callebstrom / tracking-promise

A JavaScript library for tracking the success/failure of a promise without rejecting itself.

tracking-promise

A JavaScript library for tracking the success/failure of a promise without rejecting itself.

A simple use case for tracking is to trap a promise so that you get a fulfillment regardless of whether that promise fulfills or rejects. You can then inspect the results to determine how it settled.

More generally, you can track promises, immediate values, or functions that return either. This gives you access to a Tracker object that tracks information about the current state: whether or not it is finished, whether it succeeded (returned or fulfilled) or failed (threw or rejected), and what value or error it finished with.

The Tracker object also serves as a promise (a thenable) which will fulfill (always) when the tracked value finishes, regardless of how it finishes.

Overview

Install however you install npm packages, e.g.:

npm install --save tracking-promise

Example use case:

function functionToTrack() {
    /* ... */
}

const TIMEOUT_MS = 1000;

const track = require("track");

async function main() {
    // The `track` function will invoke the given function synchronously,
    // and return a "Tracker" object for it.
    // the timeout argument is optional, and has some important limitations!
    const tracker = track(functionToTrack, TIMEOUT_MS);

    // Some tracker fields are always set immediately (synchronously) and are available
    // upong returning from `track`.
    tracker.finished; // has the tracked value ended
    tracker.synchronous; // is the tracked value is synchronous or async

    // A Tracker is a promise, it fulfills when the tracked value has ended
    const results = await tracker;

    // It fulfills with information about the tracked value:
    results.synchronous; // was the tracked value synchronous or async?
    results.failed; // did the tracked object reject or throw?
    results.error; // if failed, what was the error?
    results.value; // if succeeded, what was the resulting value?
    results.timedout; // Did the tracked value timeout (if a timeout was given)?

    // Once the tracked value is finished, the Tracker has all the same
    // fields set as well.
}

Tracked Values

You can pass any of the following in as the first argument to the track function:

1. An async function (a function that returns a promise)
2. A synchronous function (a function that returns an immediate/non-promise value, or that synchronously throws an error)
3. A promise (or any other thenable)
4. An immediate (non-promise) value.

When a function is given (options 1 and 2), it is invoked synchronously inside the track function, with any thrown errors being caught.

Options 2 and 4 are considered synchronous, while 1 and 3 are asynchronous.

Synchronous values are considered to be finished immediately (i.e., by the time the track function returns). If a synchronous function (option 2) throws an error, it is considered failed with that error. Otherwise, its value is the value it returns. An immediate value (option 4) cannot fail, and it is considered to be its own value.

Asynchronous values are considered to be finished when the promise settles (either fulfills or rejects). Either async option is considered failed if and only if the promise rejects, and its error is whatever value it rejects with. Otherwise, it's value is whatever value it fulfills with.

Fields

The following fields are available on the Tracker object returned by a call to track. Values that are set upon return from track may subsequently change values for asynchronous tracked values. No additional changes will be made once finished is set to true.

Values that are not set upon return from track are present but explicitly set to undefined.

All of these fields except for finished are also present on the object that the Tracker promise fulfills with (finished is not needed because the fact that Tracker has fulfilled implies that it is finished).

Field Name	Set upon Return from track	Immediate Values	Synchronous Functions	Promises and Async Functions
finished	yes	always true	always true	false until promise settles, then true
synchronous	yes	always true	always true	always false
value †	for synchronous only	the value itself	the returned value	the value the promise fulfills with
error ‡	for synchronous only	never set	the thrown error	the value the promise rejects with
failed	for synchronous only	always true	true if function throws, otherwise false	true if the promise rejects, otherwise false
timedout§	for synchronous only	always false	always false	true if a timeout argument is given and the promise times out§

† - Only given when failed is false, otherwise explicitly set to undefined.
‡ - Only given when failed is true, otherwise explicitly set to undefined.
§ - See "Timeouts" below for details.

Timeouts

The optional second argument to the track function is a timeout, in milliseconds. Timeouts have the following limitations:

1. Timeouts only apply to asynchronous tracked value (promises, and functions that return promises). Synchronous tracked values will never be marked as timedout, regardless of how long a synchronous function runs for.
2. Timeouts do not have any affect on already executing jobs: there is no cancel or abort or anything else.
3. There's no way to guarantee that the Tracker will settle within any finite amount of time.
4. There's no way to guarantee that the track function will return within any finite amount of time.

The last two limitations are a consequence of the JavaScript execution engine. In short: JavaScript is single threaded and there is no pre-emption. The soonest any job can execute is when all jobs that already existed on the queue have completed. If your asynchronus function pushes a long-running job to the job-queue, no other job can run until this one has completed, including the promise-handler jobs that will settle the Tracker (hence point 3).

Additionally, the given function is actually executed synchronously inside the track invocation (that's the only way we can know if it's going to return a promise or not). Thus even if it's an "asynchronous" function, but does a lot of synchronous work before returning the promise, the track function won't continue or finish before your function finishes its asynchronous work (hence point 4).

None of this should be too surprising if you're generally familiar with how timers work in JavaScript, and the subtleties generally won't matter. For when they do, this is the general flow of the track function (the parts relevant to timeout):

1. A timer T is started.
2. If the tracked value is a function, it's invoked synchronously
3. If the tracked value is a promise, or a function that returns a promise, onfulfill and onreject handlers are synchronously registered with it (by calling it's then method).
4. After timer T expires, it's callback will check if either promise handler has executed yet: if they have then the timer callback does nothing; if they have not then the timer fulfills the Tracker promise, marking it as a timeout.
5. When either promise handler is invoked, if the timer callback has already executed the handlers do nothing. Otherwise, the handler will cancel timer T and fulfill the Tracker promise appropriately (not as a timeout).