How JavaScript monitors nested pages

With the development of web applications, more and more websites are beginning to use nested pages to present data, such as nesting an iframe to display the content of another website. In this case, how can one effectively listen to events from nested pages in JavaScript?

First, we need to understand some basic concepts. In web development, a website can contain multiple pages, and each page can contain multiple iframes (inline frames). If we want to listen to the events of the nested page in JavaScript, we need to first obtain the iframe element where the nested page is located.

There are many ways to obtain iframe elements. The most common is to use the getElementById() method or querySelector() method of the document object. For example, if we want to get the iframe element with the id "myframe", we can use the following code:

var iframe = document.getElementById("myframe");

After getting the iframe element, we can monitor its events. Below are several common iframe events and corresponding monitoring methods.

1. Loading Completed Event

When the page in the iframe is loaded, the load event will be triggered. You can use the following code to listen to this event:

iframe.onload = function() {
  // iframe中的页面已经加载完成
};

2. Page loading failure event

If the page in the iframe fails to load, the error event will be triggered. You can use the following code to listen to this event:

iframe.onerror = function() {
  // iframe中的页面加载失败
};

3. Content size change event

Sometimes, the content size in the iframe may change, for example, a new page is added to the page. content or some elements have been deleted. You can use the following code to listen to this event:

iframe.contentWindow.addEventListener("resize", function() {
  // iframe中的内容大小已经发生变化
});

It should be noted that since the content in the iframe is a page on another website, we need to use the contentWindow object to obtain the DOM elements in the page. In addition, since the resize event is implemented differently in different browsers, it is best to use the addEventListener() method to listen for this event.

4. Page scroll event

If the page in the iframe supports scrolling, you can use the following code to listen for the scroll event:

iframe.contentWindow.addEventListener("scroll", function() {
  // iframe中的页面已经发生滚动
});

Similar to the resize event, we You also need to use the contentWindow object to get the DOM elements in the page.

In addition to the above events, there are many other events that can be monitored. The corresponding event type needs to be selected according to specific needs.

In general, through the above methods, we can effectively monitor various events in nested pages. Of course, before monitoring, we need to ensure that the website we are on does not use iframe sandbox technology to prohibit scripts in iframes. In addition, the restrictions of the same-origin policy must also be taken into consideration. If the page in the iframe does not belong to the same domain name as the parent page, its content cannot be directly accessed.

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