Cancelable Async Flows (CAF)


CAF (/ˈkahf/) is a wrapper for function* generators that treats them like async functions, but with support for external cancellation via tokens. In this way, you can express flows of synchronous-looking asynchronous logic that are still cancelable (Cancelable Async Flows).

Also included is CAG(..), for alternately wrapping function generators to emulate ES2018 async-generators (async function ).

Environment Support

This library uses ES2018 features. If you need to support environments prior to ES2018, transpile it first (with Babel, etc).

At A Glance

CAF (Cancelable Async Flows) wraps a function* generator so it looks and behaves like an async function, but that can be externally canceled using a cancellation token:

``js
var token = new CAF.cancelToken();

// wrap a generator to make it look like a normal async
// function that when called, returns a promise.
var main = CAF( function *main(signal,url){
var resp = yield ajax( url );

// want to be able to cancel so we never get here?!?
console.log( resp );
return resp;
} );

// run the wrapped async-looking function, listen to its
// returned promise
main( token.signal, "http://some.tld/other" )
.then( onResponse, onCancelOrError );

// only wait 5 seconds for the ajax request!
setTimeout( function onElapsed(){
token.abort( "Request took too long!" );
}, 5000 );
`

Create a cancellation token (via new CAF.cancelToken()) to pass into your wrapped function generator, and then if you cancel the token, the function generator will abort itself immediately, even if it's presently waiting on a promise to resolve.

The generator receives the cancellation token's signal, so from inside it you can call another function* generator via CAF and pass along that shared signal. In this way, a single cancellation signal can cascade across and cancel all the CAF-wrapped functions in a chain of execution:

`js
var token = new CAF.cancelToken();

var one = CAF( function *one(signal,v){
return yield two( signal, v );
} );

var two = CAF( function *two(signal,v){
return yield three( signal, v );
} );

var three = CAF( function *three(signal,v){
return yield ajax( http://some.tld/?v=${v} );
} );

one( token.signal, 42 );

// only wait 5 seconds for the request!
setTimeout( function onElapsed(){
token.abort( "Request took too long!" );
}, 5000 );
`

In this snippet, one(..) calls and waits on two(..), two(..) calls and waits on three(..), and three(..) calls and waits on ajax(..). Because the same cancellation token is used for the 3 generators, if token.abort() is executed while they're all still paused, they will all immediately abort.

Note: The cancellation token has no effect on the actual ajax(..) call itself here, since that utility ostensibly doesn't provide cancellation capability; the Ajax request itself would still run to its completion (or error or whatever). We've only canceled the one(..), two(..), and three(..) functions that were waiting to process its response. See AbortController(..) and Manual Cancellation Signal Handling below for addressing this limitation.

$3

ES2018 added "async generators", which is a pairing of async function and function -- so you can use await and yield in the same function, await for unwrapping a promise, and yield for pushing a value out. An async-generator (async function f(..) { .. }), like regular iterators, is designed to be sequentially iterated, but using the "async iteration" protocol.

For example, in ES2018:

`js
async function *stuff(urls) {
for (let url of urls) {
let resp = await fetch(url); // await a promise
yield resp.json(); // yield a value (even a promise for a value)
}
}

// async-iteration loop
for await (let v of stuff(assetURLs)) {
console.log(v);
}
`

In the same way that CAF(..) emulates an async..await function with a function generator, the CAG(..) utility emulates an async-generator with a normal function generator. You can cancel an async-iteration early (even if it's currently waiting internally on a promise) with a cancellation token.

You can also synchronously force-close an async-iterator by calling the return(..) on the iterator. With native async-iterators, return(..) is not actually synchronous, but CAG(..) patches this to allow synchronous closing.

Instead of yielding a promise the way you do with CAF(..), you use a provided pwait(..) function with yield, like yield pwait(somePromise). This allows a yield ..value.. expression for pushing out a value through the iterator, as opposed to yield pwait(..value..) to locally wait for the promise to resolve. To emulate a yield await ..value.. expression (common in async-generators), you use two yields together: yield yield pwait(..value..).

