I'm having some trouble understanding the difference between asynchronous and synchronous Javascript, and was hoping someone could shed some light on this.

I know Javascript is inherently synchronous, but you can use asynchronous events/callbacks to alter your program flow. However, what happens if you call a function with no callback that contains AJAX?

For example, if I have the following code, where foo() contains some sort of server query and foobar() contains some output text:

foo();
foobar();

Will foobar() be called before the internal logic in foo() is plete, or will the browser wait until foo() is fully executed before calling foobar()? (This seems simple, but my confusion arises from callbacks and whether or not they are absolutely necessary in all cases to control your program flow, i.e. if foo(foobar) is always necessary.)

Also, if foo() contains a server call that is quickly executed on the client side but takes a long time on the server to process, is a callback the only way I can make my program wait until foo() is pletely done executing?

I'm having some trouble understanding the difference between asynchronous and synchronous Javascript, and was hoping someone could shed some light on this.

I know Javascript is inherently synchronous, but you can use asynchronous events/callbacks to alter your program flow. However, what happens if you call a function with no callback that contains AJAX?

For example, if I have the following code, where foo() contains some sort of server query and foobar() contains some output text:

foo();
foobar();

Will foobar() be called before the internal logic in foo() is plete, or will the browser wait until foo() is fully executed before calling foobar()? (This seems simple, but my confusion arises from callbacks and whether or not they are absolutely necessary in all cases to control your program flow, i.e. if foo(foobar) is always necessary.)

Also, if foo() contains a server call that is quickly executed on the client side but takes a long time on the server to process, is a callback the only way I can make my program wait until foo() is pletely done executing?

• javascript
• ajax
• asynchronous
• dom-events
• synchronous

Share Improve this question Follow edited Aug 20, 2020 at 8:56 Brian Tompsett - 汤莱恩 5,8937272 gold badges6161 silver badges133133 bronze badges asked Jul 19, 2011 at 13:54 sichinumisichinumi 1,87533 gold badges2121 silver badges4040 bronze badges

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5 Answers 5

Sorted by: Reset to default 3

foobar() will indeed be called before the Ajax call in foo() is plete...

Unless, and this is the answer to your second question, you specify that the Ajax call should be synchronous, which is an option. Doing so will force the user to wait until the call pletes before they can do anything, so that's usually not the best choice. Using a callback is usually the best approach.

Will foobar() be called before the internal logic in foo() is plete

The answer to this question depends on what you mean by "internal logic". foobar will only be called once all of the javascript in foo is plete. It will not however wait for the AJAX call to return (kind of the whole point of the A in AJAX).

All of foo will run, then all of foobar will run.

Part of foo might say "Send an HTTP request asynchronously", in which case it won't wait for the response to e back before continuing (but since foo only triggers the sending of the request, it is considered plete).

If there was a callback, then that would run when the response arrived (which would usually be after foobar was plete).

In simple terms, foo() will entirely execute internally before foobar() starts. However, if within foo() there is a call to execute an ajax request, that request will be submitted, and will then execute in parallel.

Once the request is submitted the processing will continue. The ajax request will return with a response when it is plete, that may or may not be picked up by further processing.

Imagine, for example, you have something like this:

var something;
foo(); //Inside foo, the value of something is set to the result of an AJAX call
console.log(something);

The console.log line will print undefined, because the variable something will still be empty, until the AJAX response in foo changes it. This is the sort of situation in which a callback is useful - in the callback you can assign the value of the response to something, and you then know that something has a value.

Your example is very similar to this - foobar will be called as soon as foo returns, and if foo contains some asynchronous logic, it will return before that pletes. If you want foobar to execute after, you will need to call it from (or use it as) the callback function.

It would be possible to make foo wait until whatever it does has finished, but that would probably defeat the point of using AJAX, as you'd have to send a synchronous request.