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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()
var friends = ["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
});

How Callback Functions Work?

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.

Basic Principles when Implementing Callback Functions

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 Preserve this
- 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.

“Callback Hell”

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.

Callback Hell leads to Inversion of Control

// A
ajax( "..", 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 of trackCheckoutAjax, 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.

Callback functions - Benefits

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.

Callback functions - Use cases

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

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Promises

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
   });

Un-inversion of control using Promises

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

function finish() {
  chargeCreditCard(purchaseInfo);
  showThankYouPage(); 
}

function error(err) {
  logStatsError(err);
  finish();
}

var listener = 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.

function trackCheckout(info) {
  return new Promise(
    function(resolve, reject) {
      // attempt to track the checkout
      
      // If succesful, call resolve()
      // otherwise call reject(error)
    }
  );
}

Settling a promise

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.

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Generators (ES6)

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

Creating Generators

There are four ways in which you can create generators:

Via a generator function declaration:

function* genFunc() { ··· }
let genObj = genFunc();

Via a generator function expression:

const genFunc = function* () { ··· };
let genObj = genFunc();

Via a generator method definition in an object literal:

let obj = {
    * generatorMethod() {
        ···
    }
};
let genObj = obj.generatorMethod();

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

class MyClass {
    * generatorMethod() {
        ···
    }
}
let myInst = new MyClass();
let genObj = myInst.generatorMethod();

Roles played by generators

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).

Processing asynchronously pushed data (Generators as observers)

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.

Generators and Async

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

function getFirstName() {
  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'
}

function getSecondName() {
  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() {
  var firstName = yield getFirstName();
  var lastName = yield getSecondName();
  console.log(firstName + ' ' + lastName); // Jerry Seinfeld
}

var gen = getFullName();
gen.next(); // Initialize generator flow to first `yield`

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Async Functions (ES7)

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

Converting Promises to Async Functions

Here is an example of using async functions:

async function doAsyncOp () {
    var val = await asynchronousOperation();
    console.log(val);
    return val;
}

Here's its implementation using Promises

function doAsyncOp () {
    return asynchronousOperation().then(function(val) {
        console.log(val);
        return val;
    });
}

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.

Chaining Operations

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:

function doAsyncOp () {
    return asynchronousOperation().then(function(val) {
        return asynchronousOperation(val);
    }).then(function(val) {
        return asynchronousOperation(val);
    }).then(function(val) {
        return asynchronousOperation(val);
    });
}

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

async function doAsyncOp () {
    var val = await asynchronousOperation();
    val = await asynchronousOperation(val);
    val = await asynchronousOperation(val);
    return await asynchronousOperation(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 Operations One 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.

function doAsyncOp () {
    return Promise.all([asynchronousOperation(), asynchronousOperation()])
        .then(function(vals) {
            vals.forEach(console.log);
            return vals;
        });
}

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

async function doAsyncOp () {
    var vals = await Promise.all([asynchronousOperation(), asynchronousOperation()]);
    vals.forEach(console.log.bind(console));
    return vals;
}

Handling Rejection

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

function doAsyncOp () {
    return asynchronousOperation().then(function(val) {
        return asynchronousOperation(val);
    }).then(function(val) {
        return asynchronousOperation(val);
    }).catch(function(err) {
        console.error(err);
    });
}

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

async function doAsyncOp () {
    try {
      var val = await asynchronousOperation();
      val = await asynchronousOperation(val);
      return await asynchronousOperation(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.

Broken Promises

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:

function doAsyncOp () {
    return new Promise( function(resolve, reject) {
        if ( somethingIsBad ) {
            reject(new Error('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.

async function doAsyncOp () {
    // the next line is fine
    throw new Error('something is bad');

    if ( somethingIsBad ) {
        // this one is good too
        throw new Error('something is bad');
    }
    return 'nothing is bad';
}

// assume `doAsyncOp` does not have the killing error
async function x () {
    var val = await doAsyncOp;

    // this one will work just fine
    throw new Error("I just think an error should be here");

    return val;
}

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.

Gotchas

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:

async function getAllFiles (files) {
    return await* files.map(function(filename) {
        var file = await getFileAsync(filename);
        return parse(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.

async function getAllFiles (fileNames) {
    return await* fileNames.map(async function(fileName) {
        var file = await getFileAsync(fileName);
        return parse(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:

async function doAsyncOp () {
    try {
      var val = await asynchronousOperation();
      val = await asynchronousOperation(val);
      return await asynchronousOperation(val);
    } catch (err) {
      console.err(err);
    }
}

var a = 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.

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Videos

The Evolution Of Asynchronous JavaScript - 2017

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vasanthk / Async Javascript

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Evolution of Asynchronous JavaScript

Slides from my Talk

Callbacks

How Callback Functions Work?

Basic Principles when Implementing Callback Functions

“Callback Hell”

Callback Hell leads to Inversion of Control

Callback functions - Benefits

Callback functions - Use cases

Links

Promises

Un-inversion of control using Promises

Settling a promise

Links

Generators (ES6)

Creating Generators

Roles played by generators

Processing asynchronously pushed data (Generators as observers)

Generators and Async

Links

Async Functions (ES7)

Converting Promises to Async Functions

Chaining Operations

Handling Rejection

Broken Promises

Gotchas

Links

Videos