For example:

`js
// NOTE: this is CAG(..), not to be confused with CAF(..)
var stuff = CAG(function *stuff({ signal, pwait },urls){
for (let url of urls) {
let resp = yield pwait(fetch(url,{ signal })); // await a promise
yield resp.json(); // yield a value (even a promise for a value)
}
});

var timeout = CAF.timeout(5000,"That's enough results!");
var it = stuff(timeout,assetURLs);

cancelBtn.addEventListener("click",() => it.return("Stop!"),false);

// async-iteration loop
for await (let v of it) {
console.log(v);
}
`

In this snippet, the stuff(..) async-iteration can either be canceled if the 5-second timeout expires before iteration is complete, or the click of the cancel button can force-close the iterator early. The difference between them is that token cancellation would result in an exception bubbling out (to the consuming loop), whereas calling return(..) will simply cleanly close the iterator (and halt the loop) with no exception.

Background/Motivation

Generally speaking, an async function and a function* generator (driven with a generator-runner) look very similar. For that reason, most people just prefer the async function form since it's a little nicer syntax and doesn't require a library for the runner.

However, there are limitations to async functions that come from having the syntax and engine make implicit assumptions that otherwise would have been handled by a function* generator runner.

One unfortunate limitation is that an async function cannot be externally canceled once it starts running. If you want to be able to cancel it, you have to intrusively modify its definition to have it consult an external value source -- like a boolean or promise -- at each line that you care about being a potential cancellation point. This is ugly and error-prone.

function generators by contrast can be aborted at any time, using the iterator's return(..) method and/or by just not resuming the generator iterator instance with next(). But the downside of using function generators is either needing a runner utility or the repetitive boilerplate of handling the iterator manually.

CAF provides a useful compromise: a function* generator that can be called like a normal async function, but which supports a cancellation token.

The CAF(..) utility wraps a function* generator with a normal promise-returing function, just as if it was an async function. Other than minor syntactic aesthetics, the major observable difference is that a CAF-wrapped function must be provided a cancellation token's signal as its first argument, with any other arguments passed subsequent, as desired.

By contrast, the CAG(..) utility wraps a function generator as an ES2018 async-generator (async function ) that respects the native async-iteration protocol. Instead of await, you use yield pwait(..) in these emulated async-generators.

Overview

In the following snippet, the two functions are essentially equivalent; one(..) is an actual async function, whereas two(..) is a wrapper around a generator, but will behave like an async function in that it also returns a promise:

`js
async function one(v) {
await delay( 100 );
return v * 2;
}

var two = CAF( function *two(signal,v){
yield delay( 100 );
return v * 2;
} );
`

Both one(..) and two(..) can be called directly with argument(s), and both return a promise for their completion:

`js
one( 21 )
.then( console.log, console.error ); // 42

var token = new CAF.cancelToken();

two( token.signal, 21 )
.then( console.log, console.error ); // 42
`

If token.abort(..) is executed while two(..) is still running, the signal's promise will be rejected. If you pass a cancellation reason (any value, but typically a string) to token.abort(..), that will be the promise rejection reason:

`js
two( token, 21 )
.then( console.log, console.error ); // Took too long!

token.abort( "Took too long!" );
`

$3

One of the most common use-cases for cancellation of an async task is because too much time passes and a timeout threshold is passed.

As shown earlier, you can implement that sort of logic with a cancelToken() instance and a manual call to the environment's setTimeout(..). However, there are some subtle but important downsides to doing this kind of thing manually. These downsides are harder to spot in the browser, but are more obvious in Node.js

Consider this code:

`js
function delay(ms) {
return new Promise( function c(res){
setTimeout( res, ms );
} );
}

var token = new CAF.cancelToken();

var main = CAF( function *main(signal,ms){
yield delay( ms );
console.log( "All done!" );
} );

main( token.signal, 100 );

// only wait 5 seconds for the request!
delay( 5000 ).then( function onElapsed(){
token.abort( "Request took too long!" );
} );
`

The main(..) function delays for 100ms and then completes. But there's no logic that clears the timeout set from delay( 5000 ), so it will continue to hold pending until that amount of time expires.

