The JS event flow model

Event Capturing is a top-down propagation method. Taking the click event as an example, it starts from the outermost root element and gradually propagates inwards to reach the clicked node. It's a way to propagate from the outermost node gradually inward until the target node.

Event Bubbling is a bottom-up propagation method. Similarly taking the click event as an example, the event starts from the clicked node and then gradually propagates upwards to the highest-level node.

DOM0 Level Model

Also known as the original event model, this method is relatively simple and compatible with all browsers, but it couples the interface with the logic, leading to poor maintainability.

Example

When clicking on the <div> with the id i3, the browser will sequentially display 2 1 0.

<!DOCTYPE html>
<html>
<head>
    <title>JS event flow model</title>
</head>
<style type="text/css">
    div{
        display: flex;
        justify-content: center;
        align-items: center;
    }
</style>
<body>
    <div id="i1" style="height: 150px;width: 150px;background: red;" onclick="alert(0)">
        <div id="i2" style="height: 100px;width: 100px;background: green;" onclick="alert(1)">
            <div id="i3" style="height: 50px;width: 50px;background: blue;" onclick="alert(2)"></div>
        </div>
    </div>
</body>
</html>

IE Event Model

Versions of IE8 and earlier do not support capturing events. The IE event model consists of two processes:
Event handling phase: the event reaches the target element, triggering the listening event of the target element.
Event bubbling phase: the event bubbles from the target element to the document, sequentially executing the events bound to the nodes it passes through.

DOM2 Level Model

The DOM2 event model is a standard model established by W3C, supporting both capturing and bubbling events. The event processing phases called are in sequence: capturing, target, and bubbling.

Example

When clicking on the <div> with the id i3, the browser will sequentially display 0 1 3 2, where the third argument of the addEventListener method declares whether the bound event is capturing or bubbling, defaulting to false, which is bubbling.

<!DOCTYPE html>
<html>
<head>
    <title>JS事件流模型</title>
</head>
<style type="text/css">
    div{
        display: flex;
        justify-content: center;
        align-items: center;
    }
</style>
<body>
    <div id="i1" style="height: 150px;width: 150px;background: red;">
        <div id="i2" style="height: 100px;width: 100px;background: green;">
            <div id="i3" style="height: 50px;width: 50px;background: blue;"></div>
        </div>
    </div>
</body>

<script type="text/javascript">
    document.addEventListener('click',(e) => {
        alert(0);
    },true) 
    document.getElementById("i1").addEventListener('click',(e) => {
        alert(1);
    },true) 
    document.getElementById("i2").addEventListener('click',(e) => {
        alert(2);
    })  
    document.getElementById("i3").addEventListener('click',(e) => {
        alert(3);
    })     
</script>
</html>

The document object and the i1 node are bound to capturing listener events, while the i2 and i3 nodes are bound to bubbling-type events. The order of event propagation is as follows:

window --- document --- html --- body --- i1 --- i2 --- i3 --- i2 --- i1 --- body --- html --- document --- window

The process from window to i3 is the capture phase, sequentially executing the bound events in this example alert(0) and alert(1), then reaching the target phase of i3 and executing the event alert(3) bound to i3, followed by the bubbling phase from i3 to window, executing the bound alert(2), resulting in the execution order of 0 1 3 2.

Note

Difference in event binding

In DOM0, when directly binding functions, the later-defined function will overwrite the previously bound function. In the following example, only alert(1) is executed, not alert(0). click() is an object event that triggers the method bound to onclick(). onclick() is an object property, and when a function is bound to it, it becomes the method executed after the click() event is triggered.

<!DOCTYPE html>
<html>
<head>
    <title>JS事件流模型</title>
</head>
<style type="text/css">
    div{
        display: flex;
        justify-content: center;
        align-items: center;
    }
</style>
<body>
    <div id="i1" style="height: 150px;width: 150px;background: red;"></div>
</body>

<script type="text/javascript">


```javascript
document.getElementById("i1").onclick = function(){
    alert(0);
} // Overridden

document.getElementById("i1").onclick = function(){
    alert(1);
} // Executed

addEventListener allows you to bind multiple functions to an event, and you don't need to use on when binding. It can also take a third parameter useCapture to determine whether the event is bound to the capture phase or the bubble phase.

document.getElementById("i1").addEventListener('click',(e) => {
    alert(0);
}) // Executed

document.getElementById("i1").addEventListener('click',(e) => {
    alert(1);
}) // Executed

attachEvent allows you to bind multiple functions to an event, you need to use on when binding, and it only supports execution during the bubble phase, so there is no third parameter.

document.getElementById("i1").attachEvent('onclick',function(e){
    alert(0);
}) // Executed

document.getElementById("i1").attachEvent('onclick',function(e){
    alert(1);
}) // Executed

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