web前端面试高频考点——Vue3.x深入理解（v-model参数用法、watch和watchEffect区别、Vue3快于Vue2、Vite启动快的原因）

<text-document
  v-bind:title="doc.title"
  v-on:update:title="doc.title = $event"
>
</text-document>

<ChildComponent v-model:title="pageTitle"></ChildComponent>
<!-- 是以下的简写 -->
<ChildComponent :title="pageTitle" @update:title="pageTitle = $event"></ChildComponent>

<template>
  <p>{{ name }} {{ age }}</p>
  <UserInfo
    v-model:name="name"
    v-model:age="age"
  >
  </UserInfo>
</template>
<script>
import { reactive, toRefs } from 'vue'
import UserInfo from './UserInfo.vue';
export default {
    name: 'VModel',
    components: {UserInfo},
    setup() {
        const state = reactive({
            name: '杂货铺',
            age: '20'
        })
        return toRefs(state)
    }
};
</script>

<template>
  <input :value="name" @input="$emit('update:name', $event.target.value)">
  <input :value="age" @input="$emit('update:age', $event.target.value)">
</template>
<script>
export default {
    name: 'UserInfo',
    props: {
        name: String,
        age: String
    }
};
</script>

<template>
  <p>watch vs watchEffect</p>
  <p>{{ numberRef }}</p>
</template>
<script>
import { ref, toRefs, watch} from 'vue'
export default {
    name: 'Watch',
    setup() {
      // 定义值类型
        const numberRef = ref(100)
    // watch 监听
        watch(numberRef, (newNumber, oldNumber) => {
            console.log('ref watch', newNumber, oldNumber);
        })
    // 定时器，修改 numberRef 的值，用于监听
        setTimeout(() => {
            numberRef.value = 200
        }, 1000)
        return {
            numberRef,
        }
    }
};
</script>

<template>
  <p>watch vs watchEffect</p>
  <p>{{ name }} {{ age }}</p>
</template>
<script>
import { reactive, toRefs, watch} from 'vue'
export default {
    name: 'Watch',
    setup() {
        const state = reactive({
            name: '杂货铺',
            age: 21
        })
        watch(
            // 第一个参数，确定要监听哪个属性
            () => state.age,
            // 第二个参数，回调函数
            (newAge, oldAge) => {
                console.log('state watch', newAge, oldAge);
            },
            // 第三个参数，配置项，可选
            {
                immediate: true, // 初始换之前就监听，可选
                deep: true // 深度监听
            }
        )
        setTimeout(() => {
            state.age = 25
        }, 3000)
        setTimeout(() => {
            state.name = '前端杂货铺'
        }, 5000)
        return {
            ...toRefs(state)
        }
    }
};
</script>

<template>
  <p>watch vs watchEffect</p>
  <p>{{ name }} {{ age }}</p>
</template>
<script>
import { reactive, toRefs, watchEffect} from 'vue'
export default {
    name: 'Watch',
    setup() {
        const state = reactive({
            name: '杂货铺',
            age: 21
        })
        // 初始化时，一定会执行一次（收集要监听的数据）
        watchEffect(() => {
            console.log('hello watchEffect');
        })
        // 监听 state.name
        watchEffect(() => {
            console.log('state.name', state.name);
        })
        // 监听 state.age
        watchEffect(() => {
            console.log('state.age', state.age);
        })
        setTimeout(() => {
            state.age = 25
        }, 1500)
        setTimeout(() => {
            state.name = '前端杂货铺'
        }, 3000)
        return {
            ...toRefs(state)
        }
    }
};
</script>

<template>
  <p>setup中获取组件实例</p>
</template>
<script>
import { getCurrentInstance, onMounted } from 'vue';
export default {
    name: 'GetInstance',
    data() {
        return {
            x: 1
        }
    },
    setup() {
        // Composition API 没有 this => undefined
        console.log('this1', this);
        // 获取当前实例
        const instance = getCurrentInstance()
        console.log('instance', instance);
        // 挂载完成 => 通过 instance.data.x 获取 x 的值
        onMounted(() => {
          // 挂载完也没有 this => undefined
            console.log('this in onMounted', this);
            console.log('x', instance.data.x);
        })
    },
    // vue2.x 中有 this，能直接取到值
    mounted() {
        console.log('this2', this);
        console.log('x', this.x);
    }
};
</script>

export default function (a, b) {
    console.log(a, b)
}

import print from './print.js'
export default function add(a, b) {
    print('print', 'add')
    return a + b
}

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>ES Module demo</title>
</head>
<body>
    <p>基本演示</p>
    <script type="module">
        import add from './src/add.js'
        const res = add(1, 2)
        console.log('add res', res)
    </script>
</body>
</html>