ref(core): Remove redundant iframe check in supportsNativeFetch#19853
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ref(core): Remove redundant iframe check in supportsNativeFetch#19853
supportsNativeFetch#19853Conversation
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size-limit report 📦
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node-overhead report 🧳Note: This is a synthetic benchmark with a minimal express app and does not necessarily reflect the real-world performance impact in an application.
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supportsNativeFetch
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Semver Impact of This PR🟢 Patch (bug fixes) 📋 Changelog PreviewThis is how your changes will appear in the changelog. New Features ✨Deps
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🤖 This preview updates automatically when you update the PR. |
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packages/core/src/utils/supports.ts
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| // eslint-disable-next-line @typescript-eslint/ban-types | ||
| export function isNativeFunction(func: Function): boolean { | ||
| return func && /^function\s+\w+\(\)\s+\{\s+\[native code\]\s+\}$/.test(func.toString()); | ||
| return func && /\[native code\]/.test(func.toString()); |
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I'll revert this. It saves a few bytes but drastically reduces specificity. We have no unit tests that cover this function :(
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| if (!_isFetchSupported()) { | ||
| return false; | ||
| } | ||
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| // Fast path to avoid DOM I/O | ||
| // eslint-disable-next-line @typescript-eslint/unbound-method | ||
| if (isNativeFunction(WINDOW.fetch)) { | ||
| return true; | ||
| } | ||
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| // window.fetch is implemented, but is polyfilled or already wrapped (e.g: by a chrome extension) | ||
| // so create a "pure" iframe to see if that has native fetch | ||
| let result = false; | ||
| const doc = WINDOW.document; | ||
| // eslint-disable-next-line deprecation/deprecation | ||
| if (doc && typeof (doc.createElement as unknown) === 'function') { | ||
| try { | ||
| const sandbox = doc.createElement('iframe'); | ||
| sandbox.hidden = true; | ||
| doc.head.appendChild(sandbox); | ||
| if (sandbox.contentWindow?.fetch) { | ||
| // eslint-disable-next-line @typescript-eslint/unbound-method | ||
| result = isNativeFunction(sandbox.contentWindow.fetch); | ||
| } | ||
| doc.head.removeChild(sandbox); | ||
| } catch (err) { | ||
| DEBUG_BUILD && debug.warn('Could not create sandbox iframe for pure fetch check, bailing to window.fetch: ', err); | ||
| } | ||
| } | ||
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| return result; |
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Unfortunately, this is a behaviour change to the original function. Supporting a fetch implementation is not the same as supporting/having the native one. I'm going to experiment with moving getNativeImplementation to core so that we can actually reuse the iframe lookup but I'm not sure how much bundle size this still saves.
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Saving old size check for comparison:
size-limit report 📦
| Path | Size | % Change | Change |
|---|---|---|---|
| @sentry/browser | 25.4 kB | -0.92% | -234 B 🔽 |
| @sentry/browser - with treeshaking flags | 23.94 kB | -0.84% | -201 B 🔽 |
| @sentry/browser (incl. Tracing) | 42.4 kB | -0.52% | -220 B 🔽 |
| @sentry/browser (incl. Tracing, Profiling) | 47.06 kB | -0.48% | -223 B 🔽 |
| @sentry/browser (incl. Tracing, Replay) | 81.18 kB | -0.31% | -245 B 🔽 |
| @sentry/browser (incl. Tracing, Replay) - with treeshaking flags | 70.78 kB | -0.3% | -211 B 🔽 |
| @sentry/browser (incl. Tracing, Replay with Canvas) | 85.88 kB | -0.29% | -245 B 🔽 |
| @sentry/browser (incl. Tracing, Replay, Feedback) | 98.14 kB | -0.24% | -235 B 🔽 |
| @sentry/browser (incl. Feedback) | 42.22 kB | -0.53% | -221 B 🔽 |
| @sentry/browser (incl. sendFeedback) | 30.07 kB | -0.78% | -234 B 🔽 |
| @sentry/browser (incl. FeedbackAsync) | 35.13 kB | -0.66% | -232 B 🔽 |
| @sentry/browser (incl. Metrics) | 26.68 kB | -0.92% | -246 B 🔽 |
| @sentry/browser (incl. Logs) | 26.82 kB | -0.91% | -245 B 🔽 |
| @sentry/browser (incl. Metrics & Logs) | 27.51 kB | -0.83% | -228 B 🔽 |
| @sentry/react | 27.19 kB | -0.75% | -203 B 🔽 |
| @sentry/react (incl. Tracing) | 44.73 kB | -0.5% | -224 B 🔽 |
| @sentry/vue | 29.86 kB | -0.73% | -218 B 🔽 |
| @sentry/vue (incl. Tracing) | 44.26 kB | -0.51% | -226 B 🔽 |
| @sentry/svelte | 25.43 kB | -0.93% | -238 B 🔽 |
| CDN Bundle | 28.11 kB | -0.61% | -172 B 🔽 |
| CDN Bundle (incl. Tracing) | 43.29 kB | -0.49% | -213 B 🔽 |
| CDN Bundle (incl. Logs, Metrics) | 28.97 kB | -0.62% | -178 B 🔽 |
| CDN Bundle (incl. Tracing, Logs, Metrics) | 44.16 kB | -0.44% | -195 B 🔽 |
| CDN Bundle (incl. Replay, Logs, Metrics) | 68.03 kB | -0.27% | -184 B 🔽 |
| CDN Bundle (incl. Tracing, Replay) | 80.15 kB | -0.23% | -179 B 🔽 |
| CDN Bundle (incl. Tracing, Replay, Logs, Metrics) | 81.04 kB | -0.24% | -193 B 🔽 |
| CDN Bundle (incl. Tracing, Replay, Feedback) | 85.66 kB | -0.25% | -211 B 🔽 |
| CDN Bundle (incl. Tracing, Replay, Feedback, Logs, Metrics) | 86.55 kB | -0.25% | -215 B 🔽 |
| CDN Bundle - uncompressed | 81.97 kB | -0.8% | -655 B 🔽 |
| CDN Bundle (incl. Tracing) - uncompressed | 127.89 kB | -0.52% | -666 B 🔽 |