Of course, the token.abort(..) call at that point is moot, and is thus silently ignored. But the problem is the timer still running, which keeps a Node.js process alive even if the rest of the program has completed. The symptoms of this would be running a Node.js program from the command line and observing it "hang" for a bit at the end instead of exiting right away. Try the above code to see this in action.

There's two complications that make avoiding this downside tricky:

1. The delay(..) helper shown, which is a promisified version of setTimeout(..), is basically what you can produce by using Node.js's util.promisify(..) against setTimeout(..). However, that timer itself is not cancelable. You can't access the timer handle (return value from setTimeout(..)) to call clearTimeout(..) on it. So, you can't stop the timer early even if you wanted to.

2. If instead you set up your own timer externally, you need to keep track of the timer's handle so you can call clearTimeout(..) if the async task completes successfully before the timeout expires. This is manual and error-prone, as it's far too easy to forget.

Instead of inventing solutions to these problems, CAF provides two utilities for managing cancelable delays and timeout cancellations: CAF.delay(..) and CAF.timeout(..).

#### CAF.delay(..)

What we need is a promisified setTimeout(..), like delay(..) we saw in the previous section, but that can still be canceled. CAF.delay(..) provides us such functionality:

`js
var discardTimeout = new CAF.cancelToken();

// a promise that waits 5 seconds
CAF.delay( discardTimeout.signal, 5000 )
.then(
function onElapsed(msg){
// msg: "delayed: 5000"
},
function onInterrupted(reason){
// reason: "delay (5000) interrupted"
}
);
`

As you can see, CAF.delay(..) receives a cancellation token signal to cancel the timeout early when needed. If you need to cancel the timeout early, abort the cancellation token:

`js
discardTimeout.abort(); // cancel the CAF.delay() timeout
`

The promise returned from CAF.delay(..) is fulfilled if the full time amount elapses, with a message such as "delayed: 5000". But if the timeout is aborted via the cancellation token, the promise is rejected with a reason like "delay (5000) interrupted".

Passing the cancellation token to CAF.delay(..) is optional; if omitted, CAF.delay(..) works just like a regular promisified setTimeout(..):

`
// promise that waits 200 ms
CAF.delay( 200 )
.then( function onElapsed(){
console.log( "Some time went by!" );
} );
`

#### CAF.timeout(..)

While CAF.delay(..) provides a cancelable timeout promise, it's still overly manual to connect the dots between a CAF-wrapped function and the timeout-abort process. CAF provides CAF.timeout(..) to streamline this common use-case:

`js
var timeoutToken = CAF.timeout( 5000, "Took too long!" );

var main = CAF( function *main(signal,ms){
yield CAF.delay( signal, ms );
console.log( "All done!" );
} );

main( timeoutToken, 100 ); // NOTE: pass the whole token, not just the .signal !!
`

CAF.timeout(..) creates an instance of cancellationToken(..) that's set to abort() after the specified amount of time, optionally using the cancellation reason you provide.

Note that you should pass the full timeoutToken token to the CAF-wrapped function (main(..)), instead of just passing timeoutToken.signal. By doing so, CAF wires the token and the CAF-wrapped function together, so that each one stops the other, whichever one happens first. No more hanging timeouts!

Also note that main(..) still receives just the signal as its first argument, which is suitable to pass along to other cancelable async functions, such as CAF.delay(..) as shown.

timeoutToken is a regular cancellation token as created by CAF.cancelToken(). As such, you can call abort(..) on it directly, if necessary. You can also access timeoutToken.signal to access its signal, and timeoutToken.signal.pr to access the promise that's rejected when the signal is aborted.

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finally clauses are often attached to a try { .. } block wrapped around the entire body of a function, even if there's no catch clause defined. The most common use of this pattern is to define some clean-up operations to be performed after the function is finished, whether that was from normal completion or an early termination (such as uncaught exceptions, or cancellation).

Canceling a CAF-wrapped function* generator that is paused causes it to abort right away, but if there's a pending finally {..} clause, it will always still have a chance to run.

