I'm using bluebird and I see two ways to resolve synchronous functions into a Promise, but I don't get the differences between both ways. It looks like the stacktrace is a little bit different, so they aren't just an alias, right?
So what is the preferred way?
Way A
function someFunction(someObject) {
return new Promise(function(resolve) {
someObject.resolved = true;
resolve(someObject);
});
}
Way B
function someFunction(someObject) {
someObject.resolved = true;
return Promise.resolve(someObject);
}
Contrary to both answers in the comments - there is a difference.
While
Promise.resolve(x);
is basically the same as
new Promise(function(r){ r(x); });
there is a subtlety.
Promise returning functions should generally have the guarantee that they should not throw synchronously since they might throw asynchronously. In order to prevent unexpected results and race conditions - throws are usually converted to returned rejections.
With this in mind - when the spec was created the promise constructor is throw safe.
What if someObject is undefined?
Way A returns a rejected promise.
Way B throws synchronously.
Bluebird saw this, and Petka added Promise.method to address this issue so you can keep using return values. So the correct and easiest way to write this in Bluebird is actually neither - it is:
var someFunction = Promise.method(function someFunction(someObject){
someObject.resolved = true;
return someObject;
});
Promise.method will convert throws to rejects and returns to resolves for you. It is the most throw safe way to do this and it assimilatesthenables through return values so it'd work even if someObject is in fact a promise itself.
In general, Promise.resolve is used for casting objects and foreign promises (thenables) to promises. That's its use case.
There is another difference not mentioned by the above answers or comments:
If someObject is a Promise, new Promise(resolve) would cost two additional tick.
Compare two following code snippet:
const p = new Promise(resovle => setTimeout(resovle));
new Promise(resolve => resolve(p)).then(() => {
console.log("tick 3");
});
p.then(() => {
console.log("tick 1");
}).then(() => {
console.log("tick 2");
});
const p = new Promise(resolve => setTimeout(resolve));
Promise.resolve(p).then(() => {
console.log("tick 3");
});
p.then(() => {
console.log("tick 1");
}).then(() => {
console.log("tick 2");
});
The second snippet would print 'tick 3' firstly. Why?
If the value is a promise, Promise.resolve(value) would return value exactly. Promise.resolve(value) === value would be true. see MDN
But new Promise(resolve => resolve(value)) would return a new promise which has locked in to follow the value promise. It needs an extra one tick to make the 'locking-in'.
// something like:
addToMicroTaskQueue(() => {
p.then(() => {
/* resolve newly promise */
})
// all subsequent .then on newly promise go on from here
.then(() => {
console.log("tick 3");
});
});
The tick 1 .then call would run first.
References:
http://exploringjs.com/es6/ch_promises.html#sec_demo-promise
Related
A question asked here before, with the exact same title as this one, was answered with a "You should not use that, use this instead", I am looking to know what it does, not what else could I do, it's about understanding not a simple copy a paste.
My question is quite simple, what is the difference between these three approaches when creating a promise?
const API = (item, fail) =>
new Promise((resolve, reject) => {
if (fail) reject(item + ' ...with an error');
setTimeout(() => resolve(item), 1000);
});
(async () => {
const pro1 = Promise.resolve(API('I am inside resolve'));
const pro2 = Promise.resolve(API('I am inside resolve', true));
const pro3 = Promise.resolve().then(() => API('I am thenable'));
const pro4 = Promise.resolve().then(() => API('I am thenable', true));
const pro5 = new Promise((resolve) => resolve(API('I am a new promise')));
const pro6 = new Promise((resolve) => resolve(API('I am a new promise', true)));
const store = [pro1, pro2, pro3, pro4, pro5, pro6];
const results = await Promise.allSettled(store);
for (const { status, value, reason } of results) {
if (status === 'fulfilled') console.log(value)
else console.log(reason)
}
})();
The difference is in job to be done. While all of this methods are valid, they have different cost and predictability.
Promise.resolve() produces single resolved Promise instance, and depending on value provided to the call JS engine have information to optimize it. It makes all work to be done in a single call to underlying code of the JS engine (usually C++, but could be Java or WASM). So it's always the best choice.
