watch：可监听值的变化，旧值和新值
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<div id="app">
爱好
<select v-model="hobby">
<option value="">请选择</option>
<option value="reading">阅读</option>
<option value="swimming">游泳</option>
<option value="running">跑步</option>
</select>
<hr>
年
<select v-model="date.year">
<option value="">请选择</option>
<option value="2021">2021</option>
<option value="2022">2022</option>
<option value="2023">2023</option>
<option value="2024">2024</option>
</select>
月
<select v-model="date.month">
<option value="">请选择</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
</select>
</div>
<script type="module">
import { createApp, reactive, ref, watch } from './vue.esm-browser.js'
createApp({
setup() {
const hobby = ref("")
const date = reactive({
year: "",
month: ""
})
//监听hobby变化
watch(hobby, (newVal, oldVal) => {
console.log("oldVal ：", oldVal, "newVal ：", newVal)
})
//监听某个值，如date.year
watch(() => date.year, (newVal, oldVal) => {
console.log("oldVal ：", oldVal, "newVal ：", newVal)
})
//监听date变化
//发现date变化时，新旧值一样
//因为数组和对象通过引用传递，而不是创建一个新的对象或数组
//newVal, oldVal是指向同一个对象，所以变化时，oldVal, newVal是一样的
watch(date, (newVal, oldVal) => {
console.log("oldVal ：", oldVal, "newVal ：", newVal)
})
return {
date,
hobby
}
}
}).mount("#app")
</script>
</body>
</html>
参考
https://www.bilibili.com/video/BV1nV411Q7RX
