快速提示：如何在 JavaScript 中声明变量

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学习 JavaScript 时，基础知识之一是了解如何使用变量。变量是所有可能类型的值的容器，例如数字、字符串或数组（请参阅数据类型）。每个变量都有一个名称，稍后可以在应用程序中使用该名称（例如读取其值）。 Play Next Unmute Current Time 0:05 / Duration 2:00 Fullscreen Backward Skip 10s Play Video Forward Skip 10s 在本快速提示中，您将了解如何使用变量以及各种声明之间的差异。 声明、初始化和赋值之间的区别 在我们开始学习各种声明之前，让我们先看看变量的生命周期。 变量生命周期流程图 声明：变量在相应范围内使用给定名称进行注册（如下所述 - 例如在函数内部）。 初始化：当你声明一个变量时，它会自动初始化，这意味着 JavaScript 引擎为该变量分配内存。 赋值：这是为变量分配特定值的情况。 Learn to Code with JavaScript 声明类型 注意： whilevar自最初发布以来就在 JavaScript 中可用，let并且const仅在 ES6 (ES2015) 及更高版本中可用。请参阅此页面了解浏览器兼容性。


变量 句法这个声明可能是最流行的，因为在ECMAScript 6之前没有其他选择。用 声明的变量var在封闭函数的范围内可用。如果没有封闭函数，则它们在全局范围内可用。 例子这将导致错误ReferenceError: hello is not defined，因为该变量hello仅在函数内可用sayHello。但以 富人人数数据 下内容将起作用，因为变量将在全局范围内声明 - 位于同一范围内常量 句法： Learn to Code with JavaScript const x = "Hello World"; 从技术上讲，常量不是变量。常量的特殊性在于，声明时需要赋值，并且无法重新赋值。 Aconst仅限于封闭块的范围，例如let. 只要值在应用程序运行期间不得更改，就应该使用常量，因为在尝试覆盖它们时会收到错误通知。 意外的全球创造 您可以在全局上下文中（即在任何函数之外）编写所有上述命名声明，但即使在函数内，如果您忘记编写var,let或const在赋值之前，该变量将自动成为全局变量。 例子 上面的内容将输出Hello World到控制台，因为赋值之前没有声明hello ，因此该变量是全局可用的。




注意：为了避免意外声明全局变量，您可以使用严格模式。 提升和颞死区 Learn to Code with JavaScript var和let/之间的另一个区别与变量提升const有关。变量声明将始终在内部提升（移动）到当前作用域的顶部。这意味着以下内容 此行为的一个迹象是两个示例都将记录undefined到控制台。如果var hello;不总是在顶部，它会抛出一个ReferenceError. 这种称为提升的行为适用于/var也适用于let/ const。如上所述，var在声明之前访问变量将返回undefined，因为这是 JavaScript 在初始化变量时分配的值。 但是在声明之前访问let/const变量将会引发错误。这是因为在代码中声明之前无法访问它们。进入变量的作用域和到达其声明之间的时间段称为临时死区，即变量不可访问的时间段。 您可以在揭秘 JavaScript 变量作用域和提升一文中阅读有关提升的更多信息。 结论 为了降低对错误的敏感性，您应该尽可能使用constand 。let如果您确实需要使用，var请务必将声明移至作用域的顶部，因为这可以避免与提升相关的不良行为。

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