, not inside conditions or loops. See the useMemo and useCallback references for their rules and dependency guidance.
Should you add useMemo or useCallback everywhere?
No. Manual memoization adds code and dependency lists, and often does not solve a meaningful performance problem. Start with correct, straightforward rendering; profile when an interaction is slow; then optimize the identified work. This is also React’s guidance for useMemo and useCallback.
What React Compiler changes
React’s current documentation says React Compiler automatically memoizes values and functions, reducing the need for manually added useMemo and useCallback. It also describes manual Hooks as a way to retain control. Whether this changes what you should write depends on the project: do not assume the compiler is enabled. Check your configured toolchain and follow the guidance for that setup. React Compiler: Introduction.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
