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Trying to use `debounce` to throttle API requests, but getting "js Uncaught (in promise) TypeError: [function] is undefined" [duplicate]

I'm trying to implement a debounce function that works with a promise in javascript. That way, each caller can consume the result of the "debounced" function using a Promise. Here is the best I have been able to come up with so far:
function debounce(inner, ms = 0) {
let timer = null;
let promise = null;
const events = new EventEmitter(); // do I really need this?
return function (...args) {
if (timer == null) {
promise = new Promise(resolve => {
events.once('done', resolve);
});
} else {
clearTimeout(timer);
}
timer = setTimeout(() => {
events.emit('done', inner(...args));
timer = null;
}, ms);
return promise;
};
}
Ideally, I would like to implement this utility function without introducing a dependency on EventEmitter (or implementing my own basic version of EventEmitter), but I can't think of a way to do it. Any thoughts?

I found a better way to implement this with promises:
function debounce(inner, ms = 0) {
let timer = null;
let resolves = [];
return function (...args) {
// Run the function after a certain amount of time
clearTimeout(timer);
timer = setTimeout(() => {
// Get the result of the inner function, then apply it to the resolve function of
// each promise that has been created since the last time the inner function was run
let result = inner(...args);
resolves.forEach(r => r(result));
resolves = [];
}, ms);
return new Promise(r => resolves.push(r));
};
}
I still welcome suggestions, but the new implementation answers my original question about how to implement this function without a dependency on EventEmitter (or something like it).

In Chris's solution all calls will be resolved with delay between them, which is good, but sometimes we need resolve only last call.
In my implementation, only last call in interval will be resolved.
function debounce(f, interval) {
let timer = null;
return (...args) => {
clearTimeout(timer);
return new Promise((resolve) => {
timer = setTimeout(
() => resolve(f(...args)),
interval,
);
});
};
}
And the following typescript(>=4.5) implementation supports aborted features:
Support aborting promise via reject(). If we don't abort it, it cannot execute finally function.
Support custom reject abortValue.
If we catch error, we may need to determine if the error type is Aborted
/**
*
* #param f callback
* #param wait milliseconds
* #param abortValue if has abortValue, promise will reject it if
* #returns Promise
*/
export function debouncePromise<T extends (...args: any[]) => any>(
fn: T,
wait: number,
abortValue: any = undefined,
) {
let cancel = () => { };
// type Awaited<T> = T extends PromiseLike<infer U> ? U : T
type ReturnT = Awaited<ReturnType<T>>;
const wrapFunc = (...args: Parameters<T>): Promise<ReturnT> => {
cancel();
return new Promise((resolve, reject) => {
const timer = setTimeout(() => resolve(fn(...args)), wait);
cancel = () => {
clearTimeout(timer);
if (abortValue!==undefined) {
reject(abortValue);
}
};
});
};
return wrapFunc;
}
/**
// deno run src/utils/perf.ts
function add(a: number) {
return Promise.resolve(a + 1);
}
const wrapFn= debouncePromise(add, 500, 'Aborted');
wrapFn(2).then(console.log).catch(console.log).finally(()=>console.log('final-clean')); // Aborted + final-clean
wrapFn(3).then(console.log).catch(console.log).finally(()=>console.log('final-clean')); // 4 + final_clean
Note:
I had done some memory benchmarks,　huge number of pending promises won't cause memory leak. It seems that V8 engine GC will clean unused promises.

