What are the drawbacks of using synchronous ajax call?

Question

This question could surely be applied to jQuery but in this case I am referring to Prototype. In the Prototype doc it says,

Since synchronous usage is

Answer 1

Well, in addition to the fact that it is very not JavaScript, it has such pleasant effects as COMPLETELY BLOCKING THE BROWSER'S UI. That is not a minor issue.

Did some research, found cool things. Prototype bases "asynchronous" off of XMLHttpRequest.open. According to Wikipedia, this is not something which is not a required part of the specification and it will prevent "onreadystatechange".

Answer 2

Well, i don't find that it is a problem to block the UI if the user is generating a loading image while the synchronous call is done. It could be the same as submitting the form.

Answer 3

Let's remind you that JavaScript is single-threaded

A synchronous IO call BLOCKS THE ENTIRE THREAD

A simple fix is to use asynchronous style programming using callbacks.

Customer = Class.create({     
    initialize: function(customerId, cb) { 
        new Ajax.Request('some-url', {
            method: 'get',
            parameters: {
                customerId: customerId
            },
            onSuccess: (function() {
                this.setCustomerInfo.apply(this, arguments);
                cb.apply(this, arguments);
            }).bind(this)
        }
    },
    setCustomerInfo: function(response) {
        //for the sake of this example I will leave out the JSON validation
        this.customerInfo = response.responseText.evalJSON();
    }   
});

var c = new Customer(1, function() {
     document.write(customer.customerInfo.firstName);
});

Answer 4

Most people frown upon synchronous ajax calls because it'll freeze the UI up until it has completed, as it will not allow code to continue until it's completion. Makes for a stutter in the interface I guess you could say.