This was one of Doug Crockford's big bugaboos since
The Good Parts and JSLint and Yahoo days—the idea that lexical scope aka closures give you an unprecedented ability to actually control I/O because you can say
function main(io) {
const result = somethingThatRequiresHttp(io.fetch);
// ...
}
and as long as you don't put I/O in global scope (i.e. window.fetch) but do an injection into the main entrypoint, that entrypoint gets to control what everyone else can do. I could for example do
function main(io) {
const result = something(readonlyFetch(onlyOurAPI(io.fetch))
}
function onlyOurAPI(fetch) {
return (...args) => {
const test = /^https:\/\/api.mydomain.example\//.exec(args[0]);
if (test == null) {
throw new ValueError("must only communicate with our API");
}
return fetch(..args);
}
}
function readonlyFetch(fetch) { /* similar but allowlist only GET/HEAD methods */ }
I vaguely remember him being really passionate about "JavaScript lets you do this, we should all program in JavaScript" at the time... these days he's much more likely to say "JavaScript doesn't have any way to force you to do this and close off all the exploits from the now-leaked global scope, we should never program in JavaScript."
Shoutout to Ryan Dahl and Deno, where you write `#!/usr/bin/env deno --allow-net=api.mydomain.example` at the start of your shell script to accomplish something similar.
In my amateur programming-conlang hobby that will probably never produce anything joyful to anyone other than me, one of those programming languages has a notion of sending messages to "message-spaces" and I shamelessly steal Doug's idea -- message-spaces have handles that you can use to communicate with them, your I/O is a message sent to your main m-space containing a bunch of handles, you can then pattern-match on that message and make a new handle for a new m-space, provisioned with a pattern-matcher that only listens for, say, HTTP GET/HEAD events directed at the API, and forwards only those to the I/O handle. So then when I give this new handle to someone, they have no way of knowing that it's not fully I/O capable, requests they make to the not-API just sit there blackholed until you get an alert "there are too many unread messages in this m-space" and peek in to see why.