I landed here because I wanted to get the return value of the promise, but debounce in underscore.js was returning undefined instead. I ended up using lodash version with leading=true. It works for my case because I don't care if the execution is leading or trailing.
https://lodash.com/docs/4.17.4#debounce
_.debounce(somethingThatReturnsAPromise, 300, {
leading: true,
trailing: false
})

resolve one promise, cancel the others
Many implementations I've seen over-complicate the problem or have other hygiene issues. In this post we will write our own debounce. This implementation will -
have at most one promise pending at any given time (per debounced task)
stop memory leaks by properly cancelling pending promises
resolve only the latest promise
demonstrate proper behaviour with live code demos
We write debounce with its two parameters, the task to debounce, and the amount of milliseconds to delay, ms. We introduce a single local binding for its local state, t -
function debounce (task, ms) {
let t = { promise: null, cancel: _ => void 0 }
return async (...args) => {
try {
t.cancel()
t = deferred()
await t.promise
await task(...args)
}
catch (_) { /* prevent memory leak */ }
}
}
We depend on a reusable deferred function, which creates a new promise that resolves in ms milliseconds. It introduces two local bindings, the promise itself, an the ability to cancel it -
function deferred (ms) {
let cancel, promise = new Promise((resolve, reject) => {
cancel = reject
setTimeout(resolve, ms)
})
return { promise, cancel }
}
click counter example
In this first example, we have a button that counts the user's clicks. The event listener is attached using debounce, so the counter is only incremented after a specified duration -
// debounce, deferred
function debounce (task, ms) { let t = { promise: null, cancel: _ => void 0 }; return async (...args) => { try { t.cancel(); t = deferred(ms); await t.promise; await task(...args); } catch (_) { console.log("cleaning up cancelled promise") } } }
function deferred (ms) { let cancel, promise = new Promise((resolve, reject) => { cancel = reject; setTimeout(resolve, ms) }); return { promise, cancel } }
// dom references
const myform = document.forms.myform
const mycounter = myform.mycounter
// event handler
function clickCounter (event) {
mycounter.value = Number(mycounter.value) + 1
}
// debounced listener
myform.myclicker.addEventListener("click", debounce(clickCounter, 1000))
<form id="myform">
<input name="myclicker" type="button" value="click" />
<output name="mycounter">0</output>
</form>
live query example, "autocomplete"
In this second example, we have a form with a text input. Our search query is attached using debounce -
// debounce, deferred
function debounce (task, ms) { let t = { promise: null, cancel: _ => void 0 }; return async (...args) => { try { t.cancel(); t = deferred(ms); await t.promise; await task(...args); } catch (_) { console.log("cleaning up cancelled promise") } } }
function deferred (ms) { let cancel, promise = new Promise((resolve, reject) => { cancel = reject; setTimeout(resolve, ms) }); return { promise, cancel } }
// dom references
const myform = document.forms.myform
const myresult = myform.myresult
// event handler
function search (event) {
myresult.value = `Searching for: ${event.target.value}`
}
// debounced listener
myform.myquery.addEventListener("keypress", debounce(search, 1000))
<form id="myform">
<input name="myquery" placeholder="Enter a query..." />
<output name="myresult"></output>
</form>

Here's my version in typescript (mostly based on Chris one), if someone need it 😉
function promiseDebounce (exec: (...args: any[]) => Promise<any>, interval: number): () => ReturnType<typeof exec> {
let handle: number | undefined;
let resolves: Array<(value?: unknown) => void> = [];
return async (...args: unknown[]) => {
clearTimeout(handle);
handle = setTimeout(
() => {
const result = exec(...args);
resolves.forEach(resolve => resolve(result));
resolves = [];
},
interval
);
return new Promise(resolve => resolves.push(resolve));
};
}

