Reevaluate window.fetch each time HttpLink uses it, if not configured using options.fetch
#8603
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Should help with a variety of strategies for instrumenting window.fetch, as discussed in issue #7832.
| // #7832), or (if all else fails) the backupFetch function we saved when | ||
| // this module was first evaluated. This last option protects against the | ||
| // removal of window.fetch, which is unlikely but not impossible. | ||
| const currentFetch = preferredFetch || maybe(() => fetch) || backupFetch; |
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Question: There should no runtime difference between maybe(() => fetch) and backupFetch, as backupFetch is also maybe(() => fetch). In both cases the fetch variable is resolved during execution time of the maybe lambda. What is reason for the backupFetch fallback / the extra maybe(() => fetch)?
What is the reason for backupFetch here?
| const currentFetch = preferredFetch || maybe(() => fetch) || backupFetch; | |
| const currentFetch = preferredFetch || backupFetch; |
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@bripkens A maybe(() => fetch) expression executes the () => fetch function immediately and returns the current result, or undefined if the function threw an exception. It's the shortest way I've found to evaluate possibly-undefined global variables without obtaining a reference to the global object, and TypeScript infers the return type reliably.
To my way of thinking, this means the maybe(() => fetch) expression we evaluate here could be different from backupFetch, which was evaluated when the module was first imported (possibly long ago). I think this difference allows window.fetch to be wrapped after the HttpLink is created (the goal of #7832, IIUC). If we switch to preferredFetch || backupFetch here, then we're stuck with one of those values for all time.
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Oh, my bad, sorry @benjamn. I misunderstood what maybe is doing! :)
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A typeof fetch === "undefined" check might be clearer, but that’s a nit. I don’t like this maybe() function or relying on thrown ReferenceError in a callback, but I can tolerate it.
This should help with a variety of strategies for instrumenting
window.fetch, as discussed in issue #7832.