Promise.resolve().then(() => API(/*...*/)) Produce several Promise instances: one at Promise.resolve() and other at .then() call. It also allocates more memory and make several (3 or more) redundant jumps between JS and the engine. It's hardly optimizable and requires intensive heuristics to be performed to figure out is this call optimizable. It's the worst option.
new Promise((resolve) => resolve(API(/* ... */)) allocates one function and one Promise instance and makes two jumps between JS and the engine. It's harder to optimize this call, due to nature of JS.
Promise.resolve().then()
In your examples, the then() makes no difference as you just resolve the promise, and get its data.
then() is typically used to chain promises, take this as an example:
Promise.resolve('foo')
// 1. Receive "foo", concatenate "bar" to it, and resolve that to the next then
.then(function(string) {
return new Promise(function(resolve, reject) {
setTimeout(function() {
string += 'bar';
resolve(string);
}, 1);
});
})
// 2. receive "foobar", register a callback function to work on that string
// and print it to the console, but not before returning the unworked on
// string to the next then
.then(function(string) {
setTimeout(function() {
string += 'baz';
console.log(string); // foobarbaz
}, 1)
return string;
})
Here we chain multiple promises then() off of a previously resolved promise, while maintaining the original data from the first resolve, though in this case we modify it with every new promise.
You can read more about promises and chaining here.
This is my function
const f = (value,err) => {
if(!value === Number){
throw err;
}
return (value);
}
Using promises I want to return an object of parameter value and after some wait it will return parameter in number. Is that possible?
A promise doesn't do anything in and of itself, and it doesn't make anything asynchronous in and of itself (other than that promise reactions are always asynchronous¹). A promise is just a standardized way to report the completion of something that's (potentially) asynchronous. So the literal answer to your question is: No, you can't use a promise to do that, not on its own. You'd have to combine it with something like setTimeout or similar.
Also note that if (!value === Number) is always false. It's evaluated like this: !value, negating the value of value, and then x === Number, which will always be false because there is no value that, when negated, turns into the Number function.
But for instance, if you wanted to check whether something is a number but not respond for 100ms:
const f = (value, err) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (typeof value !== "number") {
reject(err);
} else {
resolve(value);
}
}, 100);
});
};
Other than the promise, the two significant changes there are:
Using setTimeout to introduce asynchronousness, and
Changing the if condition to something that won't always be false (I may or may not have guessed correctly what you wanted there :-) )
¹ A promise reaction is a call to a then, catch, or finally callback registered on the promise. Here's an example of what I mean by "promise reactions are asynchronous:"
console.log("Before creating the promise");
new Promise(resolve => {
console.log("Inside the promise executor function");
resolve(42);
})
.then(value => {
console.log(`Promise reaction ran, value = ${value}`);
});
console.log("After creating the promise");
That code has this output:
Before creating the promise
Inside the promise executor function
After creating the promise
Promise reaction ran, value = 42
Notice that everything was synchronous except the call to the then callback (the promise reaction), which per specification is always done asynchronously, even if (as in this case) the promise is already settled when the reaction is added to it.
I'm using bluebird and I see two ways to resolve synchronous functions into a Promise, but I don't get the differences between both ways. It looks like the stacktrace is a little bit different, so they aren't just an alias, right?
So what is the preferred way?
Way A
function someFunction(someObject) {
return new Promise(function(resolve) {
someObject.resolved = true;
resolve(someObject);
});
}
Way B
function someFunction(someObject) {
someObject.resolved = true;
return Promise.resolve(someObject);
}
Contrary to both answers in the comments - there is a difference.
While
Promise.resolve(x);
is basically the same as
new Promise(function(r){ r(x); });
there is a subtlety.
Promise returning functions should generally have the guarantee that they should not throw synchronously since they might throw asynchronously. In order to prevent unexpected results and race conditions - throws are usually converted to returned rejections.
With this in mind - when the spec was created the promise constructor is throw safe.
What if someObject is undefined?
Way A returns a rejected promise.
Way B throws synchronously.
Bluebird saw this, and Petka added Promise.method to address this issue so you can keep using return values. So the correct and easiest way to write this in Bluebird is actually neither - it is:
var someFunction = Promise.method(function someFunction(someObject){
someObject.resolved = true;
return someObject;
});
Promise.method will convert throws to rejects and returns to resolves for you. It is the most throw safe way to do this and it assimilatesthenables through return values so it'd work even if someObject is in fact a promise itself.