No clue what you are trying to accomplish as it vastly depends on what your needs are. Below is something somewhat generic though. Without a solid grasp of what is going on in the code below, you really might not want to use it though.
// Debounce state constructor
function debounce(f) {
this._f = f;
return this.run.bind(this)
}
// Debounce execution function
debounce.prototype.run = function() {
console.log('before check');
if (this._promise)
return this._promise;
console.log('after check');
return this._promise = this._f(arguments).then(function(r) {
console.log('clearing');
delete this._promise; // remove deletion to prevent new execution (or remove after timeout?)
return r;
}.bind(this)).catch(function(r) {
console.log('clearing after rejection');
delete this._promise; // Remove deletion here for as needed as noted above
return Promise.reject(r); // rethrow rejection
})
}
// Some function which returns a promise needing debouncing
function test(str) {
return new Promise(function(resolve, reject) {
setTimeout(function() {
console.log('test' + str);
resolve();
}, 1000);
});
}
a = new debounce(test); // Create debounced version of function
console.log("p1: ", p1 = a(1));
console.log("p2: ", p2 = a(2));
console.log("p1 = p2", p1 === p2);
setTimeout(function() {
console.log("p3: ", p3 = a(3));
console.log("p1 = p3 ", p1 === p3, " - p2 = p3 ", p2 === p3);
}, 2100)
View the console when running the code above. I put a few messages to show a bit about what is going on. First some function which returns a promise is passed as an argument to new debounce(). This creates a debounced version of the function.
When you run the debounced function as the code above does (a(1), a(2), and a(3)) you will notice during processing it returns the same promise instead of starting a new one. Once the promise is complete it removes the old promise. In code above I wait for the timeout manually with setTimeout before running a(3).
You can clear the promise in other ways as well, like adding a reset or clear function on debounce.prototype to clear the promise at a different time. You could also set it to timeout. The tests in the console log should show p1 and p2 get the same promise (reference comparison "===" is true) and that p3 is different.

Here is what I came up with to solve this issue. All calls to the debounced function batched to the same invocation all return the same Promise that resolves to the result of the future invocation.
function makeFuture() {
let resolve;
let reject;
let promise = new Promise((d, e) => {
resolve = d;
reject = e;
});
return [promise, resolve, reject];
}
function debounceAsync(asyncFunction, delayMs) {
let timeout;
let [promise, resolve, reject] = makeFuture();
return function(...args) {
clearTimeout(timeout);
timeout = setTimeout(async () => {
const [prevResolve, prevReject] = [resolve, reject];
[promise, resolve, reject] = makeFuture();
try {
prevResolve(await asyncFunction.apply(this, args));
} catch (error) {
prevReject(error);
}
}, delayMs);
return promise;
}
}
const start = Date.now();
const dog = {
sound: 'woof',
bark() {
const delay = Date.now() - start;
console.log(`dog says ${this.sound} after ${delay} ms`);
return delay;
},
};
dog.bark = debounceAsync(dog.bark, 50);
Promise.all([dog.bark(), dog.bark()]).then(([delay1, delay2]) => {
console.log(`Delay1: ${delay1}, Delay2: ${delay2}`);
});

Both Chris and Николай Гордеев have good solutions. The first will resolve all of them. The problem is that they all be resolved, but usually you wouldn't want all of them to run.
The second solution solved that but created a new problem - now you will have multiple awaits. If it's a function that is called a lot (like search typing) you might have a memory issue. I fixed it by creating the following asyncDebounce that will resolve the last one and reject (and the awaiting call will get an exception that they can just catch).
const debounceWithRejection = (
inner,
ms = 0,
reject = false,
rejectionBuilder
) => {
let timer = null;
let resolves = [];
return function (...args) {
clearTimeout(timer);
timer = setTimeout(() => {
const resolvesLocal = resolves;
resolves = [];
if (reject) {
const resolve = resolvesLocal.pop();
resolve.res(inner(...args));
resolvesLocal.forEach((r, i) => {
!!rejectionBuilder ? r.rej(rejectionBuilder(r.args)) : r.rej(r.args);
});
} else {
resolvesLocal.forEach((r) => r.res(inner(...args)));
}
resolves = [];
}, ms);
return new Promise((res, rej) =>
resolves.push({ res, rej, args: [...args] })
);
};
};
The rejection logic is optional, and so is the rejectionBuilder. It's an option to reject with specific builder so you will know to catch it.
You can see runing example.

