I'm stuck with two external libraries and using them together. One only has an async interface and the other requires me to return a value from the callback.
// this function is called as a callback from an external library
// it is required that the function returns a certain value
function onevent()
{
// the callback for the "thirdparty.foo" function
// with the result that must be returned from onevent
function ondone(result) {
// ???
}
// this is a call to a third party library that only has this asynchronous interface
thirdparty.foo(ondone);
// ??? return result;
}
JavaScript is pretty new for me and I was unable to find any solution for this problem.
I'm looking to something to be able to wait for the completion of ondone before returning from onevent. Comparable from C++ would be .wait or .get from a std::future.
One only has an async interface and the other requires me to return a value from the callback.
That's a problem if you have to use the one providing an async interface to satisfy a callback from the one that doesn't. If you do, you simply cannot use them together. There is no way to make the callback from the one expecting a value returned to it by the callback wait for an asynchronous event to complete.
E.g.:
doSomethingExpectingReturnValueFromCallback(function() {
// You cannot use an asynchronous API here to get the value you return
});
If the synchronous lib understood promises (similar to std::future), then you could return a promise, but it doesn't sound like it does.
Related
I am making an API call and want the code after the call to not run until the API call finishes.
I have a function, this.api_call, which calls an API and returns an array, returnValue, after the API call is complete.
let returnValue = this.api_call(data);
//This line should not run until returnValue has been set
this.otherFunction(returnValue[0], returnValue[1]);
How can I make just that one line wait before running? I want other code to still be able to run before the API call completes, it's just that one line that should wait.
that's where Promises come in, If your function this.api_call(data) returns a Promise, you can then call the then function on it, with a callback, and this callback will be run only after the promise has resolve.
this.api_call(data).then((returnValue) => {
this.otherFunction(returnValue[0], returnValue[1]);
});
You can also use RxJS's observable, which lets you alter your data before returing it using pipe function. If you are using observable, make sure your subscribe to your observable otherwise it will never be fired.
If your this.api_call(data) function is returning an observable ( usually the case if you use Angular), use it like that:
this.api_call(data).subscribe((returnValue) => {
this.otherFunction(returnValue[0], returnValue[1]);
});
There is also a way to use async / await keywords but i'm not a pro in this concept so i'll let somebody else answer that.
There are multiple ways to achieve it, the simplest one is async / await syntax of Promise:
async function myAsyncFunction(): Promise<void> {
let returnValue = await this.api_call(data);
this.otherFunction(returnValue[0], returnValue[1]);
}
this.api_call(data) have to return a Promise<ArrayLike<...>> in your example.
I was unsure how node.js was able to realize what functions where async and which were not and how to create a custom async function.
Say I wanted to create a custom asynchronous function. I would be surprised if just because I called my last argument to the async function callback or cb that it would just know its an async function:
function f(arg1, callback){
//do stuff with arg1
arg1.doStuff()
//call callback
callback(null, arg1.result());
}
I tried something like that and it did not work async. How do you tell node.js that f is actually async?
NOTE: this answer was written in 2014, before the existence of async function, and before Promises gaining popularity. While the same principles apply as well, I would recommend reading on Promises before trying to get your head around how they relate to "traditional" callback-driven async functions.
To create a function that calls its callback asynchronously, you have to use some platform-provided async primitive (typically IO-related) on it - timers, reading from the filesystem, making a request etc.
For example, this function takes a callback argument, and calls it 100ms after:
function asyncFn(callback) {
setTimeout(() => {
callback();
}, 100);
}
A possible reason for making a function async when it doesn't need to be, is for API consistency. For example, suppose you have a function that makes a network request, and caches the result for later calls:
var cache = null;
function makeRequest(callback) {
if (!cache) {
makeAjax(result => {
cache = result;
callback(result);
});
} else {
callback(cache);
}
}
The problem is, this function is inconsistent: sometimes it is asynchronous, sometimes it isn't. Suppose you have a consumer like this:
makeRequest(result => doSomethingWithResult(result));
doSomethingElse();
The doSomethingElse function may run before or after the doSomethingWithResult function, depending on whether the result was cached or not. Now, if you use an async primitive on the makeRequest function, such as process.nextTick:
var cache = null;
function makeRequest(callback) {
if(!cache) {
makeAjax(result => {
cache = result;
callback(result);
});
} else {
process.nextTick(() => callback(cache));
}
}
The call is always async, and doSomethingElse always runs before doSomethingWithResult.
