为什么我可以在javascript中更改常量的值 [英] Why can I change value of a constant in javascript

问题描述

我知道ES6尚未标准化，但目前很多浏览器支持 const 关键字在JS。

在规范中，它写成：

常量的值不能通过重新分配更改，而
常量不能重新声明。因为这样，虽然它是
可能声明一个常量没有初始化它，这将是
无用这样做。

，当我做这样的事情：

  const xxx = 6; 
 xxx = 999; 
 xxx ++; 
 const yyy = []; 
 yyy ='string'; 
 yyy = [15，'a']; 
  

我看到一切都很好 xxx 仍然 6 和 yyy 是 [] 。

但如果我做 yyy.push（6）; yyy.push（1）; ，我的常数数组已被更改。现在它是 [6，1] ，顺便说一句，我还是不能用 yyy = 1;

我这是一个bug，还是我错过了什么？我在最新的Chrome和FF29中尝试过它。

解决方案

文档说明：

...常量不能通过重新分配来更改

...常量不能重新声明

当您添加到数组或对象时，您不会重新分配或重新声明常量，它已被声明和分配，您只需添加到列表中即可

所以这样做很好

  const x = {}; 
 
 x.foo ='bar'; 
 
 console.log（x）; // {foo：'bar'} 
  

这个

  const y = []; 
 
 y.push（'foo'）; 
 
 console.log（y）; // ['foo']; 
  

但这些都不是

  const x = {}; 
 x = {foo：'bar'}; //错误 - 重新分配
 
 const y = ['foo']; 
 const y = ['bar']; //错误 - 重新声明
 
 const foo ='bar'; 
 foo ='bar2'; //错误 - 无法重新分配
 var foo ='bar3'; // error  - 已经声明
 function foo（）{}; //错误 - 已经声明
  

I know that ES6 is not standardized yet, but a lot of browsers currently support const keyword in JS.

In spec, it is written that:

The value of a constant cannot change through re-assignment, and a constant cannot be re-declared. Because of this, although it is possible to declare a constant without initializing it, it would be useless to do so.

and when I do something like this:

const xxx = 6;
xxx = 999;
xxx++;
const yyy = [];
yyy = 'string';
yyy = [15, 'a'];

I see that everything is ok xxx is still 6 and yyy is [].

But if I do yyy.push(6); yyy.push(1);, my constant array has been changed. Right now it is [6, 1] and by the way I still can not change it with yyy = 1;.

I this a bug, or am I missing something? I tried it in the latest chrome and FF29

解决方案

The documentation states :

...constant cannot change through re-assignment
...constant cannot be re-declared

When you're adding to an array or object you're not re-assigning or re-declaring the constant, it's already declared and assigned, you're just adding to the "list" that the constant points to.

So this works fine

const x = {};

x.foo = 'bar';

console.log(x); // {foo : 'bar'}

and this

const y = [];

y.push('foo');

console.log(y); // ['foo'];

but neither of these

const x = {};
x = {foo: 'bar'}; // error - re-assigning

const y = ['foo'];
const y = ['bar']; // error - re-declaring

const foo = 'bar'; 
foo = 'bar2';       // error - can not re-assign
var foo = 'bar3';   // error - already declared
function foo() {};  // error - already declared

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