Test the specific 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 itself. Use the result to choose a fallback. CSS has dedicated support queries, while JavaScript syntax support must be checked against the target runtime because unsupported syntax can prevent a file from running at all.
Start by identifying exactly what needs to be supported
“Modern JavaScript” is too broad to test meaningfully. Name the particular syntax feature, API member, or CSS declaration your code needs, then identify where it will execute: a browser, embedded webview, server-side runtime, or another host. A check is useful only when it concerns the capability the code actually depends on.
Feature detection asks whether a capability is available in the environment executing the code, rather than guessing from the browser’s name or version. That makes the result useful for choosing a supported code path or fallback. MDN’s feature detection guide demonstrates this approach.
For runtime APIs, check the owning object
Check for an API entry point on the object that provides it before calling it. For example, test for geolocation on navigator before using the Geolocation API:
#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.
if ("geolocation" in navigator) {
navigator.geolocation.getCurrentPosition(onPosition);
} else {
showStaticMap();
}The fallback should preserve a useful experience when the capability is missing; in this example, the page can show a static map instead. Avoid calling a potentially absent method first and hoping to catch an error afterward.
When presence is not enough, test the behavior
A property’s existence does not always prove that the feature works as your code requires. Depending on the feature, check whether a method exists, whether a test call returns the expected kind of result, or whether an assigned value is retained. Keep such a test narrow and safe: a probe should not trigger a costly operation or alter user data.
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.
Some capabilities cannot be reliably detected with a simple test. MDN’s feature detection guidance discusses property, method, return-value, and retained-value checks, and notes that some features require another approach, such as a polyfill where appropriate.
Syntax support is different from API availability
An API check runs after the JavaScript parser has accepted the file. Syntax support is a parser question: if the target runtime does not understand a construct, it may reject the source before any runtime check executes. A try/catch surrounding unsupported syntax in the same file is therefore not a general solution; execution may never reach the handler.
Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteRank #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, consult compatibility information for the exact language feature and target runtimes, then use a build strategy that produces compatible code or provide an alternative implementation. MDN’s compatibility tables include JavaScript features and runtime support. Compatibility data helps with planning, but it does not guarantee behavior in a customized host or an environment with feature flags.
Use CSS support queries for CSS features
If JavaScript needs to choose behavior based on whether a CSS declaration is supported, use CSS.supports(). It accepts a property/value pair or a support-condition string and returns a boolean:
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
if (CSS.supports("grid-template-columns", "subgrid")) {
loadSubgridStyles();
} else {
loadFallbackStyles();
}When the decision is only about styling, keep it in CSS with @supports:
.layout {
display: grid;
}
@supports (grid-template-columns: subgrid) {
.layout {
grid-template-columns: subgrid;
}
}MDN describes the @supports rule as the preferred approach when CSS alone can make the decision. These queries test CSS support; they do not test JavaScript syntax or general-purpose APIs.
The Tool Desk
Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →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, then test critical behavior
Check a current compatibility reference for the exact feature and every runtime/version you intend to support. MDN Browser Compatibility Data (BCD) is machine-readable and covers web APIs, JavaScript features, CSS, and browser/runtime support. Its detailed entries are updated as features ship and bugs are found, so check the relevant entry rather than relying on a broad claim that a browser supports “modern JavaScript.” See the MDN Browser Compatibility Data project.
Compatibility tables complement runtime checks; they answer different questions. Tables help plan for known targets, while a feature test tells your code what is available in the environment currently executing it. For critical behavior, validate the actual behavior in supported environments where implementations may differ. A feature-presence result is evidence of availability, not proof that every edge case behaves identically.
Choose the check that matches the question
| Approach | Best for | What it establishes | Main caution |
|---|---|---|---|
| Object or member check | Runtime APIs and properties | The relevant entry point exists on its owning object | Presence may not establish behavior, permission, or current state |
| Focused behavior test | Features where implementation behavior matters | The tested behavior works for that test | Keep the test safe and narrow; it may not cover every edge case |
CSS.supports() or @supports |
CSS declarations and 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 | Data evolves; verify the relevant versions and treat it as reference information |
| Browser or user-agent detection | Exceptional browser-specific workarounds | A clue about browser identity | Identity is not capability; user-agent strings can be ambiguous or misleading |
Why browser identity is a poor substitute
A browser may lack a feature in an older version even when a newer version includes it, while another browser may support the same capability. User-agent strings can identify multiple browsers or misrepresent the environment. Branching on browser identity can therefore select the wrong path. Prefer testing the capability itself and supplying a fallback; reserve identity-based workarounds for cases where a browser-specific difference is established. MDN explains the limits of feature detection and browser-specific handling.
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.
