When we work with web applications, there are many chances that we need to execute something asynchronously (like call 3rd party api, do some heavy stuff). To work with these asynchronous tasks, the first thing we think about is callback, but beside callback there is still one more technique to work with asynchronous is Promise. In this post, we will learn about the different between Promise and Callback.

What is asynchronous task

The main reason we need to use the asynchronous task is that we can’t determine exactly when we need to trigger something. Because it depends on something out of our control. For example we want to show a message when the end user clicks on a button, so that we can’t guess when the user will click that button. Another example is we need to log the information when a call to 3rd party service is successful, we can’t say when the call is finished. In these cases we need to `callback` to handle this kind of task.

What is callback

The main concept behind callback is we defined a method but we don’t execute it,but we will pass this method to something out of our control. Back to the example of a call to 3rd party API, we will do something like this “Hi API, when you finish your task, please help me to execute my function”. That is the way callback work. We will a callback function to someone else to trigger is instead of trigger it by ourselves.

Beside run on other context, callback can also carry value as input param which is pass from other context.

Callback example

See the Pen Callback by hieu (@phuchieuct93abc) on CodePen.light

But Callback has their own drawback

Callback helps us to handle asynchronous task but they have a big disadvantage iscallback hell . So to understand what is it, let take an example: We want to call 3rd party API, after getting the response, we will wait 5 seconds to call another API, wait for respond 2 seconds, we will print out the result.

This is the code to implement that feature

See the Pen oOLpbK by hieu (@phuchieuct93abc) on CodePen.light

As you can observe that with a chain of callback which involve many callback functions trigger in sequentialy. Our code is blooded of a ton of indentation. Besides that, the most important one is when reading this callback hell, it’s very hard to describe the process: what happens first, what happens later, what happens right after another one.

Promise was born to solve this problem of callback

What is promise

Promise in javascript has the same functionality as callback: both of them are used to handle the ansynchronous task. The main difference between them is Promise only to receive our callback function after the process is triggered. Compare to `callback, that required the function before the process. This way help us in some cases which required many callback functions need to trigger sequently in an easier way than a traditional callback.

Let take a look what is callback:

See the Pen Promise by hieu (@phuchieuct93abc) on CodePen.light

Small comparison between callback vs Promise

See the Pen Promise vs callback by hieu (@phuchieuct93abc) on CodePen.light

If you’re interested in `Promise`, you could read on this blog https://frontend.cloudaccess.host/promise-in-javascript/ to figure more features of Promise

Summary

Callback help us in some cases that required us to do something in the other context asynchronously. Promise has the same responsibility with callback but has better readability.

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