This may not what you want, but can provide you some clue:
/**
* Call a function asynchronously, as soon as possible. Makes
* use of HTML Promise to schedule the callback if available,
* otherwise falling back to `setTimeout` (mainly for IE<11).
* #type {(callback: function) => void}
*/
export const defer = typeof Promise=='function' ?
Promise.resolve().then.bind(Promise.resolve()) : setTimeout;

Calling promise method with recursion is going for infinite

I am calling a method a which is returning a promise and inside it I am calling a method which do some operation and update the count variable. I want all the promise get finish once the count is done but it is not stopping after reaching the value 10.
var count = 0;
function a(p){
return new Promise((resolve, reject) =>{
console.log(count);
if(count == 10) {console.log('sdfsdfsd'); resolve(Date.now()); }
callee().then(() => { count++; a(); } )
})
}
function callee(){ return new Promise((resolve) => resolve())}
a(1).then((res) => console.log(res)).catch((res) => console.log(res));

// So you have a function `foo` which returns a promise eventually resolved, you want to write a function
// `bar` that will call this function n times, waitinng between each call for the returned promise to be
// resolved. This function will itself return a promise
// this function returns a promise which is resolved after one second
const foo = () => new Promise(resolve => setTimeout(resolve, 1000));
// Recursively call the foo function until 0 is reached.
// This will actually create a chain of promises which settle after one second.
// It also uses the fact that if you return a promise `a` in the `then` handler the returned
// promise `b` will only settle when `a` is resolved.
const bar = n => {
if (n === 0) return Promise.resolve();
return foo().then(() => bar(n-1));
};
bar(10).then(() => console.log("done"));

var count = 0;
function a(p) {
return new Promise((resolve, reject) => {
console.log(count);
if (count == 10) { console.log('sdfsdfsd'); return resolve(Date.now()); }
return callee().then(() => {
count++;
return resolve(a());
})
})
}
function callee() { return new Promise((resolve) => resolve()) }
a(1).then((res) => console.log("res",res)).catch((res) => console.log(res))

NodeJS - Wait for return inside setTimeout

I am trying to learn async/await. I want to wait for the return statement inside my async function. I have to call it several times so I used a setTiemout inside.
EDIT:
//Processing gallery
async function somefunction(){
async function getPictureR(){
/* some code */
if($('.actions > .prev', html)[0]){
older = $('.actions > .prev', html)[0].attribs.href;
} else {
console.log('return');
return;
}
/* some code */
return new Promise((resolve, reject) => {
setTimeout(getPictureR, 1 * 1000/2);
})
}
await getPictureR();
console.log('getPictureR done');
}
I've tried await getPictureR() but it triggers right after the first call to the function. How can I wait for that return ?

You should never call a promise-returning function, like getPictureR, from an asynchronous (non-promise) callback or inside the new Promise constructor. You also were never resolving the new Promise. You are looking for
return new Promise((resolve, reject) => {
setTimeout(resolve, 1 * 1000/2);
}).then(() => {
return getPictureR(); // do the promise call in a `then` callback to properly chain it
})
But since you're using async/await, you don't need the recursive function and the then chaining anyway. Also you can factor out the setTimeout-in-promise wrapping in a separate helper function:
function delay(t) {
return new Promise(resolve => setTimeout(resolve, t));
}
async function somefunction() {
while (true)
/* some code */
const prev = $('.actions > .prev', html);
if (prev.length) {
older = prev[0].attribs.href;
} else {
console.log('return');
break;
}
/* some code */
await delay(1 * 1000/2);
// ^^^^^^^^^^^
}
console.log('getPicture done');
}

