In my classes, I've been learning indexeddb and how to handle asynchronous-ity. In the interest of stretching myself and learning more, I've been trying to use the functional paradigm in my code.
I'd been using cursors before but I've realized that my code isn't completely immutable or stateless, which bothers me. I was wondering if there was a way to use cursors without having to resort to pushing elements to an array.
Currently, I'm using something like:
async function getTable(){
return new Promise(function(resolve, reject){
const db = await connect();
const transaction = await db.transaction(["objectStore"], "readonly");
const store = await transaction.objectStore("objectStore");
var myArray = [];
store.openCursor().onsuccess = function(evt) {
var cursor = evt.target.result;
if (cursor) {
myArray.push(cursor.value);
//I don't want to use push, because it's impure. See link:
cursor.continue();
} else {
resolve(myArray);
}
}
}
//link: https://en.wikipedia.org/wiki/Purely_functional_programming
And it works fine. But it's not pure, and it uses push. I'd like to learn another way to do it, if there is one.
Thank you!
You could do several things in the spirit of functional programming, but it is probably not worth it in JavaScript.
For example, to implement immutability arrays, at least in spirit, you simply create and return a new array everytime you want to add an element to an array. I think if I recall my Scheme correctly the function was called cons.
function push(array, newValue) {
const copy = copyArray(array);
copy.push(newValue);
return copy;
}
function copyArray(array) {
const copy = [];
for(const oldValue of array) {
copy.push(oldValue);
}
return copy;
}
// Fancy spread operator syntax implementation if you are so inclined
function copyArray2(inputArray) {
return [...inputArray];
}
Now instead of mutating the input array, you are creating a modified copy of it. Keep in mind this is absolutely horrific performance, and you probably never ever ever want to do this in a real app.
You could take this further probably and use some stack based approach. Again, this is hilariously bad, but it would be something that basically creates a push function that returns a function. The stack increases in size as you append items, and then when you unwind it, it unwinds into an array of values.
My second point is that you can avoid this array building entirely by using newer, less-well-documented features of indexedDB. Specifically, IDBObjectStore.prototype.getAll. This function would create the array for you, opaquely, and provided that it is opaque, you will never know about any FP anti-patterns hidden within its abstraction, and therefore are not breaking the rules.
function getTable(){
return new Promise(function(resolve, reject){
const db = await connect();
const transaction = await db.transaction(["objectStore"], "readonly");
const store = await transaction.objectStore("objectStore");
const request = store.getAll();
request.onsuccess = () => resolve(request.result);
request.onerror = () => reject(request.error);
}
}
My third point is simple, use db.transaction("objectStore", "readonly"); instead of db.transaction(["objectStore"], "readonly");. The array parameter is optional, and it is preferable to keep it simple.
My fourth point is simple, use db.transaction("objectStore"); instead of db.transaction(["objectStore"], "readonly");. "readonly" is the default mode of a transaction, so there is no need to specify it. The purpose of your code is sufficiently clearly communicated by not specifying the parameter, and omitting the parameter is less verbose.
My fifth point is your use of the async specifier(?) in the function definition. You don't need to use it here. You have a synchronous function returning an instance of a Promise. If anything, specifying async leads to increased confusion about what your code is doing. Instead, you probably want to be using the async qualifier when using the function.
My sixth point is that you are violating some FP principles in your call to connect(). What is connect connecting to? Implied global state. This is quite the violation of the spirit of functional programming. Therefore, your connect parameters must be parameters to the function itself, so that you do not rely on the implied knowledge of which database is used.
My seventh point is that you are using a database. Functional programmers have so many problems with databases, or any I/O or interaction with the outside world, they seem to like to pretend there is no such thing. Therefore, you probably shouldn't be using a database at all if you want to use functional approach.
My eight point is that connecting within the promise (calling and awaiting connect) is definitively an anti-pattern. The goal is to chain promises, so that one starts after the other. Either the caller has to call connect and then call getTable, or getTable has to call connect and then do the rest of the promise work.
