Polyfill urls with too many features result in errors

[romainmenke]

I was wondering what the limits are because it came up in the polyfill service that there is a trade off when adding more features : https://github.com/Financial-Times/polyfill-library/pull/1187#discussion_r923445835

Maximum size that seems to work :

• ± 3086 chars
• ± 135 features

https://polyfill.io/v3/polyfill.min.js?version=3.111.0&features=AbortController%2CAggregateError%2CArray.from%2CArray.of%2CArray.prototype.%40%40iterator%2CArray.prototype.at%2CArray.prototype.copyWithin%2CArray.prototype.entries%2CArray.prototype.fill%2CArray.prototype.find%2CArray.prototype.findIndex%2CArray.prototype.flat%2CArray.prototype.flatMap%2CArray.prototype.includes%2CArray.prototype.keys%2CArray.prototype.sort%2CArray.prototype.values%2CArrayBuffer%2CArrayBuffer.isView%2CAudioContext%2CBlob%2CCSS.supports%2CCustomEvent%2CDOMRect%2CDOMRect.fromRect%2CDOMTokenList%2CDOMTokenList.prototype.%40%40iterator%2CDOMTokenList.prototype.forEach%2CDOMTokenList.prototype.replace%2CDocumentFragment%2CDocumentFragment.prototype.append%2CDocumentFragment.prototype.prepend%2CElement.prototype.after%2CElement.prototype.append%2CElement.prototype.before%2CElement.prototype.classList%2CElement.prototype.closest%2CElement.prototype.dataset%2CElement.prototype.getAttributeNames%2CElement.prototype.matches%2CElement.prototype.placeholder%2CElement.prototype.prepend%2CElement.prototype.remove%2CElement.prototype.replaceWith%2CElement.prototype.toggleAttribute%2CEvent%2CEvent.focusin%2CEvent.hashchange%2CEventSource%2CFunction.prototype.name%2CHTMLCanvasElement.prototype.toBlob%2CHTMLCollection.prototype.%40%40iterator%2CHTMLDocument%2CHTMLElement.prototype.inert%2CHTMLInputElement.prototype.valueAsDate%2CHTMLPictureElement%2CHTMLSelectElement.prototype.selectedOptions%2CHTMLTemplateElement%2CIntersectionObserver%2CIntersectionObserverEntry%2CIntl.DateTimeFormat%2CIntl.DateTimeFormat.~locale.en%2CIntl.DateTimeFormat.~timeZone.all%2CIntl.DateTimeFormat.~timeZone.golden%2CIntl.DisplayNames%2CIntl.DisplayNames.~locale.en%2CIntl.ListFormat%2CIntl.ListFormat.~locale.en%2CIntl.Locale%2CIntl.NumberFormat%2CIntl.NumberFormat.~locale.en%2CIntl.PluralRules%2CIntl.PluralRules.~locale.en%2CIntl.RelativeTimeFormat%2CIntl.RelativeTimeFormat.~locale.en%2CIntl.getCanonicalLocales%2CMap%2CMath.acosh%2CMath.asinh%2CMath.atanh%2CMath.cbrt%2CMath.clz32%2CMath.cosh%2CMath.expm1%2CMath.fround%2CMath.hypot%2CMath.imul%2CMath.log10%2CMath.log1p%2CMath.log2%2CMath.sign%2CMath.sinh%2CMath.tanh%2CMath.trunc%2CMediaQueryList.prototype.addEventListener%2CMutationObserver%2CNode.prototype.contains%2CNode.prototype.isSameNode%2CNodeList.prototype.%40%40iterator%2CNodeList.prototype.forEach%2CNumber.Epsilon%2CNumber.MAX_SAFE_INTEGER%2CNumber.MIN_SAFE_INTEGER%2CNumber.isFinite%2CNumber.isInteger%2CNumber.isNaN%2CNumber.isSafeInteger%2CNumber.parseFloat%2CNumber.parseInt%2CObject.assign%2CObject.entries%2CObject.freeze%2CObject.fromEntries%2CObject.getOwnPropertyDescriptor%2CObject.getOwnPropertyDescriptors%2CObject.getOwnPropertyNames%2CObject.is%2CObject.isExtensible%2CObject.isFrozen%2CObject.isSealed%2CObject.keys%2CObject.preventExtensions%2CObject.seal%2CObject.setPrototypeOf%2CObject.values%2CPromise%2CPromise.allSettled%2CPromise.any%2CPromise.prototype.finally%2CReflect%2CReflect.apply%2CReflect.construct%2CReflect.defineProperty%2CReflect.deleteProperty%2CReflect.get

This seems like a lot of features but it is fairly easy to reach this because of two aspects :

• Intl features have a lot of locale's and a site might use multiple Intl features with multiple locales
• well known Symbols affect many features. (e.g. @@toStringTag)

Even a very naive approach of gzip + base64 reduces the size sufficiently to include all 304 current features plus locale en with a lot of room to spare.

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Ideally polyfills would have a short ID.
Using url save characters a large alphabet (±70) can be constructed for a custom encoding.
This would make it possible to only use 3 characters per feature with space for +100k features in total. (current number is 3338 and 2 chars is limited to ±5k features)

This would still not make it possible to request all features and all locales at once, but it would significantly increase the possible number before errors are encountered.

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Building URL's could also employ some cleverness like removing features which are already listed as a dependency of other features included in the url.