Synchronous and Asynchronous Function Calls

Here is an analogy to explain the difference between synchronous and asynchronous flow:


Imagine you make a phone call to a friend and ask him to look something up for you. Although it might take a while, you wait on the phone and stare into space, until your friend gives you the answer you needed.

The same is happening when you make a function call containing “normal” code:

function findItem() {
    var item;
    while(item_not_found) {
        // search
    return item;

var item = findItem();

// Do something with item

Even though findItem might take a long time to execute, any code coming after var item = findItem(); has to wait until the function returns the result.


You call your friend again for the same reason. But this time you tell him that you are in a hurry and he should call you back on your mobile phone. You hang up, leave the house and do whatever you planned to do. Once your friend calls you back, you are dealing with the information he gave to you.

That’s exactly what’s happening when you do an Ajax request.

findItem(function(item) {
    // Do something with item

Instead of waiting for the response, the execution continues immediately and the statement after the Ajax call is executed. To get the response eventually, you provide a function to be called once the response was received, a callback (notice something? call back ?). Any statement coming after that call is executed before the callback is called.

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