My ninth point is that I am not even sure how this executes. The executor function that you pass to the Promise constructor is not qualified as async. So you are using the await modifier in a non-qualified function. This should be throwing an error. Technically, a promise swallowed exception, meaning that this promise should always be rejecting.
My tenth point is your use of async everywhere. I have no idea what is going on, unless your connect function is returning some kind of wrapper library, but calls to IDBDatabase.prototype.transaction and IDBTransaction.prototype.objectStore are synchronous. It makes no sense why you are awaiting them. They do not return promises.'
My eleventh point is that you are not watching for errors. request.onsuccess is not called back when there is an error. This could lead to your promise never settling. You need to also consider the failure case.
My twelfth point is you seem to missing a closing parentheses for your onsuccess handler function. I am not sure how this code ever is interpreted successfully.
Related
This is the function that I have:
let counter = 0;
let dbConnected = false;
async function notASingleton(params) {
if (!dbConnected) {
await new Promise(resolve => {
if (Math.random() > 0.75) throw new Error();
setTimeout((params) => {
dbConnected = true; // assume we use params to connect to DB
resolve();
}, 1000);
});
return counter++
}
};
// in another module that imports notASingleton
Promise.all([notASingleton(params), notASingleton(params), notASingleton(params), notASingleton(params)]);
or
// in another module that imports notASingleton
notASingleton(params);
notASingleton(params);
notASingleton(params);
notASingleton(params);
The problem is that apparently the notASinglton promises in might be executed concurrently and assuming they are run in parallel, the execution context for all of them will be dbConnected = false.
Note: I'm aware that we could introduce a new variable e.g. initiatingDbConnection and instead of checking for !dbConnected check for !initiatingDbConnection; however, as long as concurrently means that the context of the promises will be the same inside Promise.all, that will not change anything.
The pattern can be properly implemented in e.g. Java by utilizing the contracts of JVM for creating a class: https://stackoverflow.com/a/16106598/12144949
However, even that Java implementation cannot be used for my use case where I need to pass a variable: "The client application can’t pass any argument, so we can’t reuse it. For example, having a generic singleton class for database connection where client application supplies database server properties."
https://www.journaldev.com/171/thread-safety-in-java-singleton-classes-with-example-code
Note 2: Another possibly related issue: https://eslint.org/docs/rules/require-atomic-updates#rule-details
"On some computers, they may be executed in parallel, or in some sense concurrently, while on others they may be executed serially."
That MDN description is rubbish. I'll remove it. Promise.all is not responsible for executing anything, and promises cannot be "executed" anyway. All it does is to wait for its arguments, and you're the one who are creating those promises in the array. They would run (and concurrently open multiple connections) even if you omitted Promise.all and simply called notASingleton() multiple times.
the execution context for all of them will be dbConnected = false
Yes, but only because your dbConnected = true; is in a timeout and you are calling notASingleton() again before that happens. Not because Promise.all does anything special.
Foo is an example of a function that works "now" but in the future it will possibly throw an error or return some different value.
const foo = () => {
if (new Date() < new Date('2019-09-05') {
return 'Hello world"
}
throw new Error('failure')
}
const x = cacheLock(foo())
What I am looking for is a way to wrap the return value of foo() so that If a value is returned successfully on initial execution it will cache a snapshot of the result to a file locally and on subsequent executions of this code the stored value is automatically returned and the contents of foo() is never run.
I'm looking for a way to serialize functions results as well as async / awaited promises locally, then playback the stored results.
An ideal feature would be a way to also re-run the code and update the cache.
Is this possible? I know there are issues when it comes to serializing objects and using .toJSON() .fromJSON() methods is one way to solve
that.
Are there any libraries out there that can do this? Is there a means of achieving this without serialization?
I'd love a generic solution to this problem that's not tied to testing, or databases. But my top use case is easily mocking / stubbing API calls and Database reads. The sources of which can change with time and aren't reliable to run in tests. There are many solutions when it comes to those mocking databases or mocking api calls, but I would find that some way to cache live calls, and store them for later the most intuitive.
This idea is more of a post-runtime memoization, a cache that sustains even when the process is finished, while traditional memoization just lasts within the runtime of a process.
