除了 JS 已有的功能外，Vue还具有自己独特的作用域级别。作用域控制可以使用哪些变量以及在何处使用。它控制它们对应用程序的不同部分的“可见性”。
了解 Vue 提供的作用域级别之间的差异会帮助我们编写更清晰的代码。
下面是 vue 中4个级别的作用域：
- 全局作用域
- 子树作用域
- 组件作用域
- 实例作用域
来看看这些作用域分别是什么。
全局作用域
Vue 应用程序中的全局作用域与任何编程语言中的全局作用域类似，这些变量在应用程序中的任何地方都是可用的。
可以把全局作用域看作应用程序作用域，因为它将作用域限制为整个应用程序。
使用全局作用域
使用全局作用域的主要方式是向Vue原型添加一个值：
Vue.prototype.$globalValue = 'Global Scope!';
通过将其添加到 Vue 对象的原型中，可以在应用程序的每个组件上自动使用它。可以像这样直接从组件访问它：
export default {
mounted() {
console.log(this.$globalValue); // 'Global Scope!'
},
};使用$作为这些变量的前缀是一种标准做法，因此我们知道它们是全局值，而不是每个组件唯一的。
Vue、vue-router和vuex(以及许多其他库)都使用这种技术来定义全局值。例如，来自vue-router的$route对象是这样一个全局作用域的变量。
子树作用域
大多数时候，全局范围有点像大锤，我们需要一些更精确的东西。子树作用域中的变量作用域是应用程序的特定部分，而不是整个应用程序。
此级别的作用域可能是最少使用的，但是在确实需要使用时非常方便。 通常，一组组件需要共享很多相同的数据，并且通过props传递数据非常繁琐。
此作用域最适合用于共享上下文信息，这些上下文信息可以根据组件在应用程序中的位置进行更改，可以是这样的：
本地状态和数据-如果只有一小部分应用程序需要使用 Vuex，则无需使用 Vuex。 如果使用props传递会变得繁琐且麻烦，此时，子树作用域可能是我们最好的选择。
-
配置-有时我们需要组件以某种特定方式运行，但仅在应用程序的一部分中运行。 例如，注册表单中的所有
Input组件都需要验证，但是我们不想在整个应用程序中都要求验证。
使用子树作用域
子树作用域是通过使用provide和inject创建的。 我们提供希望可用于整个子树的值，然后将它们注入需要它们的组件中。
组件作用域
更具体一点，组件作用域使变量可用于单个组件。但是这不应该与更具体的实例作用域相混淆。
如果一个变量具有组件作用域，那么它就是一个组件的所有实例都可以使用的单个变量。我们可以拥有几个相同的组件，并且它们都能够访问相同的变量。
你可能熟悉 JS 中的模块作用域。在单个模块或文件中定义的任何内容都属于相同的模块作用域。由于组件是在单个文件中定义的，所以组件中的所有内容都在相同的模块作用域内。
使用组件作用域
要使用组件作用域，我们需要在与组件相同的文件中定义一个变量：
{{ componentScope }}<script>const componentScope = 'This is in the component scope'; export default { data() { return { componentScope: componentScope, }; }, };</script>" title="" data-original-title="复制">
<template>
<div class="new-component">
{{ componentScope }}
</div>
</template>
<script>
const componentScope = 'This is in the component scope';
export default {
data() {
return {
componentScope: componentScope,
};
},
};
</script>在此组件中渲染的变量componentScope是相同的变量，不管使用此组件多少次，componentScope 始终只有一个，没有多个副本。
如果此组件的一个实例修改了componentScope的值，则该组件的每个其他实例都会更改。 不应使用这种方式在组件之间进行通信，但这是共享数据的一种好方法。
实例作用域
实例作用域是我们可以获得的Vue作用域变量最常见形式，具有实例作用域的任何变量仅可用于组件的特定用法。 我们通常将其称为内部状态，有时也称为局部状态。
使用实例作用域
无论何时使用data()函数或使用computed props，都是在使用实例作用域。
甚至直接向组件实例添加属性也可以达到以下效果：
someMethod() {
this.newProperty = 'Instance scope';
}但是，如果以这种方式添加属性，则必须记住，它们不是响应式的。
英文原文地址：https://frontendsociety.com/why-you-shouldnt-use-vue-component-ff019fbcac2e?gi=ce7b162d53d4
为了保证的可读性，本文采用意译而非直译。
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