Entry points, with contributions from @aeleitch and @wlonk.

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+# Entry Points
+
+The following captures a conversation from IRC:
+
+> [Owen J](https://twitter.com/derspiny): Have you run across the idea
+> of an "entry point" in a runtime yet? (You've definitely used it, just
+> possibly not known it had a name.)
+>
+> [Alex L](https://twitter.com/aeleitch): I have not!
+>
+> [Owen J](https://twitter.com/derspiny): It's the point where the
+> execution of the outside system -- the OS, the browser, the Node
+> runtime, whatever -- stops and the execution of your code starts. Some
+> platforms only give you one: C on Unix is classic, where there's only
+> two entry points: main and signal handlers (and a lot of apps only use
+> main). JS gives you _a shit fucking ton_ of entry points.
+>
+> [Owen J](https://twitter.com/derspiny): In a browser, the pageload
+> process is an entry point: your code gets run when the browser
+> encounters a `<script>` tag. So is every event handler. There's none
+> of your code running when an event handler starts, only the browser
+> is running. So is every callback from an external service, like
+> `XmlHttpRequest` or `EventSource` or the `File` APIs. In Node, the top
+> level of your main script is an entry point, but so is every callback
+> from an external service.
+>
+> [Alex L](https://twitter.com/aeleitch): Ahahahahahahaha oh my
+> god. There is no way for me to contain them all. _everything the light
+> touches._
+>
+> [Owen J](https://twitter.com/derspiny): This is important for
+> reasoning about exception handling! _In JS_, exception handling only
+> propagates one direction: towards the entry point of this sequence of
+> function calls.
+>
+> [Alex L](https://twitter.com/aeleitch): Yes. This is what _I_ call a
+> stack trace.
+>
+> [Owen J](https://twitter.com/derspiny): If an exception escapes from
+> an entry point, the JS runtime logs it, and then the outside runtime
+> takes over again. That's one of the ways callbacks from external
+> services fuck up the idea of a stack trace as a map of control flow.
+>
+> [Alex L](https://twitter.com/aeleitch): Huh. Yes. Yes I can see
+> that. I mean, in my world, control flow is a somewhat handwavey idea
+> right now. I'm starting to understand why so many people hate JS-land.
+>
+> [Owen J](https://twitter.com/derspiny): Sure. But, for example, a
+> promise chain is a tool for restructuring control flow. In principle,
+> error handling should provide _some_ kind of map of that, to allow
+> programmers -- you -- to diagnose how a program reached a given error
+> state and maybe one day fix the problem. In THIS future, none of them
+> do that well, though.
+>
+> [Alex L](https://twitter.com/aeleitch): Yes. Truly the darkest
+> timeline, but this reviews why I am having these concerns.