I am very new to NodeJS. I was trying to get a function written that can simply return a configuration value from DB. I might need to call it multiple times.
In PHP or other synchronous languages, I would use the following code for it
function getConfigValue($configKeyName)
{
// DB READ OPERATIONS
return $confguration_value_fetched_from_db
}
getConfigValue("key1");
getConfigValue("key2");
etc
But in NodeJS, I found it too difficult to do this operation because of the Asynchronous nature of the code. After asking some questions here, and after spending hours to learn Callbacks, Promises, Async/await keywords, being a beginner the below is best code I could reach.
// Below function defines the 'get' function
var get = async (key) => {
var result = await COLLECTIONNAME.findOne({key}); //MongoDB in backend
return result.value;
}
// Here I am forced to define another async function so that I can await for the get function.
function async anotherfunction()
{
var value_I_am_lookingfor1 = await get("key1");
var value_I_am_lookingfor2 = await get("key2");
}
anotherfunction();
While it might work, I am not fully happy with the result, mainly because I really don't want to do all my further coding based on the fetched value within this function anotherfunction(). All I want is to fetch a single value? Also I might need to easily call it from many places within the application, not just from here (I was planning to place it in a module)
Any better or easier methods? Or should I always get the value I want, and then nest it with a 'then.' to do the subsequent operation? I even doubt the fundamental approach I take on NodeJS coding itself may be wrong.
Can anyone guide me?
mainly because I really don't want to do all my further coding based on the fetched value within this function anotherfunction(). All I want is to fetch a single value?
Because the request is asynchronous, and your code depends on having the fetched values first, there's no option other than to wait for the values to be retrieved before continuing. Somewhere in the script, control flow needs to halt until the values are retrieved before other parts of the script continue.
Also I might need to easily call it from many places within the application, not just from here (I was planning to place it in a module)
You should have that module make the requests and export a Promise that resolves to the values needed. Rather than using await (which forces requests to be processed in serial), you should probably use Promise.all, which will allow multiple requests to be sent out at once. For example:
valuegetter.js
const get = key => COLLECTIONNAME.findOne({ key }).then(res => res.value);
export default Promise.all([
get('key1'),
get('key2')
]);
main.js
import prom from './valuegetter.js';
prom.then(([val1, val2]) => {
// do stuff with val1 and val2
})
.catch((err) => {
// handle errors
});
If other modules need val1 and val2, call them from main.js with the values they need.
This is a post that might come across as quite conceptual, since I first start with a lot of pseudo code. - At the end you'll see the use case for this problem, though a solution would be a "tool I can add to my tool-belt of useful programming techniques".
The problem
Sometimes one might need to create multiple promises, and either do something after all promises have ended. Or one might create multiple promises, based on the results of the previous promises. The analogy can be made to creating an array of values instead of a single value.
There are two basic cases to be considered, where the number of promises is indepedented of the result of said promises, and the case where it is depedent. Simple pseudo code of what "could" be done.
for (let i=0; i<10; i++) {
promise(...)
.then(...)
.catch(...);
}.then(new function(result) {
//All promises finished execute this code now.
})
The basically creates n (10) promises, and the final code would be executed after all promises are done. Of course the syntax isn't working in javascript, but it shows the idea. This problem is relativelly easy, and could be called completely asynchronous.
Now the second problem is like:
while (continueFn()) {
promise(...)
.then(.. potentially changing outcome of continueFn ..)
.catch(.. potentially changing outcome of continueFn ..)
}.then(new function(result) {
//All promises finished execute this code now.
})
This is much more complex, as one can't just start all promises and then wait for them to finish: in the end you'll have to go "promise-by-promise". This second case is what I wish to figure out (if one can do the second case you can also do the first).
The (bad) solution
I do have a working "solution". This is not a good solution as can probably quickly be seen, after the code I'll talk about why I dislike this method. Basically instead of looping it uses recursion - so the "promise" (or a wrapper around a promise which is a promise) calls itself when it's fulfilled, in code:
function promiseFunction(state_obj) {
return new Promise((resolve, reject) => {
//initialize fields here
let InnerFn = (stateObj) => {
if (!stateObj.checkContinue()) {
return resolve(state_obj);
}
ActualPromise(...)
