CODEWITHSUNDEEP
javascriptasynchronouscallback
Ritwik
Ritwik

Reputation: 113

What is the use of a synchronous callback function in JavaScript?

I have been doing extensive reading on asynchronous programming for the web and use of callbacks in JavaScript and jQuery. I have understood the fundamentals of AJAX. What I cannot figure out is the use of callback functions when not used in asynchronous programming.

From my understanding, simply adding a callback to a function does not make it non-blocking/asynchronous. Asynchronous capability is actually provided by the environment (browser APIs). So adding a callback to a function that I have written will not lead to any asynchronous execution.

For example:

var X;
function Test(A, B, Callback) {
X=A+B*A*B;
Callback(X);
}

Test(99999,999999,function(Data) {
alert(Data);
});

alert("This is not printed first, as it would be in Async.");

In the above I'm taking two numbers and performing algebraic operations on them. Even though I'm using a callback function, the code execution will be blocked while the operations are performed. Then the callback will be executed, displaying an alert with the result of the computation. Then the next alert follows. Had I made an XMLHttpRequest instead of the algebraic operation, I would have got the second alert first because of asynchronous implementation. The code flow would not be blocked during the request as is happening during the mathematical operation.

Thus, what is the use of a callback in non-async calls when code execution is blocked even with a callback?

Upvotes: 11

Views: 4640

Answers (3)

Nicholas Connor
Nicholas Connor

Reputation: 26

I use callbacks in the way you describe whenever I am working with complicated access methods which I don't want to rewrite, and I use them when I know the component will be run in a context the component doesn't know about, but am not particularly interested in developing or using an event emitter.

I do not know if other people find them as easy to work with as I do. So on a team, I try to get a sense of whether it is productive to use callbacks in the way you describe.

Upvotes: 0

Tibos
Tibos

Reputation: 27823

Some very common examples of synchroneous callbacks are the methods on Array.prototype: forEach, map, filter, etc.

The role of the callback is to provide a partial implementation that can be easily swapped in a larger algorithm. Some design patterns like template method and strategy come to mind.

Upvotes: 7

SLaks
SLaks

Reputation: 887305

You're right.
There is usually no reason to have a sync callback.

Exceptions include callbacks that might sometimes be async, or callbacks that can be raised more than once (eg, [].map()).

Upvotes: 2

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