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.

Related

The Try Pattern

If all you actually care about is making sure you get a fulfillment regardless of how the promise settles, you might want to consider using the Try pattern instead, for instance using Try.fromPromise from the fp-try package.

Promise.allSettled

This is conceptually similar to the Promise.allSettled function slated for inclusion in ES2020, in that both will fulfill regardless of how the given promise settles. As of version 1.1, this package fulfills with a value that is compatible with the outcome objects provided by allSettled.

The allSettled function would essentially be a shortcut for Promise.all(promises.map(track)), except for the following limitations of allSettled:

1. Promise.allSettled does not directly support timeout.
2. Promise.allSettled does not provide information to distinguish between promises and immediate values.
3. Promise.allSettled has no polling mechanism (i.e., the finished field provided by the tracker).

Many use cases won't need any of these things and so Promise.allSettled may work fine for you.

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 | | reason ‡ | for synchronous only | never set | the thrown error | the value the promise rejects with | | error ‡ * | for synchronous only | never set | the thrown error | the value the promise rejects with | | failed | for synchronous only | always false | true if function throws, otherwise false | true if the promise rejects, otherwise false | | status | for synchronous only | always "fulfilled" | "rejected" if the function throws, otherwise "fulfilled" | "rejected" if the promise rejects, otherwise "fulfilled". | | 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.
* - Note the the error field is entirely redundant with the reason field; it was superceded by the latter in version 1.1 of this package in order to align with Promise.allSettled, however both fields will remain in order to maintain compatibility. § - 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).

API

For the most part, you will just treat a Tracker as a thennable for the tracked results; either await-ing the Tracker or using the then(...) method. The Tracker also provides the following common convenience Promise methods:

| Promise-like method | use | | --------------------------------------- | ----------------------------------------------------------------------------------------------------- | | then(fulfillHandler, [rejectHandler]) | The standard handler register for a thennable / promise | | catch(rejectHandler) | A convenience method for registering a reject-handler | | finally(finallyHandler) | A convenience method for registering a handler that will not transform the value of the promise chain |

Because the Tracker always fulfills, a reject-handler will never be called, and the catch method is actually just a near-empty function that returns the Tracker itself.

Note that, as typical, the finally method will return a promise for the same value as what the Tracker itself fulfills with; it is used for side effects, not for transforming the promise chain. However, if the finallyHandler throws or returns a promise that rejects, than the returned promise will reject with the same error.

Unpacking

You can use the unpack method to get a promise that fulfills or rejects according to whether or not the tracked thing succeeds or fails, regardless of what type of thing was being tracked. It the tracked thing succeeds, then the unpacked promise will fulfill with the value; if the tracked thing fails, then then unpacked promise will reject with the reason.