.then(new function(result) {
newState = stateObj.cloneMe(); //we'll have to clone to prevent asynchronous write problems
newState.changeStateBasedOnResult(result);
return InnerFn(newState);
})
.catch(new function(err) {
return reject(err); //forward error handling (must be done manually?)
});
}
InnerFn(initialState); //kickstart
});
}
Important to note is that the stateObj should not change during its lifetime, but it can be really easy. In my real problem (which I'll explain at the end) the stateObj was simply a counter (number), and the if (!stateObj.checkContinue()) was simply if (counter < maxNumber).
Now this solution is really bad; It is ugly, complicated, error prone and finally impossible to scale.
Ugly because the actual business logic is buried in a mess of code. It doesn't show "on the can" that is actually simply doing what the while loop above does.
Complicated because the flow of execution is impossible to follow. First of all recursive code is never "easy" to follow, but more importantly you also have to keep in mind thread safety with the state-object. (Which might also have a reference to another object to, say, store a list of results for later processing).
It's error prone since there is more redundancy than strictly necessary; You'll have to explicitly forward the rejection. Debugging tools such as a stack trace also quickly become really hard to look through.
The scalability is also a problem at some points: this is a recursive function, so at one point it will create a stackoverflow/encounter maximum recursive depth. Normally one could either optimize by tail recursion or, more common, create a virtual stack (on the heap) and transform the function to a loop using the manual stack. In this case, however, one can't change the recursive calls to a loop-with-manual-stack; simply because of how promise syntax works.
The alternative (bad) solution
A colleague suggested an alternative approach to this problem, something that initially looked much less problematic, but I discarded ultimatelly since it was against everything promises are meant to do.
What he suggested was basically looping over the promises as per above. But instead of letting the loop continue there would be a variable "finished" and an inner loop that constantly checks for this variable; so in code it would be like:
function promiseFunction(state_obj) {
return new Promise((resolve, reject) => {
while (stateObj.checkContinue()) {
let finished = false;
let err = false;
let res = null;
actualPromise(...)
.then(new function(result) {
res = result;
finished = true;
})
.catch(new function(err) {
res = err;
err = true;
finished = true;
});
while(!finished) {
sleep(100); //to not burn our cpu
}
if (err) {
return reject(err);
}
stateObj.changeStateBasedOnResult(result);
}
});
}
While this is less complicated, since it's now easy to follow the flow of execution. This has problems of its own: not for the least that it's unclear when this function will end; and it's really bad for performance.
Conclusion
Well this isn't much to conclude yet, I'd really like something as simple as in the first pseudo code above. Maybe another way of looking at things so that one doesn't have the trouble of deeply recursive functions.
So how would you rewrite a promise that is part of a loop?
The real problem used as motivation
Now this problem has roots in a real thing I had to create. While this problem is now solved (by applying the recursive method above), it might be interesting to know what spawned this; The real question however isn't about this specific case, but rather on how to do this in general with any promise.
In a sails app I had to check a database, which had orders with order-ids. I had to find the first N "non existing order-ids". My solution was to get the "first" M products from the database, find the missing numbers within it. Then if the number of missing numbers was less than N get the next batch of M products.
Now to get an item from a database, one uses a promise (or callback), thus the code won't wait for the database data to return. - So I'm basically at the "second problem:"
function GenerateEmptySpots(maxNum) {
return new Promise((resolve, reject) => {
//initialize fields
let InnerFn = (counter, r) => {
if (r > 0) {
return resolve(true);
}
let query = {
orderNr: {'>=': counter, '<': (counter + maxNum)}
};
Order.find({
where: query,
sort: 'orderNr ASC'})
.then(new function(result) {
n = findNumberOfMissingSpotsAndStoreThemInThis();
return InnerFn(newState, r - n);
}.bind(this))
.catch(new function(err) {
return reject(err);
});
}
InnerFn(maxNum); //kickstart
});
}
EDIT:
Small post scriptus: the sleep function in the alternative is just from another library which provided a non-blocking-sleep. (not that it matters).