`js
var token = new CAF.cancelToken();

var main = CAF( function *main(signal,url){
try {
return yield ajax( url );
}
finally {
// perform some clean-up operations
}
} );

main( token.signal, "http://some.tld/other" )
.catch( console.log ); // 42 <-- not "Stopped!"

token.abort( "Stopped!" );
`

Moreover, a return of any non-undefined value in a pending finally {..} clause will override the promise rejection reason:

`js
var token = new CAF.cancelToken();

var main = CAF( function *main(signal,url){
try {
return yield ajax( url );
}
finally {
return 42;
}
} );

main( token.signal, "http://some.tld/other" )
.catch( console.log ); // 42 <-- not "Stopped!"

token.abort( "Stopped!" );
`

Whatever value is passed to abort(..), if any, is normally set as the overall promise rejection reason. But in this case, return 42 overrides the "Stopped!" rejection reason.

#### signal.aborted and signal.reason

Standard AbortSignal instances have an aborted boolean property that's set to true once the signal is aborted. Recently, AbortSignal was extended to include a reason property. Prior to that change,CAFwas manually patching signal with a reason property, but nowCAFrespects the reason that's built-in to AbortSignal instances, if the environment supports it.

To set the reason for an abort-signal firing, pass a value to the AbortController's abort(..) call.

By checking the signal.aborted flag in a finally clause, you can determine whether the function was canceled, and then additionally access the signal.reason to determine more specific context information about why the cancellation occurred. This allows you to perform different clean-up operations depending on cancellation or normal completion:

`js
var token = new CAF.cancelToken();

var main = CAF( function *main(signal,url){
try {
return yield ajax( url );
}
finally {
if (signal.aborted) {
console.log( Cancellation reason: ${ signal.reason } );
// perform cancellation-specific clean-up operations
}
else {
// perform non-cancellation clean-up operations
}
}
} );

main( token.signal, "http://some.tld/other" );
// Cancellation reason: Stopped!

token.abort( "Stopped!" );
`

$3

A cancellation token fromCAFincludes a discard() method that can be called at any time to fully unset any internal state in the token to allow proper GC (garbage collection) of any attached resources.

When you are sure you're fully done with a cancellation token, it's a good idea to call discard() on it, and then unset the variable holding that reference:

`js
var token = new CAF.cancelToken();

// later
token.discard();
token = null;
`

Once a token has been discard()ed, no further calls to abort(..) will be effective -- they will silently be ignored.

$3

CAF.cancelToken(..) instantiates AbortController, the DOM standard for canceling/aborting operations like fetch(..) calls. As such, a CAF cancellation token's signal can be passed directly to a DOM method like fetch(..) to control its cancellation:

`js
var token = new CAF.cancelToken();

var main = CAF(function *main(signal,url) {
var resp = yield fetch( url, { signal } );

console.log( resp );
return resp;
});

main( token.signal, "http://some.tld/other" )
.catch( console.log ); // "Stopped!"

token.abort( "Stopped!" );
`

CAF.cancelToken(..) can optionally receive an already instantiated AbortController, though there's rarely a reason to do so:

`js
var ac = new AbortController();
var token = new CAF.cancelToken(ac);
`

Also, if you pass a raw AbortController instance into a CAF-wrapped function, it's automatically wrapped into a CAF.cancelToken(..) instance:

`js
var main = CAF(function *main(signal,url) {
var resp = yield fetch( url, { signal } );

console.log( resp );
return resp;
});

var ac = new AbortController();
main( ac, "http://some.tld/other" )
.catch( () => console.log("Stopped!") ); // "Stopped!"

ac.abort();
`

#### AbortController() Polyfill

If AbortController is not defined in the environment, use this polyfill to define a compatible stand-in. The polyfill is included in the dist/ directory.