In general, Promise.resolve is used for casting objects and foreign promises (thenables) to promises. That's its use case.
There is another difference not mentioned by the above answers or comments:
If someObject is a Promise, new Promise(resolve) would cost two additional tick.
Compare two following code snippet:
const p = new Promise(resovle => setTimeout(resovle));
new Promise(resolve => resolve(p)).then(() => {
console.log("tick 3");
});
p.then(() => {
console.log("tick 1");
}).then(() => {
console.log("tick 2");
});
const p = new Promise(resolve => setTimeout(resolve));
Promise.resolve(p).then(() => {
console.log("tick 3");
});
p.then(() => {
console.log("tick 1");
}).then(() => {
console.log("tick 2");
});
The second snippet would print 'tick 3' firstly. Why?
If the value is a promise, Promise.resolve(value) would return value exactly. Promise.resolve(value) === value would be true. see MDN
But new Promise(resolve => resolve(value)) would return a new promise which has locked in to follow the value promise. It needs an extra one tick to make the 'locking-in'.
// something like:
addToMicroTaskQueue(() => {
p.then(() => {
/* resolve newly promise */
})
// all subsequent .then on newly promise go on from here
.then(() => {
console.log("tick 3");
});
});
The tick 1 .then call would run first.
References:
http://exploringjs.com/es6/ch_promises.html#sec_demo-promise
I have a working promise chain:
function startSync(db) {
var promise = new Promise(function(resolve, reject) {
syncCats(db)
.then(syncTrees(db))
.then(syncCars(db))
...
.then(resolve());
});
return promise;
}
This works great. It performs each of those function calls, waiting for each one to complete before firing off another. Each of those functions returns a promise, like so:
function syncCaesar(db) {
var promise = new Promise(resolve, reject) {
// do aysnc db calls, and whatnot, resolving/rejecting appropriately
}
return promise;
}
I need to run this chain on a series of databases, sequentially.
I've tried other solutions here, but they would fire off syncCats() all at once.
For instance:
var promise = Promise.resolve();
dbs.forEach(function(db) {
promise = promise.then(startSync(db));
}
Fires off syncCats(db[0]) and syncCats(db[1]) simultaneously.
Edit:
Performing startSync(dbs[0]).then(startSync(dbs[1])); acts the same.
Edit2: Also performs the same:
dbs.forEach(function(db) {
promise = promise.then(function() {
return startSync(db);
}
}
You're calling startSync and then passing its return value in to then. So naturally, if you do that twice, it's going to start the process twice in parallel.
Instead, pass in a function that doesn't call startSync until it's called:
var promise = Promise.resolve();
dbs.forEach(function(db) {
promise = promise.then(function() { startSync(db); });
});
or with ES2015:
let promise = Promise.resolve();
dbs.forEach(function(db) {
promise = promise.then(_ => startSync(db));
});
Separately, three things jump out about startSync:
It starts all its operations in parallel, not sequentially
It exhibits the promise creation anti-pattern. There's no reason for startSync to create a new promise; it already has a promise it can work with
It ensures that its resolution value is undefined
If you really want the operations running in parallel like that, I suggest being more explicit about it:
function startSync(db) {
return Promise.all([
syncCats(db),
syncTrees(db),
syncCars(db)
])
.then(_ => undefined); // This does #3
}
...but you could also do to avoid the anti-pattern:
// Still run in parallel!
function startSync(db) {
return syncCats(db)
.then(syncTrees(db))
.then(syncCars(db))
.then(_ => undefined); // Does #3
}
If you meant for them to be sequential, you need to pass functions, not the result of calling them:
function startSync(db) {
return syncCats(db)
.then(_ => syncTrees(db))
.then(_ => syncCars(db))
.then(_ => undefined); // Again, does #3
}
If you made syncCats, syncTrees, and syncCars resolve their promises with db, like this:
function syncCats(db) {
return startSomethingAsync().then(_ => db);
}
...then it could be:
function startSync(db) {
return syncCats(db)
.then(syncTrees)
.then(syncCars)
.then(_ => undefined); // Only here for #3
}
...since each would receive db as its first argument.