Also, should've indicated I'm using es2015.
The alternative (bad) solution
…doesn't actually work, as there is no sleep function in JavaScript. (If you have a runtime library which provides a non-blocking-sleep, you could just have used a while loop and non-blocking-wait for the promise inside it using the same style).
The bad solution is ugly, complicated, error prone and finally impossible to scale.
Nope. The recursive approach is indeed the proper way to do this.
Ugly because the actual business logic is buried in a mess of code. And error-prone as you'll have to explicitly forward the rejection.
This is just caused by the Promise constructor antipattern! Avoid it.
Complicated because the flow of execution is impossible to follow. Recursive code is never "easy" to follow
I'll challenge that statement. You just have to get accustomed to it.
You also have to keep in mind thread safety with the state-object.
No. There is no multi-threading and shared memory access in JavaScript, if you worry about concurrency where other stuff affects your state object while the loop runs that will a problem with any approach.
The scalability is also a problem at some points: this is a recursive function, so at one point it will create a stackoverflow
No. It's asynchronous! The callback will run on a new stack, it's not actually called recursively during the function call and does not carry those stack frames around. The asynchronous event loop already provides the trampoline to make this tail-recursive.
The good solution
function promiseFunction(state) {
const initialState = state.cloneMe(); // clone once for this run
// initialize fields here
return (function recurse(localState) {
if (!localState.checkContinue())
return Promise.resolve(localState);
else
return actualPromise(…).then(result =>
recurse(localState.changeStateBasedOnResult(result))
);
}(initialState)); // kickstart
}
The modern solution
You know, async/await is available in every environment that implemented ES6, as all of them also implemented ES8 now!
async function promiseFunction(state) {
const localState = state.cloneMe(); // clone once for this run
// initialize fields here
while (!localState.checkContinue()) {
const result = await actualPromise(…);
localState = localState.changeStateBasedOnResult(result);
}
return localState;
}
Let’s begin with the simple case: You have N promises that all do some work, and you want to do something when all the promises have finished. There’s actually a built-in way to do exactly that: Promise.all. With that, the code will look like this:
let promises = [];
for (let i=0; i<10; i++) {
promises.push(doSomethingAsynchronously());
}
Promise.all(promises).then(arrayOfResults => {
// all promises finished
});
Now, the second call is a situation you encounter all the time when you want to continue doing something asynchronously depending on the previous asynchronous result. A common example (that’s a bit less abstract) would be to simply fetch pages until you hit the end.
With modern JavaScript, there’s luckily a way to write this in a really readable way: Using asynchronous functions and await:
async function readFromAllPages() {
let shouldContinue = true;
let pageId = 0;
let items = [];
while (shouldContinue) {
// fetch the next page
let result = await fetchSinglePage(pageId);
// store items
items.push.apply(items, result.items);
// evaluate whether we want to continue
if (!result.items.length) {
shouldContinue = false;
}
pageId++;
}
return items;
}
readFromAllPages().then(allItems => {
// items have been read from all pages
});
Without async/await, this will look a bit more complicated, since you need to manage all this yourself. But unless you try to make it super generic, it shouldn’t look that bad. For example, the paging one could look like this:
function readFromAllPages() {
let items = [];
function readNextPage(pageId) {
return fetchSinglePage(pageId).then(result => {
items.push.apply(items, result.items);
if (!result.items.length) {
return Promise.resolve(null);
}
return readNextPage(pageId + 1);
});
}
return readNextPage(0).then(() => items);
}
First of all recursive code is never "easy" to follow
I think the code is fine to read. As I’ve said: Unless you try to make it super generic, you can really keep it simple. And naming also helps a lot.
but more importantly you also have to keep in mind thread safety with the state-object
No, JavaScript is single-threaded. You doing things asynchronously but that does not necessarily mean that things are happening at the same time. JavaScript uses an event loop to work off asynchronous processes, where only one code block runs at a single time.
The scalability is also a problem at some points: this is a recursive function, so at one point it will create a stackoverflow/encounter maximum recursive depth.
