JavaScript 中的 const:何时使用它，是否有必要?

2022-01-23 00:00:00 node.js immutability constants javascript

我最近遇到了 JavaScript 中的 const 关键字.据我所知，它用于创建 不可变变量，以及我已经测试以确保它不能被重新定义(在 Node.js 中):

I've recently come across the const keyword in JavaScript. From what I can tell, it is used to create immutable variables, and I've tested to ensure that it cannot be redefined (in Node.js):

const x = 'const';
const x = 'not-const';

// Will give an error: 'constant 'x' has already been defined'

我意识到它尚未在所有浏览器中标准化 - 但我只对 Node.js V8，我注意到某些 var 关键字可以用于相同的效果时，" rel="noreferrer">开发人员/项目 似乎非常喜欢它.

I realise that it is not yet standardized across all browsers - but I'm only interested in the context of Node.js V8, and I've noticed that certain developers / projects seem to favor it heavily when the var keyword could be used to the same effect.

  • 什么时候适合使用 const 代替 var?
  • 是否应该每次都使用一个不会出现的变量re-assigned 被声明了吗?
  • 如果使用 varconst 还是相反?
  • When is it appropriate to use const in place of var?
  • Should it be used every time a variable which is not going to be re-assigned is declared?
  • Does it actually make any difference if var is used in place of const or vice-versa?

推荐答案

您的问题有两个方面:使用 const 代替 var 的技术方面是什么以及这样做与人类相关的方面是什么.

There are two aspects to your questions: what are the technical aspects of using const instead of var and what are the human-related aspects of doing so.

技术差异很大.在编译语言中，常量将在编译时被替换，并且它的使用将允许其他优化，例如删除死代码，以进一步提高代码的运行时效率.最近(广泛使用的术语)JavaScript 引擎实际上编译 JS 代码以获得更好的性能，因此使用 const 关键字会告知它们上述优化是可能的并且应该进行.这会带来更好的性能.

The technical difference is significant. In compiled languages, a constant will be replaced at compile-time and its use will allow for other optimizations like dead code removal to further increase the runtime efficiency of the code. Recent (loosely used term) JavaScript engines actually compile JS code to get better performance, so using the const keyword would inform them that the optimizations described above are possible and should be done. This results in better performance.

与人相关的方面是关于关键字的语义.变量是包含预期会改变的信息的数据结构.常量是包含永远不会改变的信息的数据结构.如果有出错的余地，则应始终使用 var.然而，并不是所有在程序生命周期中永远不会改变的信息都需要用 const 声明.如果在不同的情况下信息应该改变，使用 var 来表示，即使实际的改变没有出现在你的代码中.

The human-related aspect is about the semantics of the keyword. A variable is a data structure that contains information that is expected to change. A constant is a data structure that contains information that will never change. If there is room for error, var should always be used. However, not all information that never changes in the lifetime of a program needs to be declared with const. If under different circumstances the information should change, use var to indicate that, even if the actual change doesn't appear in your code.

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