Test the capability in the environment that will run your code. For a runtime API, check the relevant property or method on its owning object; for behavior-sensitive features, test the behavior you need. Use the result to select a fallback. CSS has separate feature queries, while JavaScript syntax must be accepted by the parser before any runtime check can execute.
First identify what “support” means
“Modern JavaScript” is too broad to test. Name the exact syntax feature or API member, then identify the target host: a browser, embedded webview, server-side runtime, or another JavaScript environment. Different hosts and versions can provide different capabilities, so test the feature itself rather than assuming support from a browser brand.
There are two distinct questions:
- Does this API or property exist at runtime? Check the relevant member on the object that owns it.
- Can this runtime parse this syntax? Check compatibility for the exact target and use a build strategy or alternative implementation that produces code the target can parse.
Check an API on its owning object
For an API entry point, test the property on the object where the API is exposed. MDN uses the Geolocation API as an example: the API is accessed through navigator.geolocation.
if ("geolocation" in navigator) {
navigator.geolocation.getCurrentPosition(onPosition);
} else {
showStaticMap();
}The in operator checks whether the property exists on the object or its prototype chain. You can also check whether a required method is a function when that is the specific capability your code needs. Do not call a potentially missing member before checking it.
#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.
A detected entry point does not establish that an operation will succeed: permission, device availability, user settings, or other runtime conditions may still matter. Handle those outcomes in the API call itself, separately from feature detection.
When presence is not enough, test behavior
A property can exist without providing the behavior your application depends on. MDN’s feature-detection guidance describes checking a property or method, examining a return value, or seeing whether an assigned value is retained. Choose the narrowest safe test that represents the capability you actually need.
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.
- Check a method’s presence before calling it.
- Where a result matters, verify a meaningful return value or observable outcome.
- Keep behavior tests scoped so they do not trigger unwanted side effects.
- For implementation differences that could affect critical behavior, test in the supported environments rather than treating a presence check as a guarantee.
Some features do not have a reliable simple detection test. In those cases, consider an appropriate polyfill or a different implementation instead of relying on a misleading check.
Do not use runtime checks to protect unsupported syntax
An API check executes after the source has been parsed. If the target runtime cannot parse syntax in the file, execution may fail before it reaches a surrounding if or try block. A member-presence test therefore cannot make unsupported grammar safe in that same source file.
Do these 3 things before closing this tab:
1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesRank #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.
For syntax-dependent code, look up the exact feature and target runtime versions in compatibility data, then configure your build to transform syntax as needed or provide an alternative source path. Test the resulting code in the environments you intend to support.
Use CSS feature queries for CSS
CSS support is separate from JavaScript grammar and general API detection. If the decision is only about styling, MDN recommends CSS @supports rather than moving the decision into JavaScript.
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
.layout {
display: grid;
}
@supports (grid-template-columns: subgrid) {
.layout {
grid-template-columns: subgrid;
}
}When JavaScript itself needs to choose behavior based on a CSS declaration, use CSS.supports(). It accepts a property/value pair or a support-condition string and returns a boolean.
if (CSS.supports("grid-template-columns", "subgrid")) {
loadSubgridStyles();
} else {
loadFallbackStyles();
}These queries answer whether the CSS declaration or condition is supported; they do not test JavaScript syntax or the availability of an unrelated API. See MDN’s reference for the static CSS.supports() method.
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.
Use compatibility data to plan targets
Runtime feature detection tells you what is available where the code runs. Compatibility references help you decide which runtimes to support and what to build for them. MDN Browser Compatibility Data (BCD) provides machine-readable information about web APIs, JavaScript features, CSS, and browser/runtime support. Its detailed entries are updated as features ship and bugs are found, so check the exact feature and relevant runtime versions when making deployment decisions.
Compatibility tables are reference data, not a guarantee about a customized host, embedded webview, or runtime with unusual feature flags. For critical behavior, combine compatibility planning with tests in the actual supported environments.
Why browser-name checks are a poor substitute
A browser’s identity does not establish that a particular capability exists. An older version may lack it, another browser may support it, and user-agent strings can identify multiple or pretend browsers. A branch based on identity can therefore take the wrong path even when the feature itself is available—or assume support when it is not.
Use browser-specific detection only for an exceptional browser-specific workaround. For ordinary capability decisions, test the feature and keep a useful fallback. MDN explains the limitations of browser detection compared with feature detection.
Quick Recap
Choose the right check
| Approach | Best for | What it establishes | Main caution |
|---|---|---|---|
| Object or member check | Runtime APIs and properties | The relevant entry point exists on the object | Presence may not establish behavior, permission, or state. |
| Focused behavior test | Features where implementation behavior matters | The tested behavior works for that specific test | Keep it safe and narrow; it may not cover every edge case. |
CSS.supports() or @supports |
CSS declarations or values | The CSS feature query is accepted | It does not test JavaScript grammar or a general API. |
| MDN BCD or compatibility tables | Planning support for target runtimes | Documented compatibility by feature and runtime | Entries evolve; check versions and treat the data as a reference. |
| Browser or user-agent detection | Exceptional browser-specific workarounds | A clue about browser identity | Identity is not equivalent to capability and can select the wrong branch. |
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.
