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Table of Contents

• Callbacks
• Promises (ES6)
• Generators (ES6)
• Async Functions (ES7)

In JavaScript, functions are first-class objects; that is, functions are of the type Object and they can be used in a first-class manner like any other object. They can be “stored in variables, passed as arguments to functions, created within functions, and returned from functions”.

Because functions are first-class objects, we can pass a function as an argument in another function and later execute that passed-in function or even return it to be executed later. This is the essence of using callback functions in JavaScript.

A callback function, is a function that is passed to another function (let’s call this other function “otherFunction”) as a parameter, and the callback function is called (or executed) inside the otherFunction.

//Note that the item in the click method's parameter is a function, not a variable.​ The item is a callback function$("#btn_1").click(function(){alert("Btn 1 Clicked");});// forEach()varfriends=["Mike","Stacy","Andy","Rick"];friends.forEach(function(eachName,index){// Inside callback function.console.log(index+1+". "+eachName);// 1. Mike, 2. Stacy, 3. Andy, 4. Rick​});

We can pass functions around like variables and return them in functions and use them in other functions. When we pass a callback function as an argument to another function, we are only passing the function definition. We are not executing the function in the parameter. In other words, we aren’t passing the function with the trailing pair of executing parenthesis () like we do when we are executing a function.

And since the containing function has the callback function in its parameter as a function definition, it can execute the callback anytime. Note that the callback function is not executed immediately. It is “called back” (hence the name) at some specified point inside the containing function’s body.

When we pass a callback function as an argument to another function, the callback is executed at some point inside the containing function’s body just as if the callback were defined in the containing function. This means the callback is a closure. As we know, closures have access to the containing function’s scope, so the callback function can access the containing functions’ variables, and even the variables from the global scope.

• Use Named OR Anonymous Functions as Callbacks
• Can pass Parameters to Callback Functions
  • Since the callback function is just a normal function when it is executed, we can pass parameters to it. We can pass any of the containing function’s properties (or global properties) as parameters to the callback function.
• Make Sure Callback is a Function Before Executing It
  • It is always wise to check that the callback function passed in the parameter is indeed a function before calling it. Also, it is good practice to make the callback function optional.
• Use the Call or Apply Function To Preservethis
  • When the callback function is a method that uses the this object, we have to modify how we execute the callback function to preserve the this object context. Or else the this object will either point to the global window object (in the browser), if callback was passed to a global function. Or it will point to the object of the containing method.
  • We can fix the preceding problem by using the Call or Apply function. Call and Apply are used to set the this object inside the function and to pass arguments to the functions.
• Multiple Callback Functions Allowed
  • We can pass more than one callback functions into the parameter of a function, just like we can pass more than one variable.

In asynchronous code execution, which is simply execution of code in any order, sometimes it is common to have numerous levels of callback functions to the extent that it is messy and hard to comprehend.

getData(function(a){getMoreData(a,function(b){getMoreData(b,function(c){getMoreData(c,function(d){getMoreData(d,function(e){console.log('Callback Hell');});});});});});

Here are two solutions to this problem:

• Name your functions and declare them and pass just the name of the function as the callback, instead of defining an anonymous function in the parameter of the main function. It makes code easier to read and also helps to get better stack traces when exeptions happen.
• Modularity: Separate your code into modules, so you can export a section of code that does a particular job. Then you can import that module into your larger application.

// Aajax("..",function(..){// C});// B

// A and // B happen now, under the direct control of the main JS program. But // C gets deferred to happen later, and under the control of another party -- in this case, the ajax(..) function.In a basic sense, that sort of hand-off of control doesn't regularly cause lots of problems for programs.

A real world example would be

trackCheckoutAjax(purchaseInfo,function(){chargeCreditCard(purchaseInfo);showThankYouPage();}

But don't be fooled by its infrequency that this control switch isn't a big deal. In fact, it's one of the worst (and yet most subtle) problems about callback-driven design.It revolves around the idea that sometimes ajax(..) (i.e., the "party" you hand your callback continuation to) is not a function that you wrote, or that you directly control. Many times it's a utility provided by some third party.We call this "inversion of control," when you take part of your program and give over control of its execution to another third party. There's an unspoken "contract" that exists between your code and the third-party utility -- a set of things you expect to be maintained.In the case oftrackCheckoutAjax, the 3rd party trackCheckoutAjax could potentially call the passed callback multiple times -- since we passed over the callback and really have no control over it being called.

