Two questions 1. How is jQuery’s chain operation implemented? 2. Why use chain operation? Which one of these two questions do you think is easier to answer?
Chain operation I believe Baidu has a lot of information on the principle. In fact, the chain operation is just through the method on the object. Finally,
return this
replace the object. After returning, the object can of course continue to call methods, so chain operations can be performed. Then,
simply implement a :
//Define a JS class
function Demo() {
}
//Extend its prototype
Demo.prototype ={
setName:function (name) {
this. name = name;
return this;
},
getName:function () {
return this.name;
},
setAge:function (age) {
this.age = age;
return this;
}
};
////Factory function
function D() {
return new Demo();
}
//To implement chainable calls
D().setName("CJ").setAge(18).setName();
But... why? What to use?
General explanation:
Save the amount of code and the code looks more elegant.
For example, if there is no chaining, you may need to write the code like this:
document.getElementById("ele").dosomething();
document.getElementById("ele").dootherthing();
This code is called twice document.getElementById is used to obtain the elements of the DOM tree, which consumes a lot of money and requires writing two lines. However, the chain method only needs to write one line, which saves code...
But we can also use cached elements. For example:
var ele = document.getElementById("ele" ; .
The worst thing is that all object methods return the object itself, which means there is no return value, which may not be suitable in any environment.
For example, we want to create a very large integer BigInteger (meaning an integer that may overflow if Javascript's Number is used to save it), and by extending its operation method, is it suitable to use chain operations?
For example, if the operation is 31415926535 * 4 - 271828182, if it is designed in a chain style, the method may look like this:
Copy code
This seems elegant, but what if we want intermediate results? Maybe it will be written like this:
Copy code
console.log("result1 == " result1 ", result2 == " result2);
This does not seem elegant at all, it is no different from not using chain operations!
So what if the requirement is that the original BigInteger cannot be changed? Well, chain operations seem to be unable to meet this requirement.
So why do we need to use chain operations? For a better asynchronous experience
Javascript is a non-blocking language, so it is not that it does not block, but it cannot block, so it needs to be driven by events and asynchronously complete some operations that need to block the process. .
But asynchronous programming is a crazy thing... It doesn’t matter if it is separated when running, but it is also separated when writing code...
What are the common asynchronous programming models? Woolen cloth? •
Callback function The so-called callback function means to first register the function somewhere in the system to let the system know the existence of this function, and then in the future, when someone When an event occurs, call this function to respond to the event.
function f(num, callback){
if (num<0) {
alert("Call low-level function processing!");
alert("Score cannot be negative, input error!");
}else if(num==0){
alert("Call low-level function processing!");
alert("The student may not have taken the exam!");
}else{
alert("Call high-level function processing!");
setTimeout(function(){callback();}, 1000);
}
}
Here callback is the callback function. It can be found that callback will only be called when num is a non-negative number.
But the problem is, if we don’t look inside the function, we don’t know when the callback will be called and under what circumstances. There will be a certain coupling between the codes and a certain amount of confusion in the process.
Although the callback function is a simple and easy-to-deploy method to achieve asynchronous implementation, it is not good enough in terms of programming experience.
•
Event monitoring That is, it is event-driven, and the order of execution depends on the order of events.
function EventTarget(){
this.handlers = {};
}
EventTarget.prototype = {
constructor: EventTarget,
addHandler: function(type, handler){
this.handlers[type] = [];
},
fire: function(){
if(!event.target){
event.target = this;
}
if(this.handlers[event.type instanceof Array] ){
var handlers = this.handlers[event.type];
for(var i = 0, len = handlers.length, i < len; i ){
handlers[i](event );
}
}
},
removeHandler: function(type, handler){
if(this.handlers[type] instanceof Array){
var handlers = this. handlers[type];
for(var i = 0, le = handlers.length; i < len; i ){
if(handlers[i] === handler){
break;
}
}
handlers.splice(i, 1);
}
}
};
The above is from "JavaScript Advanced Programming" Custom event implementation. So we can bind the event processing function through addHandler, use fire to trigger the event, and use removeHandler to delete the event processing function.
Although it is decoupled through events, the process sequence is even more confusing.
•
Chained Asynchrony Personally, the most praiseworthy thing about chained operation is that it solves the problem of unclear execution process of asynchronous programming model. $(document).ready in jQuery illustrates this concept very well. DOMCotentLoaded is an event. Most of jQuery's operations will not work before the DOM is loaded. However, the designers of jQuery did not treat it as an event. Instead, it turned it into a "select its object and select it." operation" idea. $ selects the document object, and ready is its method for operation. In this way, the process problem is very clear, and the method at the later position in the chain will be executed later.
(function(){
var isReady=false; // Determine whether the onDOMReady method has been executed
var readyList= []; // Temporarily store the methods that need to be executed in this array inside
var timer;//timer handle
ready=function(fn) {
if (isReady)
fn.call( document);
else
readyList.push( function() { return fn.call(this);});
return this;
}
var onDOMReady=function(){
for(var i=0;ireadyList[i].apply(document);
}
readyList = null;
}
var bindReady = function(evt){
if(isReady) return;
isReady=true;
onDOMReady.call(window);
if(document.removeEventListener){
document.removeEventListener("DOMContentLoaded", bindReady, false);
}else if(document.attachEvent){
document.detachEvent("onreadystatechange", bindReady);
if(window == window.top){
clearInterval(timer);
timer = null;
}
}
};
if(document.addEventListener){
document.addEventListener("DOMContentLoaded", bindReady, false);
}else if(document.attachEvent){
document.attachEvent("onreadystatechange", function(){
if((/loaded|complete/).test(document.readyState))
bindReady();
});
if(window == window.top){
timer = setInterval(function(){
try{
isReady||document.documentElement.doScroll('left');//Use under IE Can you execute doScroll to determine whether the dom is loaded?
}catch(e){
return;
}
bindReady();
},5);
}
}
})();
The above code cannot use $(document).ready, but should be window.ready.
•
Promise The asynchronous programming model in CommonJS also continues this idea. Each asynchronous task returns a Promise object, which has a then method that allows a callback function to be specified.
So we can write like this:
f1().then(f2).then(f3);
In this method we don’t need to pay too much attention to the implementation, nor do we need to understand it. Asynchrony, as long as you know how to select objects through functions and operate through then, you can perform asynchronous programming.
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