If you load CAF in Node using its CJS format (with require(..)) and use the main package entry point (require("caf")), the polyfill is automatically loaded (in the global namespace). If you don't use this entry point, but instead load something more directly, like require("caf/core") or require("caf/cag"), then you need to manually load the polyfill first:

`js
require("/path/to/caf/dist/abortcontroller-polyfill-only.js");

var CAF = require("caf/core");
var CAG = require("caf/cag");
`

When using the ESM format of CAF in Node, the polyfill is not loaded automatically. Node 15/16+ includes AbortController natively, but in prior versions of Node (12-14) while using the ESM format, you need to manually require(..) the polyfill (before importing CAF) like this:

`js
import { createRequire } from "module";
const require = createRequire(import.meta.url);
require("/path/to/caf/dist/abortcontroller-polyfill-only.js");

import CAF from "caf/core";
// ..
`

Be aware that if any environment needs this polyfill, utilities in that environment like fetch(..) won't know about AbortController so they won't recognize or respond to it. They won't break in its presence, just won't use it.

$3

Even if you aren't calling a cancellation signal-aware utility (like fetch(..)), you can still manually respond to the cancellation signal via its attached promise:

`js
var token = new CAF.cancelToken();

var main = CAF( function *main(signal,url){
// listen to the signal's promise rejection directly
signal.pr.catch( reason => {
// reason == "Stopped!"
} );

var resp = yield ajax( url );

console.log( resp );
return resp;
} );

main( token.signal, "http://some.tld/other" )
.catch( console.log ); // "Stopped!"

token.abort( "Stopped!" );
`

Note: The catch(..) handler inside of main(..) will still run, even though main(..) itself will be aborted at its waiting yield statement. If there was a way to manually cancel the ajax(..) call, that code should be placed in the catch(..) handler.

Even if you aren't running a CAF-wrapped function, you can still respond to the cancellation signal's promise manually to affect flow control:

`js
var token = new CAF.cancelToken();

// normal async function, not CAF-wrapped
async function main(signal,url) {
try {
var resp = await Promise.race( [
ajax( url ),
signal.pr // listening to the cancellation
] );

// this won't run if signal.pr rejects
console.log( resp );
return resp;
}
catch (err) {
// err == "Stopped!"
}
}

main( token.signal, "http://some.tld/other" )
.catch( console.log ); // "Stopped!"

token.abort( "Stopped!" );
`

Note: As discussed earlier, the ajax(..) call itself is not cancellation-aware, and is thus not being canceled here. But we are ending our waiting on the ajax(..) call. When signal.pr wins the Promise.race(..) race and creates an exception from its promise rejection, flow control jumps straight to the catch (err) { .. } clause.

$3

You may want to combine two or more signals, similar to how you combine promises with Promise.race(..) and Promise.all(..). CAF provides two corresponding helpers for this purpose:

`js
var timeout = CAF.timeout(5000,"Took too long!");
var canceled = new CAF.cancelToken();
var exit = new AbortController();

var anySignal = CAF.signalRace([
timeout.signal,
canceled.signal,
exit.signal
]);

var allSignals = CAF.signalAll([
timeout.signal,
canceled.signal,
exit.signal
]);

main( anySignal, "http://some.tld/other" );

// or

main( allSignals, "http://some.tld/other" );
`

CAF.signalRace(..) expects an array of one or more signals, and returns a new signal (anySignal) that will fire as soon as any of the constituent signals have fired.

CAF.signalAll(..) expects an array of one or more signals, and returns a new signal (allSignals) that will fire only once all of the constituent signals have fired.

Warning: This pattern (combining signals) has a potential downside. CAF typically cleans up timer-based cancel tokens to make sure resources aren't being wasted and programs aren't hanging with open timer handles. But in this pattern, signalRace(..) / signalAll(..) only receive reference(s) to the signal(s), not the cancel tokens themselves, so it cannot do the manual cleanup. In the above example, you should manually clean up the 5000ms timer by calling timeout.abort() if the operation finishes before that timeout has fired the cancellation.

$3

Beware of creating a single cancellation token that is reused for separate chains of function calls. Unexpected results are likely, and they can be extremely difficult to debug.