• Do not repeat code (DRY—Do Not Repeat Yourself)
• Implement better abstraction where you can have more generic functions that are versatile (can handle all sorts of functionalities)
• Have better maintainability
• Have more readable code
• Have more specialized functions.

• For asynchronous execution (such as reading files, and making HTTP requests)
• In Event Listeners/Handlers
• In setTimeout and setInterval methods
• For Generalization: code conciseness

More Code

• Understand JavaScript Callback Functions and Use Them
• Callback Hell
• Avoiding Callback hell in Node.js
• Syncing Async by Kyle Simpson
• Callbacks - YDKJS

Promises are usually vaguely defined as “a proxy for a value that will eventually become available”. They can be used for both synchronous and asynchronous code flows, although they make asynchronous flows easier to reason about.

Promises can be chained “arbitrarily”, that is to say - you can save a reference to any point in the promise chain and then tack more promises on top of it. This is one of the fundamental points to understanding promises.

Promises can be created from scratch by using new Promise(resolver). The resolver parameter is a method that will be used to resolve the promise. It takes two arguments, a resolve method and a reject method. These promises are fulfilled and rejected, respectively, on the next tick.Usually promises will resolve to some result, like the response from an AJAX call. Similarly, you’ll probably want to state the reason for your rejections – typically using an Error object.

// There can be one or more then() method calls that don’t provide an error handler.// Then the error is passed on until there is an error handler.asyncFunc1().then(asyncFunc2).then(asyncFunc3).catch(function(reason){// Something went wrong above});

Let's look at the sametrackCheckoutAjax example code and see how we can un-invert the control by using some sort of event listener.

functionfinish(){chargeCreditCard(purchaseInfo);showThankYouPage();}functionerror(err){logStatsError(err);finish();}varlistener=trackCheckoutAjax(purchaseInfo);listener.on('completion',finish);listener.on('error',error);

In essense, promises are a more formalized way of doing the above - there uninverting the control.

functiontrackCheckout(info){returnnewPromise(function(resolve,reject){// attempt to track the checkout// If succesful, call resolve()// otherwise call reject(error)});}

Promises can exist in three states: pending, fulfilled, and rejected. Pending is the default state. From there, a promise can be “settled” into either fulfillment or rejection. Once a promise is settled, all reactions that are waiting on it are evaluated. Those on the correct branch – .then for fulfillment and .catch for rejections – are executed.

From this point on, the promise is settled. If at a later point in time another reaction is chained onto the settled promise, the appropriate branch for that reaction is executed in the next tick of the program. Interestingly, if a .catch branch goes smoothly without errors, then it will be fulfilled with the returned value.

Promises already make the “run this after this other thing in series” use case very easy, using .then as we saw in several examples earlier. For the “run these things concurrently” use case, we can use Promise.all()

Promise.all has two possible outcomes.

• Settle with a single rejection reason as soon as one of its dependencies is rejected.
• Settle with all fulfillment results as soon as all of its dependencies are fulfilled.

Promise.race() is similar to Promise.all, except the first promise to settle will “win” the race, and its value will be passed along to branches of the race.Rejections will also finish the race, and the race promise will be rejected. This could be useful for scenarios where we want to time out a promise we otherwise have no control over.

More Code

• ES6 Promises in Depth
• Promises for asynchronous programming
• ECMAScript 6 promises: the API
• JavaScript Promises ... In Wicked Detail
• ES6 Promises
• Promise Patterns & Anti-Patterns
• Javascript Promise Chaining & Error Handling
• 3 Interesting experiments of Promise.all()
• What’s the deal with jQuery Deferred objects and Promises?

Generators, a new feature of ES6, are functions that can be paused and resumed. This helps with many applications: iterators, asynchronous programming, etc.

