Promises in JavaScript: Part 2

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Before proceeding into what I've for this article, let's recap on what was discussed at my previous article: Promises in JavaScript: Part 1.

Whenever an asynchronous instruction is given to be executed, it's goes to the Web Browser features (API) and there wasn't a way of telling JavaScript what's going on in the background of the Browser until Promise. So Promises makes JavaScript aware of what's going on in the background of the Browser.

Promise is a special object that notifies JavaScript of the progress that goes on at the Web Browser background when fetching a data from the internet or making a promise object.

Also note, all Synchronous Instructions must execute first before any Asynchronous Instruction


Here are points to note using the .then method:

  • Any code (function) you want to run on the returned response data must be saved on the promise object.

  • The response data is added to the function to be invoked using .then method to the hidden property onFulfilled.

  • Promise object will automatically trigger the attached function to run with the returned response data.

Now let's look at an example having multiple Asynchronous instruction to be executed. The example below is to fetch a data from the internet fetch and also display a string after a given duration setTimeout().

function display(data) {
   console.log(data)
}

function printHello() {
   console.log("Hello");
}

function blockFor200ms() {
   // Multiple loops occurring for 200ms
}

setTimeout(printHello, 0);

const futureData = fetch('https://twitter.com/will/tweets/1')
futureData.then(display)

blockFor200ms()

console.log("Me first!");

The example above is gotten from the course: Javascript: The Hard Parts on Frontend Masters

Looking at the example, knowing what I've written about in previous articles, you'll expect all synchronous instructions to be executed first, then setTimeout() and finally fetch.

Hmm! Will it be so? Lets good through the explanation and see if it's so...

If you'll prefer an illustration first please click here

  • Firstly, the function display is stored in Global Memory waiting to be used.

  • Then the function printHello is also stored in Global Memory.

  • Also the function blockFor200ms is stored in Global Memory.

    To know about the Global Memory read this article here

  • setTimeout(printHello,0) is executed next. Since it isn't of Javascript but a facade function which then gets added to the Web Browser API.

    Check out Asynchronous JavaScript to know more facade function.

  • The duration is 0 in the setTimeout(printHello,0) function. So immediately the callback function printHello gets added to the Callback Queue.

  • Then the variable futureData is assigned to hold the promise object returned from fetch which is to get some data (response data) at a given URL (Uniform Resource Locator).

    Recap, from Promises in JavaScript: Part 1 the promise object has some properties which are: value andonFulfilled.

  • futureData.then(display): this line of code is saying, whatever response data that'll be received and added to the value property of the promise object, should be added to the function display in the onFulfilled property array.

  • blockFor200ms() in then invoked and gets added to the Call Stack.

    The block of imaginary instructions above will be running for 200ms

Meanwhile, blockFor200ms() is executing, Twitter's API which is to return a response data, has returned the response data which is "Hi everyone, how's your day?".

It took 100ms to get the response data from Twitter, meanwhile, blockFor200ms() is still executing. Then the response data gets added the value property in the promise object and automatically sent to the display function in the onFulfilled property.

At this point, display function is ready to be executed and get added to Call Stack but unfortunately it can't.

Why? Well blockFor200ms() isn't done executing and is in the Call Stack already. So where does it get added?

Well display function gets added to Microtask Queue

  • After 200ms blockFor200ms() is done executing and popped off the Call Stack, then

  • console.log("Me first!") is executed and displays "Me first!" at the console of the browser.

Remember printHello and display hasn't been executed, for JavaScript engine to know which of the functions is to be added to the Call Stack, the Event Loop is needed. The Event Loop checks if Call Stack is empty or not.

Event Loop has been discussed here

At this point the Call Stack is empty, so the Event Loop checks first, if the Microtask Queue has a reference to a function to be executed.

I made use of the word "has a reference to a function to be executed"

Meaning the function itself isn't in Mirotask Queue, its just a reference to label display at the Global Memory where it is stored to be used, same goes to functions I have addressed without the parenthesis ()

Yeah, it does have reference to display function, so display() gets added (pushed) to the Call Stack. When done executing, it gets popped off the Call Stack.

Then finally, the Event Loop checks if the Callback Queue has a reference to a function to be executed.

Yeah, also the Callback queue has a reference to printHello function, so finally printHello() gets added (pushed) to the Call Stack and pops out from the Call Stack when done executing.

promise


If you've read my previous article I mentioned that they're three properties in a promise object, which are value and onfulfilled. The last one is onrejection. This is when there is some sought of errors while fetching a data or you just want to display an error.

In order to display an error in a much more presentable way, you'll have to make use of these methods:

// errorFunction is the function that's to be called on error
.catch(errorFunction)

// OR

.then(nonerrorFunction, errorFunction)

// The first callback is executed onfulfillment while
// The second callback is executed onrejection

Wow! The explanation is lengthy. But I hope it's helpful

Conclusion

Just to be clear on everything that I've discussed, whenever a facade function e.g. setTimeout(), setInterval() and so on, takes a callback function and a duration in this because it's get added to the Callback Queue.

Meanwhile, a two pronged facade function fetch and a promise object created new Promise(...) having a function to be executed gets added to the Microtask Queue not the Callback Queue.

The Event Loop has a higher priority to check the Microtask Queue firstly before the Callback Queue to push any function in them to the Call Stack.

Thanks for Reading if you've read so far. I'll see you at the next article.

Comments (2)

Tapas Adhikary's photo

Jome Favourite, Great Series.

Favourite Jome's photo

Thank you very much, I'm just sharing what I've learnt 😊