As illustrated earlier, it's totally OK and intended that a single cancellation token signal be shared across all the functions in one chain of calls (A -> B -> C). But reusing the same token across two or more chains of calls (A -> B -> C and D -> E -> F) is asking for trouble.

Imagine a scenario where you make two separate fetch(..) calls, one after the other, and the second one runs too long so you cancel it via a timeout:

`js
var one = CAF( function *one(signal){
signal.pr.catch( reason => {
console.log( one: ${reason} );
} );

return yield fetch( "http://some.tld/", {signal} );
} );

var two = CAF( function *two(signal,v){
signal.pr.catch( reason => {
console.log( two: ${reason} );
} );

return yield fetch( http://other.tld/?v=${v}, {signal} );
} );

var token = CAF.cancelToken();

one( token.signal )
.then( function(v){
// only wait 3 seconds for this request
setTimeout( function(){
token.abort( "Second response too slow." );
}, 3000 );

return two( token.signal, v );
} )
.then( console.log, console.error );

// one: Second response too slow. <-- Oops!
// two: Second response too slow.
// Second response too slow.
`

When you call token.abort(..) to cancel the second fetch(..) call in two(..), the signal.pr.catch(..) handler in one(..) still gets called, even though one(..) is already finished. That's why "one: Second response too slow." prints unexpectedly.

The underlying gotcha is that a cancellation token's signal has a single pr promise associated with it, and there's no way to reset a promise or "unregister" then(..) / catch(..) handlers attached to it once you don't need them anymore. So if you reuse the token, you're reusing the pr promise, and all registered promise handlers will be fired, even old ones you likely don't intend.

The above snippet illustrates this problem with signal.pr.catch(..), but any of the other ways of listening to a promise -- such as yield / await, Promise.all(..) / Promise.race(..), etc -- are all susceptible to the unexpected behavior.

The safe and proper approach is to always create a new cancellation token for each chain of CAF-wrapped function calls. For good measure, always unset any references to a token once it's no longer needed, and make sure to call discard(); thus, you won't accidentally reuse the token, and the JS engine can properly GC (garbage collect) it.

$3

A common use case in managing async operations is when a currently pending operation needs to be canceled only because it's being replaced by a subsequent operation.

For example, imagine a button on a page that requests some remote data to display. If the user clicks the button again while a previous request is still pending, you can likely discard/cancel the previous request and start up a new fresh request in its place.

In these sorts of cases, you may find yourself "cycling" through cancellation tokens, where you store a reference to the current token, then when a new one is needed, the former token is aborted (to cancel all its chained operations) and replaced with the new token instance. This sort of logic is not too complex, but it does require keeping the token around across async operations, which unfortunately pollutes an outer scope.

This use case is common enough to warrant a standard helper shipped with this library to reduce the friction/impact of managing these cycles of tokens. CAF ships with tokenCycle() for this purpose:

`js
// create a token cycle
var getReqToken = CAF.tokenCycle();

btn.addEventListener("click",function onClick(){
// get a new cancellation token, and
// cancel the previous token (if any)
//
// note: this function optionally takes a
// reason for aborting the previous token
var cancelToken = getReqToken();

requestUpdatedData(cancelToken,"my-data");
});
`

The tokenCycle() function creates a separate instance of the token cycle manager, so you can create as many independent cycles as your app needs. It returns a function (named getReqToken() in the above snippet) which, when called, will produce a new token and cancel the previous token (if one is pending). This function also optionally takes a single argument to use as the reason passed in to abort the previous token.

You can of course keep these tokens around and use them for other cancellation controls. But in that situation you likely don't need tokenCycle(). This helper is designed for the lightweight use case where you wouldn't otherwise need to keep the token other than to make sure the previous operation is canceled before being replaced with the new operation.

CAG: Emulating Async Generators

Where CAF(..) emulates a promise-returning async function using a generator, CAG(..) is provided to emulate an async-iterator returning async-generator (async function*).

