JavaScript中的常量:什么时候使用它，有必要吗? [英] Const in JavaScript: when to use it and is it necessary?

问题描述

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 ，我已进行测试以确保无法重新定义它(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-但是我只对开发人员/项目在可以使用var关键字达到相同效果时似乎非常喜欢它.

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.

• 何时使用const代替var?
• 是否应在每次将不会成为变量的变量时使用 重新分配的声明?
• 如果使用var代替，实际上有什么区别吗? const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?进行操作时与人相关的方面是什么?如此.

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，常量将在编译时替换，并且其使用将允许进行其他优化(例如删除无效代码)，以进一步提高代码的运行效率.最近的(松散使用的术语)JavaScript引擎实际上会编译JS代码以获得更好的性能，因此使用const关键字将告知它们上述优化是可行的，应该完成.这样可以带来更好的性能.

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.

与人有关的方面与关键字的语义有关.变量是一种数据结构，其中包含预期会更改的信息.常数是一种数据结构，其中包含永远不变的信息.如果有错误的余地，应始终使用var.但是，并非必须使用const声明程序生命周期中从未改变过的所有信息.如果在不同情况下信息应该更改，则即使实际更改未出现在代码中，也请使用var进行指示.

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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