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data为何以函数形式返回.md

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Why does data return as a function?

When building component-based applications with Vue, the data property of each component is returned as a function. This is mainly because in the implementation of componentization, each instance can maintain an independent copy of the returned object, rather than sharing the reference to the same object.

Simple Vue Instance

In a simple Vue instance, when not using componentization, data can be an object since there is only one instance and no issue with multiple instances sharing the same data.

<!DOCTYPE html>
<html>
<head>
    <title>Vue</title>
</head>
<body>
    <div id="app">
        <div>{{msg}}</div>
    </div>
</body>
<script src="https://cdn.bootcss.com/vue/2.4.2/vue.js"></script>
<script type="text/javascript">
    var vm = new Vue({
        el: '#app',
        data: {
            msg: 'Vue Instance'
        }
    })
</script>
</html>

Component-based Instance

If using Vue with component-based instances, the data property must be returned as a function. If not using a function to return data, unexpected situations may occur. For example, in the following example, the button component is being reused. When clicking the first button, it should only increment the count for that specific button, but all buttons end up getting incremented. It's important to note that using a function to return the data is still necessary in this case, as without it Vue would throw an error in componentization. Even though the data is returned as a function, the count property within the returned object still points to the reference of the counter object. Essentially, it still leads to multiple component instances sharing one object, producing the same effect as directly returning an object.

<!DOCTYPE html>
<html>
<head>
    <title>Vue</title>
</head>
<body>
    <div id="app">
        <button-counter></button-counter>
        <button-counter></button-counter>
        <button-counter></button-counter>
    </div>
</body>
<script src="https://cdn.bootcss.com/vue/2.4.2/vue.js"></script>
<script type="text/javascript">
    var counter = {
        count: 0
    }
    Vue.component('button-counter', {
      data: function(){
          return counter;
      },
      template: '<button v-on:click="count++">You clicked me {{ count }} times.</button>'
    })
    var vm = new Vue({
        el: '#app'
    })
</script>
</html>

To prevent this phenomenon, in the implementation of componentization, the data property must be returned as a function so that each instance can maintain an independent copy of the returned object, rather than sharing the reference to the same object.

<!DOCTYPE html>
<html>
<head>
    <title>Vue</title>
</head>
<body>
    <div id="app">
        <button-counter></button-counter>
        <button-counter></button-counter>
        <button-counter></button-counter>
    </div>
</body>
<script src="https://cdn.bootcss.com/vue/2.4.2/vue.js"></script>
<script type="text/javascript">
    Vue.component('button-counter', {
      data: function(){
          return {
            count: 0
          }
      },
      template: '<button v-on:click="count++">You clicked me {{ count }} times.</button>'
    })
    var vm = new Vue({
        el: '#app'
    })
</script>
</html>

Daily Question

https://github.com/WindrunnerMax/EveryDay

References

https://segmentfault.com/a/1190000019318483
https://cn.vuejs.org/v2/guide/components.html
https://blog.csdn.net/fengjingyu168/article/details/72900624