manual-javascript
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JavaScript 是一种具有函数优先的轻量级、解释型、即时编译型的编程语言。
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# 事件监听
执行下面的代码，监听元素的点击事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<button>Click</button>
<script type="text/javascript">
const btn = document.querySelector('button');
btn.addEventListener('click', function() {
console.log('Hello World!');
});
</script>
</body>
</html>
```
还可以为同一个元素绑定多个相同类型的监听事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<button>Click</button>
<script type="text/javascript">
const btn = document.querySelector('button');
btn.addEventListener('click', function() {
console.log('Hello World!');
});
btn.addEventListener('click', function() {
console.log('Hello JavaScript!');
});
</script>
</body>
</html>
```
执行下面的代码，监听元素的鼠标移动事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
<style type="text/css">
.box {
width: 500px;
height: 500px;
background-color: red;
}
</style>
</head>
<body>
<div class="box">Hello World!</div>
<script type="text/javascript">
const box = document.querySelector('.box');
box.addEventListener('mouseenter', function() {
console.log('enter');
});
box.addEventListener('mouseleave', function() {
console.log('leave');
});
</script>
</body>
</html>
```
执行下面的代码，监听表单的输入操作事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<form action="#" method="post">
<input type="text" name="username">
</form>
<script type="text/javascript">
const input = document.querySelector('input');
input.addEventListener('keydown', function() {
console.log('down');
});
input.addEventListener('keyup', function(e) {
console.log('up');
console.log(e.key);
});
</script>
</body>
</html>
```
事件冒泡是指当一个元素的事件被触发时，同样的事件将会在该元素的所有祖先元素中依次被触发。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<div class="box1">
<div class="box2">
<div class="box3">Hello World!</div>
</div>
</div>
<script type="text/javascript">
const box1 = document.querySelector('.box1');
box1.addEventListener('click', function() {
console.log('box1');
});
const box2 = document.querySelector('.box2');
box2.addEventListener('click', function() {
console.log('box2');
});
const box3 = document.querySelector('.box3');
box3.addEventListener('click', function() {
console.log('box3');
});
</script>
</body>
</html>
```
执行下面的代码，阻止冒泡事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<div class="box1">
<div class="box2">
<div class="box3">Hello World!</div>
</div>
</div>
<script type="text/javascript">
const box1 = document.querySelector('.box1');
box1.addEventListener('click', function() {
console.log('box1');
});
const box2 = document.querySelector('.box2');
box2.addEventListener('click', function() {
console.log('box2');
});
const box3 = document.querySelector('.box3');
box3.addEventListener('click', function(e) {
console.log('box3');
e.stopPropagation();
});
</script>
</body>
</html>
```
执行下面的代码，利用事件冒泡实现事件委托。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<ul>
<li>1</li>
<li>2</li>
<li>3</li>
<p>Hello World!</p>
</ul>
<script type="text/javascript">
const ul = document.querySelector('ul');
ul.addEventListener('click', function(e) {
if (e.target.tagName === 'LI') {
e.target.style.color = 'red';
}
});
</script>
</body>
</html>
```
执行下面的代码，阻止元素的默认行为。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<form action="#" method="post">
<input type="text" name="username">
</form>
<a href="#">Click</a>
<script type="text/javascript">
const form = document.querySelector('form');
form.addEventListener('submit', function(e) {
e.preventDefault();
});
const a = document.querySelector('a');
a.addEventListener('click', function(e) {
e.preventDefault();
});
</script>
</body>
</html>
```
执行下面的代码，监听页面加载事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<script type="text/javascript">
window.addEventListener('load', function() {
console.log('Hello JavaScript!');
});
document.addEventListener('DOMContentLoaded', function() {
console.log('Hello World!');
});
</script>
</body>
</html>
```
执行下面的代码，监听页面滚动事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
<style>
.box {
height: 5000px;
}
</style>
</head>
<body>
<div class="box"></div>
<script type="text/javascript">
window.addEventListener('scroll', function() {
console.log('scroll');
});
</script>
</body>
</html>
```
执行下面的代码，监听窗口尺寸改变事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<script type="text/javascript">
window.addEventListener('resize', function() {
console.log('resize');
});
</script>
</body>
</html>
```
执行下面的代码，监听移动端事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<script type="text/javascript">
document.addEventListener('touchstart', function() {
console.log('touchstart');
});
document.addEventListener('touchmove', function() {
console.log('touchmove');
});
document.addEventListener('touchend', function() {
console.log('touchend');
});
</script>
</body>
</html>
```
执行下面的代码，解绑元素的监听事件。
```html
<html lang="zh-Hans">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript</title>
</head>
<body>
<button>Click</button>
<script type="text/javascript">
function func() {
console.log('Hello World!');
}
const btn = document.querySelector('button');
btn.addEventListener('click', func);
btn.removeEventListener('click', func);
</script>
</body>
</html>
```