Only native functions (with access to the event loop) are asynchronous. You would need to call one of them to get asynchronity for your callback. See What is a simple example of an asynchronous javascript function?.
If you aren't using any, there's hardly a reason to make your function asynchronous.
Note: I'm bootstrapping a reactjs app but this is a general JavaScript question.
I have a special module "locationUtils" that I am trying to keep in it's own package, but keeping that code separate is causing an eyesore with callbacks.
When I access one of it's methods I have to send a callback with it that only has one of its parameters initially defined, and in that method I get the other data parameter to initalize the other parameter.
Can I add in undefined parameters later like that in JavaScript, and is it good practice to initial parameters for a callback method as you go down the callback chain in general, or am I making a convoluted newbie mistake?
/********************Module 1******************************/
var bootStrapUI = function(callback) {
locationUtils.findData(findOtherData(callback));
}
//This gets called last to finalize bootstraping
var findOtherData = function(callback,originalFetchedData){
//use originalFetchedData to get more data
//bootStraping program with all rendering data
callback() //sends back a boolean confirming all fetched
}
/**********************Module2**********************************/
var findData = function(findOtherData){
var data = magicGetData();
findOtherData(findOtherData,data);//I initialized a param late here!
}
It's a good Javascript question, callbacks can become a serious hell for the uninitiated, particularly when they are nested and / or the order in which they return is important.
This is where promises come in: they are an essential tool for Javascript development and about to become part of the standard (in EcmaScript 6).
In essence: a promise is an object that is returned from a function with a method (callback) that is called when the asynchronous action (e.g. API call) has been completed. The difference between a promise and a callback is that promises allow you to structure how you handle the callbacks and, importantly, in what order.
I recently wrote a method that had to make 30 api calls with each call dependent on the results of the previous one (this was not a well designed api). Can you imagine trying to do that with callbacks? As it was, I created an array of promises and used jQuery.when() to handle things when all the api calls had completed.
For the moment we need to use a library for promises. jQuery: https://api.jquery.com/jquery.deferred/ is the obvious one but there are various other implementations that do much the same thing.
Update:
The question relates more specifically to the passing of arguments between callbacks and modifying the arguments as execution moves between them. This can be done easily by passing whatever info you need as an argument to your resolve method.
Typically it looks something like this (using jQuery):
var myAsyncMethod = function(info){
var deferred = $.Deferred();
$.getJSON(myUrl,
function(dataFromServer) {
// Do stuff with data
var newData = doSomething(dataFromServer);
deferred.resolve(newData);
});
});
return deferred.promise();
};
// Make initial method call
myAsyncMethod(myInitialData).then(
function(transformedData){
// transformed data from server is returned here.
}
);
I want to create a function, let's say a readConfig function.
readConfig(path [, callback(err, config)])
The callback here is optional. If there is no callback when called, the function will only use synchronous methods (readFile(path) and configParser(string) which return promises) to read the file and directly return the parsed config object. Exceptions are thrown directly. However when callback is provided, the function will use asynchronous methods and call the callback after finished. Exceptions are directly raised to callback but not thrown.
There are many similar codes to do sync and async work, so I want to use one method for both. How can a promise like function detect an async or sync call according to the caller's callback argument? And how can we ensure that the promise will act in synchronous way? Please show me a pattern for that.
I can't speak to the promise acting in a synchronous way, but in JavaScript you can do arguments.length to get the number of arguments passed to a function. If that equals 2 and typeof arguments[1] === 'function', then you have a second argument that is a function.
As for personal taste, I'd recommend just having 2 different functions, one of them ending in Sync, à la http://nodejs.org/api/fs.html. This module is replete with function pairs where one is async and the other isn't.
