JS basics day12

Addition, deletion and modification of DOM:

Add innerHTML + = appendChild() insertBefore()

Delete} innerHTML = 'remove() removeChild()

Modify replaceChild()


Know node - line breaks, text, comments, labels are all node lists

Tag: HTMLCollection

Array array

Array 2} Arguments

Array 3 # NodeList

Array 4} HTMLCollection

Each of the above arrays belongs to a different category, so it has different properties and methods

How to convert a pseudo array to a true array


        var arr = new Array() ;

        function fn() {  

        var oPs = document.querySelectorAll('p') ;

        // oPs.push('a')

        var oPs2 = document.getElementsByTagName('p') ;

        // oPs2.pop()

        var arr = [] ;
        for(var i = 0 ; i < oPs.length ; i++) {



Event: something happened and was handled

Event: event source Event type = event handler

Event handlers are asynchronous

This anonymous function is called by the system after the event occurs

General event handling functions do not have a specific return value, but return can also be used

document.onclick = function () {  

The first parameter of the event handler function represents the event object

The event object is the whole process in which the user records what happened

document.onclick = function (a , b) {  
            // The event handler function has only one parameter
            console.log(b);  // undefined
        document.onclick = function (e) {  
            // Most browsers support the first parameter of the event handler as an event object
            // console.log(e);

            // Older browsers can only use event to record event objects
            // console.log(event);

            e = e || event ;

            // Target - which tag triggered the target
            // this points to the event source



How to bind events

1 inline js ο nclick="fn()"

2. Event binding: odiv onclick = function(){}

3. Event monitoring: odiv Addeventlistener (type, event handler) --- will not be overwritten

Remove event

Both in-line js and ordinary event bindings can be removed in the way of overwriting by means of re assignment

The event listening method needs to be written as a named function before it can be removed (Note: when removing, it needs to be removed one by one) removeEventListener()

        // onclick re assignment (any value is OK). It is generally recommended to write null
        oDiv.onclick = null ;

        function fn2() {  

        function fn() {  

        // Add event listener
        function addEvent(ele , type , cb) {  
            if(window.addEventListener) {
                ele.addEventListener(type , cb)
            } else {
                ele.attachEvent('on' + type , cb)

        // Remove event listener
        function removeEvent(ele , type , cb) {  
            if(window.removeEventListener) {
                ele.removeEventListener(type , cb)
            } else {
                ele.detachEvent('on' + type , cb)

Event flow:

Event bubbling: from the inside out

Event capture: from outside to inside

Mainly use event bubbling

How to implement event capture

The third parameter of addeventlistener (type, FN, true) indicates whether event capture is supported

What is an event flow:

Event flow describes the order in which events are received from the page

Interestingly, the development teams of Microsoft (ie) and Netscape (Netscape) have proposed two completely opposite concepts of event flow. The event flow of IE is event bubbling, while the event flow of Netscape is event capturing

Event capture: the idea of event capture flow is that less specific DOM nodes should receive events earlier, while the most specific nodes should receive events last

Event bubbling: the event flow proposed by IE is called event bubbling, that is, the event is received by the most specific element at the beginning, and then propagated upward level by level to less specific nodes


Ie proposes bubbling stream, while Netscape proposes capture stream. Later, under the unification of W3C organization, JS supports bubbling stream and capture stream. However, at present, lower versions of IE browsers can only support bubbling stream (IE6, IE7 and IE8 only support bubbling stream). Therefore, in order to be compatible with more browsers, it is recommended that you use bubbling stream.

Event flow includes three stages: event capture stage, target stage and event bubble stage

Prevent event bubbling

Using event objects


              e.cancelBubble = true

        oP.onclick = function (e) {  
            e = e || event ;
            // Prevent event bubbling
            // How mainstream browsers are written
            // e.stopPropagation()
            // Method for canceling event bubbling in low version IE
            // e.cancelBubble = true ;
            if(e.stopPropagation) {
            } else {
                e.cancelBubble = true ;

Event delegation

If there are too many event handlers on the page, performance will be affected

If you need to make every li have click events and bind events circularly - there will be many event handling functions on the page

Circular binding events cannot bind events to future objects


Solution: event delegation

The event of the child element will trigger the event of the parent element

Implementation of event delegation:

Find all the parent elements that need to bind events, bind events to the parent element, and then judge who triggered them according to the target

What is an event delegate

The same type event of the child element is delegated to the parent element

Why use event delegates

Reduce event handler

You can bind events to future objects

Principle of event delegation

Event bubbling --- events of the same type of child elements will trigger events of the same type of parent elements

Keywords: Javascript Front-end linq

Added by mainewoods on Sun, 26 Dec 2021 15:55:30 +0200