ES6 promises with timeout interval

I'm trying to convert some of my code to promises, but I can't figure out how to chain a new promise inside a promise.
My promise function should check the content of an array every second or so, and if there is any item inside it should resolve. Otherwise it should wait 1s and check again and so on.
function get(){
return new Promise((resolve) => {
if(c.length > 0){
resolve(c.shift());
}else{
setTimeout(get.bind(this), 1000);
}
});
}
let c = [];
setTimeout(function(){
c.push('test');
}, 2000);
This is how I expect my get() promise function to work, it should print "test" after 2 or 3 seconds max:
get().then((value) => {
console.log(value);
});
Obviously it doesn't work, nothing is ever printed

setTimeout has terrible chaining and error-handling characteristics on its own, so always wrap it:
const wait = ms => new Promise(resolve => setTimeout(resolve, ms));
function get(c) {
if (c.length) {
return Promise.resolve(c.shift());
}
return wait(1000).then(() => get(c)); // try again
}
let c = [];
get(c).then(val => console.log(val));
wait(2000).then(() => c.push('test'));
While you didn't ask, for the benefit of others, this is a great case where async/await shines:
const wait = ms => new Promise(r => setTimeout(r, ms));
async function get(c) {
while (!c.length) {
await wait(1000);
}
return c.shift();
}
let c = [];
get(c).then(val => console.log(val));
wait(2000).then(() => c.push('test'));
Note how we didn't need Promise.resolve() this time, since async functions do this implicitly.

The problem is that your recursive call doesn't pass the resolve function along, so the else branch can never call resolve.
One way to fix this would be to create a closure inside the promise's callback so that the recursive call will have access to the same resolve variable as the initial call to get.
function get() {
return new Promise((resolve) => {
function loop() {
if (c.length > 0) {
resolve(c.shift());
} else {
setTimeout(loop, 1000);
}
}
loop();
});
}
let c = [];
setTimeout(function() {
c.push('test');
}, 2000);
get().then(val => console.log(val));

In the else case, you never resolve that promise. get might create another one, but it is returned to nowhere.
You should promisify your asynchronous function (setTimeout) on the lowest level, and then only chain your promises. By returning the result of the recursive call from a then callback, the resulting promise will resolve with the same result:
function delayAsync(time) {
return new Promise(resolve => {
setTimeout(resolve, time);
});
}
function get(c) {
if (c.length > 0){
return Promise.resolve(c.shift());
} else {
return delay(1000).then(() => {
return get(c); // try again
});
}
}

What you need is a polling service, which checks periodically for specific condition prior proceeding with promise resolution. Currently when you run setTimeout(get.bind(this), 1000); you are creating a new instance of the promise without actually resolving the initial promise, because you don't reference to the initial resolve function that you created.
Solution:
Create a new callback function that you can reference to it inside the promise
Pass the resolve & reject as params in the setTimeout invocation e.g. setTimeout(HandlePromise, 1000, resolve, reject, param3, param4 ..); setTimeout API
function get() {
var handlerFunction = resolve => {
if (c.length > 0) {
resolve(c.shift());
} else {
setTimeout(handlerFunction, 1000, resolve);
}
};
return new Promise(handlerFunction);
}
let c = [];
setTimeout(function() {
c.push("test");
}, 2000);
get().then(value => {
console.log(value);
});
For more information look into javascript polling article

You could try this solution. Since JS needs to free itself to download the images, I use await within an asynchronous function and an asynchronous call to wake up JS after a delay
private async onBeforeDoingSomething() : void {
await this.delay(1000);
console.log("All images are loaded");
}
private delay (ms : number = 500) : Promise<number> {
return new Promise((resolve,reject) => {
const t = setTimeout( () => this.areImgsLoaded(resolve), ms);
});
}
private async areImgsLoaded (resolve) {
let reload = false;
const img = document.querySelectorAll('img');
console.log("total of images: ",img.length);
for (let i = 0; i < img.length; i++){
if (!img[i]["complete"]) {
console.log("img not load yet");
reload = true;
break;
}
}
if (reload) {
await this.delay();
}
resolve();
}

Use setInterval to check every second. Run this script to understand.
let c = [];
function get(){
return new Promise((resolve) => {
var i = setInterval(function(){
if(c.length > 0){
resolve(c.shift());
clearInterval(i);
}
}, 1000);
});
}
setTimeout(function(){
c.push('test');
}, 2000);
get().then((value) => {
console.log(value);
});

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.

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);
})();
});
}

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;
}

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).

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()
}

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);