Use Promise to wait until polled condition is satisfied

I need to create a JavaScript Promise that will not resolve until a specific condition is true. Let's say I have a 3rd party library, and I need to wait until a certain data condition exists within that library.

The scenario I am interested in is one where there is no way to know when this condition is satisfied other than by simply polling.

I can create a promise that waits on it - and this code works, but is there a better or more concise approach to this problem?

function ensureFooIsSet() {
return new Promise(function (resolve, reject) {
waitForFoo(resolve);
});
}


function waitForFoo(resolve) {
if (!lib.foo) {
setTimeout(waitForFoo.bind(this, resolve), 30);
} else {
resolve();
}
}

Usage:

ensureFooIsSet().then(function(){
...
});

I would normally implement a max poll time, but didn't want that to cloud the issue here.

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最佳答案

A small variation would be to use a named IIFE so that your code is a little more concise and avoids polluting the external scope:

function ensureFooIsSet() {
return new Promise(function (resolve, reject) {
(function waitForFoo(){
if (lib.foo) return resolve();
setTimeout(waitForFoo, 30);
})();
});
}
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Is there a more concise approach to this problem?

Well, with that waitForFoo function you don't need an anonymous function in your constructor at all:

function ensureFooIsSet() {
return new Promise(waitForFoo);
}

To avoid polluting the scope, I would recommend to either wrap both in an IIFE or to move the waitForFoo function inside the ensureFooIsSet scope:

function ensureFooIsSet(timeout) {
var start = Date.now();
return new Promise(waitForFoo);
function waitForFoo(resolve, reject) {
if (window.lib && window.lib.foo)
resolve(window.lib.foo);
else if (timeout && (Date.now() - start) >= timeout)
reject(new Error("timeout"));
else
setTimeout(waitForFoo.bind(this, resolve, reject), 30);
}
}

Alternatively, to avoid the binding that is needed to pass around resolve and reject you could move it inside the Promise constructor callback like @DenysSéguret suggested.

Is there a better approach?

Like @BenjaminGruenbaum commented, you could watch the .foo property to be assigned, e.g. using a setter:

function waitFor(obj, prop, timeout, expected) {
if (!obj) return Promise.reject(new TypeError("waitFor expects an object"));
if (!expected) expected = Boolean;
var value = obj[prop];
if (expected(value)) return Promise.resolve(value);
return new Promise(function(resolve, reject) {
if (timeout)
timeout = setTimeout(function() {
Object.defineProperty(obj, prop, {value: value, writable:true});
reject(new Error("waitFor timed out"));
}, timeout);
Object.defineProperty(obj, prop, {
enumerable: true,
configurable: true,
get: function() { return value; },
set: function(v) {
if (expected(v)) {
if (timeout) cancelTimeout(timeout);
Object.defineProperty(obj, prop, {value: v, writable:true});
resolve(v);
} else {
value = v;
}
}
});
});
// could be shortened a bit using "native" .finally and .timeout Promise methods
}

You can use it like waitFor(lib, "foo", 5000).

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function getReportURL(reportID) {
return () => viewReportsStatus(reportID)
.then(res => JSON.parse(res.body).d.url);
}


function pollForUrl(pollFnThatReturnsAPromise, target) {
if (target) return P.resolve(target);
return pollFnThatReturnsAPromise().then(someOrNone => pollForUrl(pollFnThatReturnsAPromise, someOrNone));
}


pollForUrl(getReportURL(id), null);
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Here's a utility function using async/await and default ES6 promises. The promiseFunction is an async function (or just a function that returns a promise) that returns a truthy value if the requirement is fulfilled (example below).

const promisePoll = (promiseFunction, { pollIntervalMs = 2000 } = {}) => {
const startPoll = async resolve => {
const startTime = new Date()
const result = await promiseFunction()


if (result) return resolve()


const timeUntilNext = Math.max(pollIntervalMs - (new Date() - startTime), 0)
setTimeout(() => startPoll(resolve), timeUntilNext)
}


return new Promise(startPoll)
}

Example usage:

// async function which returns truthy if done
const checkIfOrderDoneAsync = async (orderID) => {
const order = await axios.get(`/order/${orderID}`)
return order.isDone
}


// can also use a sync function if you return a resolved promise
const checkIfOrderDoneSync = order => {
return Promise.resolve(order.isDone)
}


const doStuff = () => {
await promisePoll(() => checkIfOrderDone(orderID))
// will wait until the poll result is truthy before
// continuing to execute code
somethingElse()
}
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Here's a waitFor function that I use quite a bit. You pass it a function, and it checks and waits until the function returns a truthy value, or until it times out.

  • This is a simple version which illustrates what the function does, but you might want to use the full version, added further in the answer
let sleep = ms => new Promise(r => setTimeout(r, ms));
let waitFor = async function waitFor(f){
while(!f()) await sleep(1000);
return f();
};

Example usages:

  • wait for an element to exist, then assign it to a variable
let bed = await waitFor(() => document.getElementById('bedId'))
if(!bed) doSomeErrorHandling();
  • wait for a variable to be truthy
await waitFor(() => el.loaded)
  • wait for some test to be true
await waitFor(() => video.currentTime > 21)
  • add a specific timeout to stop waiting
await waitFor(() => video.currentTime > 21, 60*1000)
  • pass it some other test function
if(await waitFor(someTest)) console.log('test passed')
else console.log("test didn't pass after 20 seconds")

Full Version:

This version takes cares of more cases than the simple version, null, undefined, empty array, etc., has a timeout, a frequency can be passed as an argument, and logs to the console what it is doing with some nice colors

function sleep(ms) { return new Promise(resolve => setTimeout(resolve, ms));}


/**
* Waits for the test function to return a truthy value
* example usage:
*    wait for an element to exist, then save it to a variable
*        let el = await waitFor(() => document.querySelector('#el_id')))
*    timeout_ms and frequency are optional parameters
*/
async function waitFor(test, timeout_ms = 20 * 1000, frequency = 200) {
if (typeof (test) != "function")     throw new Error("test should be a function in waitFor(test, [timeout_ms], [frequency])")
if (typeof (timeout_ms) != "number") throw new Error("timeout argument should be a number in waitFor(test, [timeout_ms], [frequency])");
if (typeof (frequency) != "number")  throw new Error("frequency argument should be a number in waitFor(test, [timeout_ms], [frequency])");
let logPassed = () => console.log('Passed: ', test);
let logTimedout = () => console.log('%c' + 'Timeout : ' + test, 'color:#cc2900');
let last = Date.now();
let logWaiting = () => {
if(Date.now() - last > 1000) {
last = Date.now();
console.log('%c' + 'waiting for: ' + test, 'color:#809fff');
}
}


let endTime = Date.now() + timeout_ms;
let isNotTruthy = (val) => val === undefined || val === false || val === null || val.length === 0; // for non arrays, length is undefined, so != 0
let result = test();
while (isNotTruthy(result)) {
if (Date.now() > endTime) {
logTimedout();
return false;
}
logWaiting();
await sleep(frequency);
result = test();
}
logPassed();
return result;
}

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