Async iteration is similar to streams (or primitive observables), where the values are consumed asynchronously (typically using an ES2018 for await (..) loop):

`js
for await (let v of someAsyncGenerator()) {
// ..
}

// or:
var it = someAsyncGenerator();
for await (let v of it) {
// ..
}
`

For all the same reasons that async functions being non-cancelable is troublesome, async-generators are similarly susceptible. An async-generator can be "stuck" awaiting internally on a promise, and the outer consuming code cannot do anything to force it to stop.

That's why CAG(..) is useful:

`js
// instead of:
async function *stuff(urls) {
for (let url of urls) {
let resp = await fetch(url); // await a promise
yield resp.json(); // yield a value (even a promise for a value)
}
}

// do this:
var stuff = CAG(function *stuff({ signal, pwait },urls){
for (let url of urls) {
let resp = yield pwait(fetch(url,{ signal })); // await a promise
yield resp.json(); // yield a value (even a promise for a value)
}
});
`

Like CAF(..), functions wrapped by CAG(..) expect to receive a special value in their first parameter position. Here, the object is destructured to reveal it contains both the cancellation-token signal (as with CAF(..)) and the pwait(..) function, which enables emulating local await ..promise.. expressions as yield pwait(..promise..).

The return from a CAG(..) wrapped function is an async-iterator (exactly as if a real native async-generator had been invoked). As with CAF(..) values, the first argument passed should always be the mandatory cancellation token (or its signal):

`js
var stuff = CAG(function *stuff(..){ .. });

var timeout = CAF.timeout(5000,"Took too long.");
var it = stuff(timeout);
`

The returned async-iterator (it above) can be iterated manually with it.next(..) calls -- each returns a promise for an iterator-result -- or more preferably with an ES2018 for await (..) loop:

`js
var timeout = CAF.timeout(5000,"Took too long.");
var it = stuff(timeout);

var { value, done } = await it.next();
// ..do that repeatedly..

// or preferably:
for await (let value of it) {
// ..
}
`

In addition to being able to abort(..) the cancellation token passed into a CAG(..)-wrapped generator, async-iterators also can be closed forcibly by calling their return(..) method.

`js
var timeout = CAF.timeout(5000,"Took too long.");
var it = stuff(timeout);

// later (e.g. in a timer or event handler):
it.return("all done");
// Promise<{ value: "all done", done: true }>
`

Typically, the return(..) call on an async-iterator (from an async-generator) will have "wait" for the attached async-generator to be "ready" to be closed -- in case an await promise expression is currently pending. This means you cannot actually synchronously force-close them. But since CAG(..) emulates async-generators with regular sync-generators, this nuance is "fixed". For consistency, return(..) still returns a Promise, but it's an already-resolved promise with the associated iterator-result.

CAG(..)-wrapped functions also follow these behaviors of CAF(..)-wrapped functions:

* Aborting the cancellation token results in an exception (which can be trapped by try..catch) propagating out from the most recent for await (..) (or it.next(..)) consumption point.

* The reason provided when aborting a cancellation token is (by default) set as the exception that propagates out. This can be overriden by a return .. statement in a finally clause of the wrapped generator function.

#### Event Streams

One of the most common use cases for async iterators (aka, streams) is to subscribe to an event source (DOM element events, Node.js event emitters, etc) and iterate the received events.

CAG provides two helpers for event stream subscription: onEvent(..) and onceEvent(..). As the name implies, onEvent(..) listens for all events, whereas onceEvent(..) will listen only for a single event to fire (and then close the stream and unsubscribe from the event emitter).

onEvent(..) returns an ES2018 async iterator, but onceEvent(..) returns a promise that resolves (with the event value, if any) when the event fires.

`js
var cancel = new CAF.cancelToken();

var DOMReady = CAG.onceEvent(cancel,document,"DOMContentLoaded",/evtOpts=/false);
var clicks = CAG.onEvent(cancel,myBtn,"click",/evtOpts=/false);

// wait for this one-time event to fire
await DOMReady;

for await (let click of clicks) {
console.log("Button clicked!");
}
`

onEvent(..) event subscriptions are lazy, meaning that they don't actually attach to the emitter element until the first attempt to consume an event (via for await..of or a manual next(..) call on the async iterator). So, in the above snippet, the clicks event stream is not yet subscribed to any click events that happen until the for await..of loop starts (e.g., while waiting for the prior DOMReady event to fire).

