One mystery solved, *so* many more revealed: https://twitter.com/slightlylate/status/1184958694523453440 …pic.twitter.com/qHj0uyb9lh
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*PERHAPS* the developer said to themselves "I don't want to include a full fetch polyfill" (although the same file pulls in rando core-js polyfill this-and-that) and leaned on Babel or whatevs to desugar arrows/promises?...on a site that loads more than 850KiB of (compressed) JS
There definitely were version ranges where it was true, but I believe all these browsers updated now. Depends on when that code was written.
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