Also no. This is recursive in the sense that the function references itself. But it will not call itself directly. Instead it will register itself as a callback when an asynchronous process finishes. So the current execution of the function will finish first, then at some point the asynchronous process finishes, and then the callback will eventually run. These are (at least) three separate steps from the event loop, which all run independently from another, so you do no have a problem with recursion depth here.
The crux of the matter seems to be that "the actual business logic is buried in a mess of code".
Yes it is ... in both solutions.
Things can be separated out by :
having an asyncRecursor function that simply knows how to (asynchronously) recurse.
allowing the recursor's caller(s) to specify the business logic (the terminal test to apply, and the work to be performed).
It is also better to allow caller(s) to be responsible for cloning the original object rather than resolver() assuming cloning always to be necessary. The caller really needs to be in charge in this regard.
function asyncRecursor(subject, testFn, workFn) {
// asyncRecursor orchestrates the recursion
if(testFn(subject)) {
return Promise.resolve(workFn(subject)).then(result => asyncRecursor(result, testFn, workFn));
// the `Promise.resolve()` wrapper safeguards against workFn() not being thenable.
} else {
return Promise.resolve(subject);
// the `Promise.resolve()` wrapper safeguards against `testFn(subject)` failing at the first call of asyncRecursor().
}
}
Now you can write your caller as follows :
// example caller
function someBusinessOrientedCallerFn(state_obj) {
// ... preamble ...
return asyncRecursor(
state_obj, // or state_obj.cloneMe() if necessary
(obj) => obj.checkContinue(), // testFn
(obj) => somethingAsync(...).then((result) => { // workFn
obj.changeStateBasedOnResult(result);
return obj; // return `obj` or anything you like providing it makes a valid parameter to be passed to `testFn()` and `workFn()` at next recursion.
});
);
}
You could theoretically incorporate your terminal test inside the workFn but keeping them separate will help enforce the discipline, in writers of the business-logic, to remember to include a test. Otherwise they will consider it optional and sure as you like, they will leave it out!
Sorry, this doesn't use Promises, but sometimes abstractions just get in the way.
This example, which builds from #poke's answer, is short and easy to comprehend.
function readFromAllPages(done=function(){}, pageId=0, res=[]) {
fetchSinglePage(pageId, res => {
if (res.items.length) {
readFromAllPages(done, ++pageId, items.concat(res.items));
} else {
done(items);
}
});
}
readFromAllPages(allItems => {
// items have been read from all pages
});
This has only a single depth of nested functions. In general, you can solve the nested callback problem without resorting to a subsystem that manages things for you.
If we drop the parameter defaults and change the arrow functions, we get code that runs in legacy ES3 browsers.
I'm aware of the power of promises, however I have several old functions that are synchronous:
function getSomething() {
return someExternalLibrary.functionReturnsAValue()
}
console.log(getSomething()); // eg prints 'foo'
Unfortunately, when someExternalLibrary updated, it has removed functionReturnsAValue() and has lumped me with functionReturnsAPromise():
function getSomething() {
return someExternalLibrary.functionReturnsAPromise()
}
console.log(getSomething()); // now prints '[object]'
This of course, breaks absolutely everything written that depends on what used to be a simple value.
Obviously, I'd prefer two things:
ask the original library to keep a synchronous return value. (Not going to happen -- b/c they have refused)
A way to actually wait for a value
I have read numerous articles on why promises are great, ad nauseam, but the simple fact is: If I embrace promises, all I really do is shuffle promises onto some other part of the code, which then must deal with the promise of a value...
Is there a way (in nodejs) to actually wait for a promise to get itself together?
The best I can find is to use coroutines and yield, but really, it's still passing the buck. To be clear, I want the function getSomething to continue to return a value. Is there a way to do it?
Clearly, I fear I've misunderstood something about Promises...
The app is for non-browser implementations and runs purely from the command line. I've been trying to understand how bluebird's reflect() might help, to no avail.
(Yes, I'm aware this question has been asked many times in various formats, but I can't find a suitable answer to the core issue. If anything, I'm looking for the opposite of this question. The closest related (but unhelpful) question I can find is: Managing promise dependencies.)
