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‎25 - Event Related/README.md

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# 25 Event Related 中文指南
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> 本篇作者：©[大史快跑Dashrun](https://github.com/dashrun)——Chinasoft Frontend Developer
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> 简介：[JavaScript30](https://javascript30.com) 是 [Wes Bos](https://github.com/wesbos) 推出的一个 30 天挑战。项目免费提供了 30 个视频教程、30 个挑战的起始文档和 30 个挑战解决方案源代码。目的是帮助人们用纯 JavaScript 来写东西，不借助框架和库，也不使用编译器和引用。现在你看到的是这系列指南的第 25 篇。完整指南在 [GitHub](https://github.com/soyaine/JavaScript30)，喜欢请 Star 哦♪(^∇^*)
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> 创建时间：2017-09-28
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最后更新：2017-09-28
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## 挑战任务
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初始文档`index-start.html`提供了3个尺寸不一的`<div>`元素，本次挑战是一次学习任务，主要了解学习DOM的事件机制，包括事件捕获，事件冒泡，单次触发等。
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## 结果展示
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![结果展示](https://github.com/dashrun/vanilla-javascript-30/blob/master/25%20-%20Event%20Related/effects.png)
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## 相关知识
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[addEventListener文档](https://developer.mozilla.org/zh-CN/docs/Web/API/EventTarget/addEventListener)
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## 过程指南
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1.如下设置时，当点击某个`div`时，自该`div`起以及其外层的`div`也将监听到点击事件，且点击事件只触发一次，当once参数值为false时，点击事件可多次触发。
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```js
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divs.forEach(div => div.addEventListener('click', logText, {
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once: true,
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capture: false
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}));
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```
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2.将`capture`参数设置为`true`后，运行程序后点击内层`div`可以看到，事件的触发顺序为由外向内，即在事件捕获阶段监听到事件。
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```js
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one.addEventListener('click', logText1, {
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capture: true
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});
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two.addEventListener('click', logText2, {
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capture: true
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});
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three.addEventListener('click', logText3, {
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capture: true
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});
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function logText1(e) {
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console.log(this.classList.value);
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// e.stopPropagation();
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}
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function logText2(e) {
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console.log(this.classList.value);
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e.stopPropagation();
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}
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function logText3(e) {
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console.log(this.classList.value);
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//e.stopPropagation();
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}
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```
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3.在事件回调函数中调用`e.stopPropagation()`方法后，即可看到在该处监听到事件后不再继续传递事件、
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## 延伸知识
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由于事件冒泡机制的存在，实际应用中常在父元素来监听众多同类子元素的点击事件，如在`<ul>`元素上监听多个`<li>`元素的点击事件，任何一个`<li>`元素被点击后，父元素都会监听到点击事件，更常用的方法是jQuery中的事件委托机制，感兴趣的小伙伴可以自行了解。
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‎25 - Event Related/effects.png

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‎25 - Event Related/index-finished-Dashrun.html

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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>Understanding JavaScript's Capture</title>
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</head>
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<body class="bod">
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<div class="one">
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<div class="two">
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<div class="three">
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</div>
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</div>
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</div>
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<style>
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html {
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box-sizing: border-box;
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}
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*,
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*:before,
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*:after {
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box-sizing: inherit;
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}
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div {
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width: 100%;
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padding: 100px;
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}
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.one {
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background: thistle;
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}
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.two {
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background: mistyrose;
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}
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.three {
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background: coral;
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}
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</style>
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<script>
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let divs = document.querySelectorAll('div');
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let one = document.querySelector('.one');
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let two = document.querySelector('.two');
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let three = document.querySelector('.three');
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/* divs.forEach(div => div.addEventListener('click', logText, {
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once: false,
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capture: false
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}));*/
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one.addEventListener('click', logText1, {
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capture: true
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});
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two.addEventListener('click', logText2, {
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capture: true
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});
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three.addEventListener('click', logText3, {
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capture: true
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});
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function logText(e) {
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console.log(this.classList.value);
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// e.stopPropagation();
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}
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function logText1(e) {
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console.log(this.classList.value);
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// e.stopPropagation();
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}
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function logText2(e) {
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console.log(this.classList.value);
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e.stopPropagation();
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}
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function logText3(e) {
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console.log(this.classList.value);
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// e.stopPropagation();
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}
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</script>
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</body>
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</html>

‎25 - Event Related/index-start.html

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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>Understanding JavaScript's Capture</title>
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</head>
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<body class="bod">
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<div class="one">
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<div class="two">
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<div class="three">
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</div>
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</div>
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</div>
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<style>
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html {
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box-sizing: border-box;
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}
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*,
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*:before,
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*:after {
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box-sizing: inherit;
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}
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div {
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width: 100%;
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padding: 100px;
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}
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.one {
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background: thistle;
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}
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.two {
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background: mistyrose;
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}
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.three {
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background: coral;
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}
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</style>
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<script>
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let divs = document.querySelectorAll('div');
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let one = document.querySelector('.one');
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let two = document.querySelector('.two');
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let three = document.querySelector('.three');
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divs.forEach(div => div.addEventListener('click', logText, {
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once: true,
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capture: false
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}));
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// one.addEventListener('click', logText1, {
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// // capture: true
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// });
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// two.addEventListener('click', logText2, {
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// // capture: true
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// });
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// three.addEventListener('click', logText3, {
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// capture: true
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// });
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function logText(e) {
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console.log(this.classList.value);
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// e.stopPropagation();
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}
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function logText1(e) {
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console.log(this.classList.value);
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// e.stopPropagation();
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}
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function logText2(e) {
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console.log(this.classList.value);
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// e.stopPropagation();
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}
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function logText3(e) {
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console.log(this.classList.value);
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e.stopPropagation();
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}
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</script>
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</body>
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</html>

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