Javascript encapsulates DOMContentLoaded event instance_javascript skills

I am writing a Javascript framework recently, and I have just encapsulated the DOMContentLoaded event. With a little excitement, I took notes on the principles and compatibility issues encountered during the development process, so as not to forget to look for it everywhere.

When we write js code, we usually add the window.onload event, mainly so that after the DOM is loaded, we can use getElementById, getElementsByTagName and other methods to select DOM elements for operation, but window.load will wait until the DOM is loaded. Scripts, CSS, and all resources in the iframe will not be triggered until the final image or even the iframe is loaded. Many times, web pages have many and large images. It is obviously too late to execute the js until the last image, which is a time-consuming task, is loaded. Many times, it is too late. will affect the user experience.

Many js frameworks have a document.ready function, such as JQuery’s $(document).ready() method, which can execute the js code immediately after the DOM is loaded and let the image load slowly.

The core of document.ready is the DOMContentLoaded event. Firefox, chrome, opera, safari, and ie9 can all use addEventListener('DOMContentLoaded',fn,false) for event binding, but ie6~8 does not support the DOMContentLoaded event, so you need to Compatibility processing for ie6~8.

According to the information, ie6~8 can use the document.onreadystatechange event to monitor whether the document.readyState status is equal to complete to determine whether the DOM has been loaded. If there is an iframe embedded in the page, the document.readyState of ie6~8 will wait until the iframe. It will become complete only after all resources are loaded. At this time, iframe becomes a time-consuming one. However, after testing, even if there is no iframe in the page, when readyState equals complete, the onload event is actually triggered instead of the DOMContentLoaded event. I am surprised by this.

Fortunately, IE has a unique doScroll method. When the page DOM has not been loaded, an error will be reported when the doScroll method is called. On the contrary, as long as doScroll is called at intervals until no error is reported, it means that the page DOM has been loaded. , this method is valid regardless of whether the content in the image and iframe has been loaded.

If there are multiple js files bound to the document.ready event, in order to prevent the browser from binding repeatedly and execute them in an orderly manner, an event queue mechanism can be introduced to solve the problem.

The above is the principle and compatibility issues of the document.ready event. Below is a sample code. In order to facilitate understanding of the execution process, the function encapsulation mode is used. The execution process is written in comments. If there is anything wrong, welcome Give advice.

Copy code The code is as follows:

//Save the event queue of domReady
eventQueue = [];

//Judge whether the DOM is loaded
isReady = false;

//Judge Whether DOMReady is bound
isBind = false;

/*Execute domReady()
*
*@param {function}
*@execute Push the event handler into the event queue , and bind DOMContentLoaded
* If DOM loading has been completed, execute immediately
*@caller
*/
function domReady(fn){
if (isReady) {
fn .call(window);
}
else{
eventQueue.push(fn);
};

bindReady();
};

/*domReady event binding
*
*@param null
*@execute Modern browsers bind DOMContentLoaded through addEvListener, including ie9
ie6-8 determines whether the DOM is loaded by judging doScroll
*@caller domReady()
*/
function bindReady(){
if (isReady) return;
if (isBind) return;
isBind = true;

if (window.addEventListener) {
document.addEventListener('DOMContentLoaded',execFn,false);
}
else if (window.attachEvent) {
doScroll();
};
};

/*doScroll determines whether the DOM of ie6-8 is loaded
*
*@param null
*@execute doScroll determines whether the DOM is loaded
*@caller bindReady()
*/
function doScroll(){
try{
document.documentElement.doScroll('left');
}
catch(error) {
return setTimeout(doScroll,20);
};
execFn();
};

/*Execution event queue
*
*@param null
*@execute Loop the event handler in the execution queue
*@caller bindReady()
*/
function execFn(){
if (!isReady) {
isReady = true;
for (var i = 0; i < eventQueue.length; i ) {
eventQueue[i].call(window);
};
eventQueue = [];
};
};

//js file 1
domReady(function(){
...
});
//js file 2
domReady(function(){
...
});

//Note, if it is asynchronously loaded js, do not bind the domReady method, otherwise the function will not be executed,
//Because DOMContentLoaded has been triggered before the asynchronously loaded js is downloaded, addEventListener can no longer listen when it is executed

Test page: Two large images are loaded. Onload requires the images to be loaded before js can be executed. DOMContentLoaded only needs to wait until the DOM is loaded before executing js. You can open firebug to view the loading process. Remember to clear the browser cache before each test.