Finally, if you don't need to force undefined as the promise resolution value, I suggest dropping that part from the above. :-)
With appreciation to T.J and Jaromanda, I was able to solve it by fixing how startSync() was resolved:
function startSync(db) {
var promise = new Promise(function(resolve, reject) {
syncCats(db)
.then(syncTrees(db)) // not correct way to call, see below
.then(syncCars(db))
...
.then(function() { resolve(); });
});
return promise;
}
Now, as T.J points out, the way each of the .then's is being called is incorrect. He explains it better in his answer. I'm being saved by some misunderstood database layer queueing. syncTrees() and syncCars() should be running in parallel.
If I understood it correctly, you will be having an array of databases and you want to sync them one by one i.e. sync for dbs[0], once that is complete sync for dbs[1], and so on. If that's correct you can do something like this.
var syncMultipleDBs = (dataBases) {
var db = dataBases.shift();
if (!db) {
return;
}
startSync(db).then(syncMultipleDBs.bind(null, dataBases));
};
syncsyncMultipleDBs(dbs.slice());
I hope this will help.
I need to throw in a userID param in the middle of a promise chain(it is the only promise that needs it). All the promises should execute in a synchronous order.
SideNote- All the similar examples on stackoverflow are all a bit different- like using lambda functions(I use declared functions).So I'm still not quite sure.
var initialize = function(userID) {
var firstPromise = Module2.getFirstPromise();
firstPromise.then(getSecondPromise)
.then(getThirdPromise)
.then(getFourthPromise) //<----Fourth promise needs userID
.then(getFifthPromise)
.then(Utils.initializeComplete);
}
All the promises are functions that look like this:
var thirdPromise = function() {
return new Promise(function(resolve, reject) {
//fetch data
//On Return, Store it
resolve() //Nothing needed to passed down from this promise
});
});
}
I'm trying this, and it "works", but I'm not sure if that is how I am "suppose" to handle something like this. :)
var initialize = function(userID) {
var firstPromise = Module2.getFirstPromise();
firstPromise.then(getSecondPromise)
.then(getThirdPromise)
.then(function(){ return fourthPromise(userID)})
.then(getFourthPromise)
.then(Utils.initializeComplete);
}
Note: getFirstPromise is coming from a different module in my code. That shouldn't be important to the question though :)
Assuming that firstPromise is really a promise but secondPromise and so on are actually functions returning promises, then yes, what you've done is how you're supposed to do that. Here's a live example on Babel's REPL, which looks like this:
function doSomething(userID) {
getFirstPromise()
.then(getSecondPromise)
.then(getThirdPromise)
.then(() => getFourthPromise(userID))
// Or .then(function() { return getFourthPromise(userID); })
.then(getFifthPromise)
.catch(err => {
console.log("Error: " + err.message);
});
}
doSomething("foo");
function getFirstPromise() {
console.log("getFirstPromise called");
return new Promise(resolve => {
setTimeout(() => {
resolve("one");
}, 0);
});
}
// and then second, third, fourth (with user ID), and fifth
(If you don't use arrow functions, just replace them with the function form.)
Note the catch in the example above. Unless you have a really good reason not to, a promise chain should always have a .catch if you don't return the result.
Your solution is perfectly fine. It might be easier to understand with concrete signatures.
If your thirdPromise doesn't take anything and doesn't return anything its signature might be written (pseudocode assuming a -> b is a function from a to b) as _ -> Promise (_). If it returns some value a, it would be _ -> Promise (a). If it took something and returned something it might be a -> Promise (b)
So you can reason about your promise chains as about functions taking some value and returning some other value wrapped in a promise. However, your fourthPromise looks differently:
fourthPromise : UserId -> a -> Promise (b)
Which can be written as:
fourthPromise : UserId -> (a -> Promise (b))
It takes one parameter before becoming an actual promise you can chain. In a way, it's a template of a promise.
If you want the plain .then chain in the main function, try to write getFourthPromise as a factory function
function getSomethingByUserID(userId) {
return function() {
return new Promise(function(resolve) {
//your actual async job
resolve('a result');
});
};
}
Then you will get plan list of thens
var firstPromise = Module2.getFirstPromise()
.then(getSecondPromise)
.then(getSomethingByUserID(userId))
.then(Utils.initializeComplete);
Do not forget that if you missed to provide userId to getSomethingByUserID, it will not work.