<div id="app">
    <post-item :post-title="title"></post-item>
</div>

<script src="https://unpkg.com/vue@next"></script>
<script>
const app = Vue.createApp({
    data() {
        return {
            title: 'Java无难事'
        }
    }
});

app. component('PostItem', {
    props: ['PostTitle'],
    setup(props) {
        Vue.watchEffect(() => {
            console.log(props.postTitle);
        })
    },
    template: '<h3>{{ postTitle }}</h3>'
});

const vm = app.mount('#app')
</script>

老是有小错误啊

<div id="app">
    <post-item :post-title="title"></post-item>
</div>

<script src="https://unpkg.com/vue@next"></script>
<script>
const app = Vue.createApp({
    data() {
        return {
            title: 'Java无难事'
        }
    }
});

app.component('PostItem', {
    props: ['postTitle'],
    setup(props) {
        Vue.watchEffect(() => {
            console.log(props.postTitle);
        })
    },
    template: '<h3>{{ postTitle }}</h3>'
});

const vm = app.mount('#app')
</script>