If you have started you carrier around 7-10 years back as
web developer then probably you have experienced the evaluation practically.
Because of huge advantage of AJAX, more or less 70-80 % of
modern web application has adapted this technology. The new style of AJAX
implementation came in market so quickly that it’s sometime hard to remember
what things were before that?
Anyway, Long ago,
Netscape added a feature in browser called live script which was capable to
give flavor of offline processing like form validation in client side and bit
more. Slowly Live script became JavaScript with more functionality and flexibility.
The inception of DynamicHTML has started in development
world, slowly XML gain its popularity in AJAX world when Microsoft added a
small function in IE5 to perform XMLHttpRequest call to server.
The classic AJAX
using XMLHttpRequest
This is classic style to make AJAX call using vanilla JavaScript.
If you worked in very old ASP, JSP or PHP application, probably you are familiar
with this style of AJAX call.
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
// Typical action to be performed when the document is ready:
document.getElementById("demo").innerHTML = xhttp.responseText;
}
};
xhttp.open("GET", "yourfilename.extension", true);
xhttp.send();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
// Typical action to be performed when the document is ready:
document.getElementById("demo").innerHTML = xhttp.responseText;
}
};
xhttp.open("GET", "yourfilename.extension", true);
xhttp.send();
Oh, did I miss something? Yes, we need to detect browser
prior to AJAX call, Just to make sure we are using proper XMLHttpRequest
object. Yes, XMLHttpRequest object is browser specific.
JQuery changed the
whole game
Then JQuery came in market, it played major role to change
face of client side app development. It
combine and packed the above code into few lines and introduce the concept of
Callback.
$.ajax({url: "yoururl.com", success: function(result){
//enjoy with result variable
//enjoy with result variable
}});
And that’s all, simple, compact, no code smell and maintainable.
Now, in mean time other library too started wrapping classic XMLHttpRequest
request using very similar syntax.
So, as a result the concept of callback has introduced in development
world. The moto of callback is, call me once you have prepared the result, or
call error in mess!.
Let’s return promise
rather than actual result
People started to realize that, as it is asynchronous call,
it should return something by which we can take decision. Let’s give some
example. The below code is based on promise and then callback function.
requestSomeData("http://example.com ") // returns
a promise for the response
.then(function(response){
// ‘then’ is used to provide a promise handler
return JSON.parse(response.body);
// parse the body
}) // returns a
promise for the parsed body
.then(function(data){
return data.something;
// get the price
}) // returns a
promise for the price
.then(function(price){
// print out the price when it is fulfilled
//do something
with the data
});
We can consider this as fully asynchronous execution plan,
even if my JSON.parse() take huge time, the execution flow will not hang the
thread because I have attached then callback to ensure that someone will take
care once parse completed.
Angular and Other
library utilize it best
Angular 1.X introduced a way to sync all asyn call using “$q”
service. This is huge power to developer to process more than one asynchronous
call and print consolidated result.
Want to have more
control on AJAX call? Use Observable.
RxJs introduced the concept called Observable to have more
grip on AJAX call. Let’s think that we need to cancel an AJAX call in middle. Assume,
the situation where need to handle multiple AJAX call. Observable is preferred
than promise here.
So, an observable is like a stream and allow to pass zero or
more events where the callback is called for each event. The Observable covers
more including promise.
For more info, please check Observable in Angular2