However, there may be cases where you want to force the event subscription to start early even before consuming its events. Use start() to do so:

`js
var cancel = new CAF.cancelToken();

var clicks = CAG.onEvent(cancel,myBtn,"click",/evtOpts=/false);

// force eager listening to events
clicks.start();

// .. consume the stream later ..
`

Event streams internally buffer received events that haven't yet been consumed. This buffer grows unbounded, so responsible memory management implies always consuming events from a stream that is subscribed and actively receving events.

Once an event stream is closed (e.g., token cancellation, breaking out of a for await..of loop, manually calling return(..) on the async iterator), the underlying event is unsubscribed.

npm Package

![npm Module](https://www.npmjs.org/package/caf)
![Modules](https://nodejs.org/api/packages.html#dual-commonjses-module-packages)

To install this package from npm:

`
npm install caf
`

IMPORTANT: The CAF library relies on AbortController being present in the JS environment. If the environment does not already define AbortController natively, CAF needs a polyfill for AbortController. In some cases, the polyfill is automatically loaded, and in other cases it must be manually required/imported. See the linked section for more details.

As of version 12.0.0, the package is available as an ES Module (in addition to CJS/UMD), and can be imported as so:

`js
// named imports
import { CAF, CAG } from "caf";

// or, default imports:
import CAF from "caf/core";
import CAG from "caf/cag";
`

Note: Starting in version 15.0.0, the (somewhat confusing) ESM specifier "caf/caf" (which imports only CAF as a default-import) has been deprecated and will eventually be removed. Use "caf/core" to default-import only the CAF module, or use just "caf" for named imports ({ CAF, CAG }).

Also Note: Starting in version 11.x, CAF was also available in ESM format, but required an ESM import specifier segment /esm in CAF import paths. As of version 15.0.0, this has been removed, in favor of unified import specifier paths via Node Conditional Exports. For ESM import statements, always use the specifier style "caf" or "caf/cag", instead of "caf/esm" and "caf/esm/cag", respectively.

To use CAF in Node via CJS format (with require(..)):

`js
var CAF = require("caf");
var CAG = require("caf/cag");
`

Builds

![Build Status](https://travis-ci.org/getify/CAF)
![npm Module](https://www.npmjs.org/package/caf)
![Modules](https://nodejs.org/api/packages.html#dual-commonjses-module-packages)

The distribution files come pre-built with the npm package distribution, so you shouldn't need to rebuild it under normal circumstances.

However, if you download this repository via Git:

1. The included build utility (scripts/build-core.js) builds (and minifies) the dist/* files.

2. To install the build and test dependencies, run npm install from the project root directory.

3. To manually run the build utility with npm:

`
npm run build
`

4. To run the build utility directly without npm:

`
node scripts/build-core.js
`

Tests

A test suite is included in this repository, as well as the npm package distribution. The default test behavior runs the test suite using the files in src/.

1. The tests are run with QUnit.

2. You can run the tests in a browser by opening up tests/index.html.

3. To run the test utility:

`
npm test
`

Other npm test scripts:

* npm run test:package will run the test suite as if the package had just been installed via npm. This ensures package.json:main and exports entry points are properly configured.

npm run test:umd will run the test suite against the dist/umd/ files instead of the src/* files.

npm run test:esm will run the test suite against the dist/esm/ files instead of the src/* files.

* npm run test:all will run all four modes of the test suite.

$3

![Coverage Status](https://coveralls.io/github/getify/caf?branch=master)

If you have NYC (Istanbul) already installed on your system (requires v14.1+), you can use it to check the test coverage:

`
npm run coverage
`

Then open up coverage/lcov-report/index.html in a browser to view the report.

Note: The npm script coverage:report` is only intended for use by project maintainers. It sends coverage reports to Coveralls.

License


All code and documentation are (c) 2022 Kyle Simpson and released under the MIT License. A copy of the MIT License is also included.