| CDN Bundle (incl. Logs, Metrics) - uncompressed | 84.83 kB | -0.77% | -655 B 🔽 |
| CDN Bundle (incl. Tracing, Logs, Metrics) - uncompressed | 130.76 kB | -0.51% | -666 B 🔽 |
| CDN Bundle (incl. Replay, Logs, Metrics) - uncompressed | 208.47 kB | -0.32% | -655 B 🔽 |
| CDN Bundle (incl. Tracing, Replay) - uncompressed | 244.75 kB | -0.28% | -666 B 🔽 |
| CDN Bundle (incl. Tracing, Replay, Logs, Metrics) - uncompressed | 247.6 kB | -0.27% | -666 B 🔽 |
| CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed | 257.66 kB | -0.26% | -666 B 🔽 |
| CDN Bundle (incl. Tracing, Replay, Feedback, Logs, Metrics) - uncompressed | 260.5 kB | -0.26% | -666 B 🔽 |
| @sentry/nextjs (client) | 47.16 kB | -0.44% | -204 B 🔽 |
| @sentry/sveltekit (client) | 42.82 kB | -0.6% | -255 B 🔽 |
| @sentry/node-core | 56.28 kB | -0.12% | -63 B 🔽 |
| @sentry/node | 173.21 kB | +0.03% | +51 B 🔺 |
| @sentry/node - without tracing | 96.27 kB | -0.08% | -76 B 🔽 |
| @sentry/aws-serverless | 113.28 kB | -0.06% | -62 B 🔽 |
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|
trying one more approach here to only extract the iframe stuff |
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Lms24
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Mar 20, 2026
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| // This is a really weird fallback but here's what's going on: | ||
| // We're just being extra careful here. According to types, windowImpl is _always_ defined. | ||
| // However, in some very rare cases (for example test environments), it may in fact not be defined. | ||
| // In exactly this case, if we fail to get an iframeImpl, AND no windowImpl either, | ||
| // we skip caching and just return effectively undefined (despite types saying it's always defined) | ||
| // This basically tricks TS into thinking this function never returns `undefined` which | ||
| // for the most part is true. | ||
| if (!windowImpl) { | ||
| return windowImpl; // but actually return undefined | ||
| } | ||
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| return (cachedImplementations[name] = impl.bind(WINDOW) as CacheableImplementations[T]); | ||
| // If _only_ iframeImpl is undefined and windowImpl is defined and not not native, we end up here | ||
| // In this case, we deliberately cache the windowImpl. | ||
| return cacheAndReturn(windowImpl); |
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Writing this down in case we ever wonder why the hell this code path exists:
This PR DID NOT change any prior behavior. I think this function was already flawed but for the scope of this PR, I'll not fix it right away. Two sketchy things:
- We have a code path where we effectively return undefined, despite TS assuming we always return a defined implementation (see 1st comment why)
- We return and cache a non-native, potentially patched version of an implementation in a function that's called
getNativeImplementation.
This PR just more clearly call out this behaviour as it was quite hidden away in the prior implementation. I'll investigate if we can fix at least the latter flaw by returning undefined.
supportsNativeFetch() created a sandboxed iframe to check if the Fetch API
is natively implemented — identical logic to getNativeImplementation('fetch')
in browser-utils. The iframe approach adds ~30 lines of DOM manipulation code.
The function is only called behind a `skipNativeFetchCheck` guard that is
never set to true in the base CDN bundle, making the iframe code dead weight
that cannot be tree-shaken by terser (it can't prove the parameter is always
falsy across the program).
Simplify to delegate to `_isFetchSupported()` which checks if `window.fetch`
exists. The actual native-vs-polyfill distinction is already handled by
`getNativeImplementation` at the call sites that need it.
Also simplifies the `isNativeFunction` regex from an exact whitespace pattern
to a simpler `/[native code]/` check, which is more permissive across
different JS engine toString() formats.
Saves ~200 bytes gzipped on the base browser bundle.
Co-Authored-By: Claude claude@anthropic.com
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lol, so after making far to many changes to still get this in without behaviour changes but less duplication, I ended up with a larger bundle size than before. Code is arguably cleaner now but honestly not worth pursuing further. Closing this experiment 😅 |
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Summary
Remove ~30 lines of iframe-based DOM manipulation from
supportsNativeFetch(). Saves ~200 bytes gzipped — the single biggest win in this bundle size effort.Problem
supportsNativeFetch()created a sandboxed iframe to check iffetchis natively implemented. This is identical logic togetNativeImplementation("fetch")inbrowser-utils, creating two separate iframe-based checks in the CDN bundle.The function is only called behind a
skipNativeFetchCheckguard:In the base CDN bundle,
skipNativeFetchCheckis never set totrue, making the iframe code dead weight. Terser cannot tree-shake it because it cannot prove the parameter is always falsy.Solution
Simplify
supportsNativeFetch()to just delegate to_isFetchSupported()(checks ifwindow.fetchexists). The native-vs-polyfill distinction is handled bygetNativeImplementationat call sites that need it.Also simplifies
isNativeFunctionfrom/^function\s+\w+\(\)\s+\{\s+\[native code\]\s+\}$/to/\[native code\]/— more permissive across different JS enginetoString()formats.Part of #19833.
Co-Authored-By: Claude claude@anthropic.com