Also, remember that this is not async code:
function notAsync(cb) {
cb(null)
}
That callback executes in the same tick. You need to wrap it like so:
function async(cb) {
process.nextTick(function(){ cb(null)})
}
or something like that.
Whatever you decide, happy coding.
In this example, I'm using jQuery's deferred api. I wrap the callback in .when which allows you to attach a .then() to the callback, whether it is synchronous or asynchronous with a promise.
The solution should be to use your promise api's when. http://howtonode.org/promises
Live demo (click).
$('#sync').click(function() {
foo(function() {
});
});
$('#async').click(function() {
foo(function() {
var deferred = new $.Deferred();
setTimeout(function() {
deferred.resolve();
}, 500);
return deferred.promise();
});
});
function foo(callback) {
$.when(callback()).then(function() {
console.log('done!');
});
}
In node.js, is it possible to determine (using a function) whether a method is synchronous or asynchronous?
I'd like to write a function that does the following:
function isSynchonous(methodName) {
//if the method is synchronous, return true. Otherwise, return false.
}
From a language standpoint this is not possible, which I believe llambda's answer proves.
Functions can do things asynchronously but return something synchronously; say, return the number of async tasks that were fired off.
Functions can synchronously return promises... that represent asynchronous information. I would call such a method asynchronous but the language can't really tell that.
In some perverse sense every asynchronous function "returns" something... undefined if nothing else.
From an engineering standpoint:
read the documentation.
If the function accepts a callback, it is likely asynchronous. Look at the function signature.
Read the code.
Use "common sense." If the function does IO and returns a result from IO it must be, in some case, asynchronous. This includes file reading, reading from standard input, saving to a database, and HTTP/AJAX requests. Note streams are often used for this, which represents an asynchronous task, but the callbacks are special.
Furthermore there are functions that mix the two.
function(callback) {
if(ready) {
callback();
}
else {
setTimeout(callback, 5000);
}
}
Arguably this is very evil, and correct practice would be
if(ready) {
process.nextTick(callback);
}
so the function has uniform behavior.
However there is a hacky way to tell if anything asynchronous happened, at least in Node.js. See this discussion.
// untested!! please read the documentation on these functions before using yourself
var work = process._getActiveHandles().length + process._getActiveCallbacks().length;
foo;
var newWork = (process._getActiveHandles().length + process._getActiveCallbacks().length) - work;
if(newWork > 0) {
console.log("asynchronous work took place.");
}
This works because asynchronous work cannot resolve on the same tick, by definition, and because Node.js is single threaded.
You don't necessarily know. A function could even be randomly synchronous or asynchronous.
For example, a function that takes another function could execute that function immediately, or it could schedule to execute it at a later time using setImmediate or nextTick. The function could even randomly choose to call its passed function synchronously or asynchronous, such as:
console.log('Start')
function maybeSynchOrAsync(fun) {
var rand = Math.floor((Math.random() * 2))
if (rand == 0) {
console.log('Executing passed function synchronously')
fun()
console.log('Done.')
} else {
console.log('Executing passed function asynchronously via setImmediate')
setImmediate(fun)
console.log('Done.')
}
}
maybeSynchOrAsync(function () { console.log('The passed function has executed.') });
Further, technically speaking, every function call is synchronous. If you call a function F, and F queues a callback function to be invoked later (using setTimeout or whatever mechanism), the original function F still has a synchronous return value (whether it's undefined, a promise, a thunk, or whatever).
No, that's impossible. The methods aren't just marked synchronous or asynchronous, they either use callbacks or they don't.
Why exactly do you need to know, is the real question.
Having said that; that's possible with the new JS abstractions. Nowadays for the async functions those explicitly defined by a preceding async keyword you can perform a test like;
async function test(){}
var type = Object.getPrototypeOf(test).constructor.name;
console.log(type); // <- 'AsyncFunction'
cool..!
As for the normal functions those happen to return a promise, we must remember that it's just a sync function returning a promise object right away, synchronously. That means you have to check dynamically if a function returns a promise in advance, before invoking the function. I think that takes you to a type level programming adventure which exists in sophisticated languages like Haskell etc.