Two important applications of generators are:

• Implementing iterables
• Blocking on asynchronous function calls

There are four ways in which you can create generators:

• Via a generator function declaration:

  function*genFunc(){···}letgenObj=genFunc();

• Via a generator function expression:

  constgenFunc=function*(){···};letgenObj=genFunc();

• Via a generator method definition in an object literal:

  letobj={*generatorMethod(){···}};letgenObj=obj.generatorMethod();

• Via a generator method definition in a class definition (which can be a class declaration or a class expression):

  classMyClass{*generatorMethod(){···}}letmyInst=newMyClass();letgenObj=myInst.generatorMethod();

• Iterators (data producers):
  • Each yield can return a value via next(), which means that generators can produce sequences of values via loops and recursion. Due to generator objects implementing the interface Iterable, these sequences can be processed by any ES6 construct that supports iterables. Two examples are: for-of loops and the spread operator (...).
• Observers (data consumers):
  • yield can also receive a value from next() (via a parameter). That means that generators become data consumers that pause until a new value is pushed into them via next().
• Coroutines (data producers and consumers):
  • Given that generators are pausable and can be both data producers and data consumers, not much work is needed to turn them into coroutines (cooperatively multitasked tasks).

The fact that generators-as-observers pause while they wait for input makes them perfect for on-demand processing of data that is received asynchronously. The pattern for setting up a chain of generators for processing is as follows:

• First chain member: A normal function that has a parameter target, which is the generator object of the next element in the chain of generators. The function makes an asynchronous request and pushes the results to the target via target.next().
• Intermediate chain members: Generators that have a parameter target. They receive data via yield and send data via target.next().
• Last chain member: A generator that has no parameter target and only receives data.

Here's an example to explain the concepts via setTimeouts as a replacement for an async operation.

functiongetFirstName(){setTimeout(function(){gen.next('Jerry');},2000);// returns undefined// But next() is not called until the async activity is finished// After which var a is set to 'Jerry'}functiongetSecondName(){setTimeout(function(){gen.next('Seinfeld');},3000);// Same as getFirstName(), fn is paused until next() is called// And then the value is assigned to var b}function*getFullName(){varfirstName=yieldgetFirstName();varlastName=yieldgetSecondName();console.log(firstName+' '+lastName);// Jerry Seinfeld}vargen=getFullName();gen.next();// Initialize generator flow to first `yield`

More Code

• ES6 Generators in depth
• The Basics Of ES6 Generators
• Writing simple asynchronous code with JavaScript generators
• No promises: asynchronous JavaScript with only generators
• ES6 Generators in Depth

Async functions take the idea of using generators for asynchronous programming and give them their own simple and semantic syntax.

Here is an example of using async functions:

asyncfunctiondoAsyncOp(){varval=awaitasynchronousOperation();console.log(val);returnval;}

Here's its implementation using Promises

functiondoAsyncOp(){returnasynchronousOperation().then(function(val){console.log(val);returnval;});}

This has the same number of lines, but there is plenty of extra code due to then and the callback function passed to it. The other nuisance is the duplication of the return keyword, it makes it difficult to figure out exactly what is being returned from a function that uses promises.Also, Whenever you return a value from and async function, you are actually implicitly returning a promise that resolves to that value. If you don’t return anything at all, you are implicitly returning a promise that resolves to undefined.

One of the aspects of promises that hooks many people is the ability to chain multiple asynchronous operations without running into nested callbacks. This is one of the areas in which async functions excel even more than promises.

Using promises:

functiondoAsyncOp(){returnasynchronousOperation().then(function(val){returnasynchronousOperation(val);}).then(function(val){returnasynchronousOperation(val);}).then(function(val){returnasynchronousOperation(val);});}

Using Async functions, we can just act like asynchronousOperation is synchronous.

asyncfunctiondoAsyncOp(){varval=awaitasynchronousOperation();val=awaitasynchronousOperation(val);val=awaitasynchronousOperation(val);returnawaitasynchronousOperation(val);}

You don’t even need the await keyword on that return statement because either way it will return a promise resolving to the final value.

