A callback is a function sent as an argument to another function. The callback is executed when a particular task is completed or when an event occurs. It is a predecessor to promises. Promises, though perform the same functionality as callback, are ,much more easier to read and compact. Older modules use callbacks whereas, the newer ones work well with promises. This creates makes them difficult to work with each other. Also, the functional modules that use callbacks cannot be used with async/await. One of the ways to use callback functions in such scenarios, is to convert them to promises.
The ‘async’ keyword is added before a function during its definition. Any function defined this way returns a promise. If any other value is returned, it is wrapped within a resolved promise.
In recent times, promises are the most commonly used solutions to asynchronous problems. They are used when a certain task has to be carried out without halting or freezing the entire program: same reason as callback functions.
Promises are, simply put, asynchronous or deferred tasks.
A promise is eager — i.e., it immediately starts executing once called. It is easier to use, makes the code readable and can be used in most of the places where a callback function is used.
The syntax of a promise declaration is:
The setbacks of using callback functions inspired the ‘Async’ library that is the point of this post. It was initially developed to be used in NodeJS programs. But currently, it is very widely used, even in browsers.
The Async library can be installed via npm or yarn:
npm install async / yarn add async
It provides certain functions like each, map, reduce etc that provides solutions to the usual requirements: a set of values or iteratees on which the tasks are performed. …
Asynchronous programming allows waiting for a certain event or completion of a certain task without completely freezing or halting the entire program. It allows all unrelated tasks of the program to be carried out without any pause, while the related tasks are being paused.
One of the earliest or common ways of ensuring this is using callback functions or call-after functions.
A callback function is…