There's the concept of generator functions. These are a special kind of function in both syntax (asterisk notation) and semantics. Unlike regular functions, generator functions return something that's also new to ECMAScript: iterators. Iterators happen to be objects made specifically to be iterated on, e.g. with the all new for...of loop. They can be also iterated on manually by calling their 'next' method. Each such call produces an object containing two properties: 'value' (iterator's current value) and 'done' (a boolean indicating whether we reached the last value of the iterable). However, the best thing about generator functions is their ability to suspend their execution each time a keyword 'yield' is encountered. Let's have a glimpse of how it all works together:
'use strict';
let asyncTask = () =>
new Promise((resolve, reject) => {
if (Math.random() > 0.5) {
resolve(1);
} else {
reject(new Error('Something went wrong'));
}
});
let makeMeLookSync = fn => {
let iterator = fn();
let loop = result => {
!result.done && result.value.then(
res => loop(iterator.next(res)),
err => loop(iterator.throw(err))
);
};
loop(iterator.next());
};
makeMeLookSync(function* () {
try {
let result = yield asyncTask();
console.log(result);
} catch (err) {
console.log(err.message);
}
});
The short answer
I am told repeatedly: You can't undo functions that have been promisified.
Edit: An upcoming solution
It appears that the ES2017 (although still draft), goes a long way in making promisified code easier to work with:
https://ponyfoo.com/articles/understanding-javascript-async-await
It seems that there is also a node library ready for this support too: https://github.com/normalize/mz.
Using this methodology, having apis converted to Promises won't be so bad (although it still appears that promises still poison the rest of the codebase):
const fs = require('mz/fs')
async function doSomething () {
if (await fs.exists(__filename)) // do something
}
The rest of this answer is just a general commentary on the problem.
Why we need a solution
Let's start with a sample piece of traditional synchronous code, in 3 flavours from more 'older-fashioned' to 'newer':
This is the traditional javascript way, requiring exception based programming to handle unexpected errors:
function getSomething() {
if (someproblem) throw new Error('There is a problem');
return 'foo';
}
However, adding try/ catch statements becomes very laborious and tedious, very quickly.
With the advent of node.js, callbacks were made popular, which nicely circumvented the issue, since each caller was explicitly forced to deal with error conditions in the same callback. This meant less errors in the caller's code:
function getSomething(callback) {
if (callback) {
if (someproblem)
callback(new Error('There is a problem'), null);
else
callback(null, 'foo');
}
return 'foo';
}
Then, the after some teething issues, node.js quickly proved itself for server-side communications, and people were amazed at the speed that asynchronous solutions provided. Node application frameworks like Express and Meteor grew, which focused on this.
Unfortunately, using the same callback scheme quickly became troublesome and the developers dealing in asynchronous code started using Promises in an effort to linearize the code, to make it readable, like the traditional (try/catch) code was.
The problem is that it got evangenlized too much. Everyone started thinking that Promises are the way to go. Personally, I call it a poison on a codebase. Once you have anything that uses Promises, your whole codebase must become asynchronous. This is not always a sensible nor a practical solution, IMHO.
The worst of all side effects is that the above function, even though it is completely synchronous, can be written in Promises too:
var bluebird = require('bluebird');
function getSomething() {
// IMHO, this is ridiculous code, but is increasingly popular.
if (someproblem) return Promise.reject(new Error('There is a problem'));
return Promise.resolve('foo');
}
For those who doubt this is a problem, perhaps should look at the SO question: How do I convert an existing callback API to promises?. Pay particular attention to #3, Node-style callback.
So, for anyone who cares, I would like to suggest that there needs to be a 'pill' for Promises. I urge that we need more than promises: we need results, and sometimes in a timely manner.
Take a look at the default node.js api. It does not use Promises. It also provides both synchronous and asynchronous calls to appropriate parts of the api (eg File System).
For those of you who feel tempted to downvote this answer: that is your prerogative, but there are clear issues on when Promises are not the answer, and I feel strongly that there are cases when we need to be able to re-synchronize decoupled code.
I also apologize for this 'blog-post' styled answer.