JS Support Checker — Feature Detection
Generate a JavaScript snippet that tests 30 JavaScript features from ES6 to latest proposals.
How it works
- 1Configure your options above
- 2Click "Generate Detection Script" — processing happens in your browser
- 3Copy or download the result
What to do next
About JS Support Checker
JS Support Checker is a free, in-browser web utility tool. Generate a JavaScript snippet that tests 30 JavaScript features from ES6 to latest proposals. The page exposes a small surface — input, controls, output — so a first-time visitor can complete the job without reading documentation.
JS Support Checker works well as a bookmarked utility you reach for when you need it. The first visit shows you what the tool does; the second is when you realise it is a low-friction option for the task and worth keeping in your tab list.
JS Support Checker runs the entire transformation inside your browser. The file is read by JavaScript running in the page, processed in-memory by standard browser APIs, and written back as a download. The browser is the runtime; the page is the interface. You can confirm what the tool does by opening the developer-tools Network tab during a run — the only requests are for the page's own static assets.
Behind the controls you see, standard browser APIs is doing the actual web and productivity utility. Formats are detected on load and the engine produces a deterministic output for any given input + options combination — useful when you need to re-run a job and expect identical results.
A practical note on limits: JS Support Checker accepts inputs up to 0 MB per run, and the tool processes one input at a time to keep memory usage predictable. If you ever bump into the ceiling, the cause is the size of the input.
If you fit any of these descriptions, JS Support Checker should slot cleanly into your workflow: product managers comparing options; teachers building resource lists; researchers gathering quick references. The tool keeps the controls focused on what matters for each of these use cases.
The output handed back by JS Support Checker is the output file. If you would prefer to keep the result in the browser instead of downloading it, you can copy it from the result panel and paste it directly into another tab — useful when the next tool in your workflow expects pasted text rather than a file.
JS Support Checker is intentionally narrow in what it does, which makes it easy to slot into a longer workflow. Take its output, hand it to whichever next tool fits the job, and JS Support Checker stays out of your way until the next time you need it.
JS Support Checker is honest about scope: it handles a single, well-defined web and productivity utility step. Specialist edge-case work — uncommon formats, very large inputs, or pipelines that need scripting — is what dedicated desktop apps are for. This page handles the common case quickly.
Some background on the design choices behind JS Support Checker: every option you see on the page is there because a real workflow needs it, and every option that is not shown has been deliberately omitted to keep the common case fast. The bias is toward minimal-but-complete.
If you also use a command-line tool for js support checker, JS Support Checker is a convenient alternative for the times you are on a different machine or helping someone who is not comfortable in a terminal. The output is a standard file in the format documented above.
If you want to get the most out of JS Support Checker, three small habits help. Drag-and-drop is faster than the file picker once you get used to it. The keyboard shortcut for downloading the result is whatever your browser uses for "save link as," because the result is a normal download. And if you are working on a sensitive file, processing in an Incognito or Private window is a good extra layer — it leaves no trace in browser history when the tab closes.
Common gotchas worth flagging: the supported formats are listed in the upload area. The 0 MB ceiling is per-file, not per-session; you can run as many separate jobs as you like, but a single oversized input will be rejected on load.
Open the workspace above to start using JS Support Checker. The engine loads on the first interaction so the page itself stays light, and once the tool is warm it processes subsequent jobs quickly. The moment the page is interactive, the tool is ready to do real work on your file.
How it works
- 1Reach the JS Support Checker page in your browser to begin.
- 2Select the web utility file you want to process — drag-and-drop and the file picker both work.
- 3Pick any non-default settings you need. Most users leave the defaults alone for the first run and only revisit if the result needs tuning.
- 4Click to start the job. The engine (standard browser APIs) processes the input in the page; you can watch the progress indicator until it completes.
- 5Save the output when it is ready.
- 6Re-run with different settings as often as you want. Each run produces a fresh output and the original file on disk is never modified.
Common use cases
- Compare two product variations side by side using JS Support Checker.
- Pull a quick reference number for a status update.
- Plan content without paying for a SaaS dashboard.
- Sanity-check a webhook response while debugging.
- Audit a marketing page before launch.
- Generate a campaign asset in seconds for a quick test.
- Run a fast accessibility check before publishing.
- Preview how a result looks before deploying it.
FAQ
How many features?
Tests 30 JavaScript features from ES6 basics to cutting-edge proposals like Temporal and Iterator Helpers.
How does it work?
Each feature is tested with a try-catch to check if the syntax or API is available.
Which ES versions?
Covers ES2015 (ES6) through ES2024+ including classes, async/await, optional chaining, and more.
Private?
Yes — runs locally.
Transpilation?
Features showing NO may need Babel/SWC transpilation or polyfills for your target browsers.
Node.js?
Most features also apply to Node.js. Run the script in Node REPL to check server-side support.
Can I use JS Support Checker on documents that contain personal data?
Your file is processed inside your browser by standard browser APIs. The engine reads the file's bytes from your tab's memory, computes the result, and writes the result back into the tab. You can confirm what the page does by opening developer tools and watching the Network tab during a run — the requests you see are for the tool's static assets only.
Can I use JS Support Checker for commercial work?
JS Support Checker can be used for personal and commercial work alike — there is no separate "business" licence to purchase. The output you generate is yours to use however you want, including in client deliverables, internal documents, or commercial products. Favtoo's only ask is fair, individual use; the tool is not designed to be embedded as a backend service or wrapped behind an API for resale.
Does JS Support Checker need an internet connection to run?
Once the page is loaded, JS Support Checker can complete jobs without an active internet connection — the engine is bundled with the page, so there is no per-job network call. The initial page load does require a connection (to fetch the static assets), but after that you can disconnect entirely and the tool will still work. This is a side-effect of the local-first architecture, not a deliberate "offline mode" feature.
What is the maximum file size for JS Support Checker?
Inputs are capped at 0 MB per file, which keeps memory usage stable across phones, tablets and older laptops. You can run JS Support Checker as often as you need; every run produces a full-quality result.
Are there any hidden fees with JS Support Checker?
JS Support Checker is free to use. The processing runs in your browser, which keeps the per-user cost low enough that the tool can be offered openly. The download is the same file the engine produced — you can use it for as many runs as you need.
What permissions does JS Support Checker need to function?
JS Support Checker only needs the standard web platform — file picker access for the inputs you choose to load, and optionally clipboard access if you copy the result rather than downloading it. There is no microphone, camera, geolocation or background-permission request, because none of those are needed for the work the tool does.
Can I self-host JS Support Checker for my team?
JS Support Checker is a static page running an open-source engine in your browser, so a typical corporate firewall does not get in the way as long as it allows JavaScript to load from Favtoo. For teams that need to host it themselves on an internal network, the underlying engine (standard browser APIs) is open-source and can be packaged into a private build with the same behaviour. Reach out via the Contact page if that is something you are exploring.
Will I notice a difference in the output from JS Support Checker?
JS Support Checker is built to preserve quality wherever the underlying web utility format allows it. Operations that are mathematically lossless (e.g. structural transformations, lossless re-encoding) round-trip with no perceptible change. Operations that involve a lossy codec inevitably introduce small artefacts at the byte level, but the defaults aim at the sweet spot where output looks or sounds the same to a normal viewer or listener while still being meaningfully smaller or faster than the input.
Does JS Support Checker support batch processing?
JS Support Checker processes one input at a time by design — it keeps memory usage predictable on lower-end devices and makes results easier to verify. To handle a folder, run the tool once per file; the page stays loaded between runs and remembers your last-used settings, so the second run is essentially instant.