###Parallel OperationsOne of the other great features of promises is the ability to run multiple asynchronous operations at once and continue on your way once all of them have completed. Promise.all is the way to do this according to the new ES6 spec.

functiondoAsyncOp(){returnPromise.all([asynchronousOperation(),asynchronousOperation()]).then(function(vals){vals.forEach(console.log);returnvals;});}

This is also possible with async functions, though you may still need to use Promise directly:

asyncfunctiondoAsyncOp(){varvals=awaitPromise.all([asynchronousOperation(),asynchronousOperation()]);vals.forEach(console.log.bind(console));returnvals;}

Promises have the ability to be resolved or rejected. Rejected promises can be handled with the second function passed to then or with the catch method.Since we’re not using any Promise API methods, how would we handle a rejection? We do it with a try and catch.When using async functions, rejections are passed around as errors and this allows them to be handled with built-in JavaScript error handling code.

Using promises

functiondoAsyncOp(){returnasynchronousOperation().then(function(val){returnasynchronousOperation(val);}).then(function(val){returnasynchronousOperation(val);}).catch(function(err){console.error(err);});}

Here’s what it would look like with async functions.

asyncfunctiondoAsyncOp(){try{varval=awaitasynchronousOperation();val=awaitasynchronousOperation(val);returnawaitasynchronousOperation(val);}catch(err){console.err(err);}}

If you don’t catch the error here, it’ll bubble up until it is caught in the caller functions, or it will just not be caught and you’ll kill execution with a run-time error.

To reject an ES6 promises you can use reject inside the Promise constructor, or you can throw an error—either inside the Promise constructor or within a then or catch callback. If an error is thrown outside of that scope, it won’t be contained in the promise.

Here are some examples of ways to reject ES6 promises:

functiondoAsyncOp(){returnnewPromise(function(resolve,reject){if(somethingIsBad){reject(newError('something is bad'));// OR// reject('something is bad');// OR// throw new Error('something is bad');}resolve('nothing is bad');});}

Generally it is best to use the new Error whenever you can because it will contain additional information about the error, such as the line number where it was thrown, and a potentially useful stack trace.

With async functions promises are rejected by throwing errors. The scope issue doesn’t arise—you can throw an error anywhere within an async function and it will be caught by the promise.

asyncfunctiondoAsyncOp(){// the next line is finethrownewError('something is bad');if(somethingIsBad){// this one is good toothrownewError('something is bad');}return'nothing is bad';}// assume `doAsyncOp` does not have the killing errorasyncfunctionx(){varval=awaitdoAsyncOp;// this one will work just finethrownewError("I just think an error should be here");returnval;}

Of course, we’ll never get to that second error or to the return inside the doAsyncOp function because the error will be thrown and will stop execution within that function.

• One gotcha to be aware of is using nested functions. For example, if you have another function within your async function (generally as a callback to something), you may think that you can just use await from within that function.You can’t. You can only use await directly within an async function. This does not work:

asyncfunctiongetAllFiles(files){returnawait*files.map(function(filename){varfile=awaitgetFileAsync(filename);returnparse(file);});}

The await on line 3 is invalid because it is used inside a normal function. Instead, the callback function must have the async keyword attached to it.

asyncfunctiongetAllFiles(fileNames){returnawait*fileNames.map(asyncfunction(fileName){varfile=awaitgetFileAsync(fileName);returnparse(file);});}

• The next gotcha relates to people thinking that async functions are synchronous functions.Remember, the code inside the async function will run as if it is synchronous, but it will still immediately return a promise and allow other code to execute outside of it while it works to fulfillment.For example:

asyncfunctiondoAsyncOp(){try{varval=awaitasynchronousOperation();val=awaitasynchronousOperation(val);returnawaitasynchronousOperation(val);}catch(err){console.err(err);}}vara=doAsyncOp();console.log(a);a.then(function(){console.log('`a` finished');});console.log('hello');/* -- will output -- */// Promise Object// hello// `a` finished

You can see that async functions still utilize built-in promises, but they do so under the hood.This gives us the ability to think synchronously while within an async function, although others can invoke our async functions using the normal promises API or using async functions of their own.

More Code

• Simplifying Asynchronous Coding with ES7 Async Functions
• Jafar Husain: Async Programming in ES7 | JSConf US 2015
• Taming the asynchronous beast with ES7
• Async Await Pitfalls

The Evolution Of Asynchronous JavaScript - 2017