Javascript 中的 const 关键字范围
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const keyword scope in Javascript
提问by JohnJohnGa
1. >>> const a = 2
2. >>> var a = 3
3. >>> a = 4
4. >>> a // print 2
Why the operation line 3 is allowed? const seems more "global" than without any keyword...
为什么操作线3是允许的？const 似乎比没有任何关键字更“全局”......
采纳答案by JohnJohnGa
This is is just how constworks(or doesn't work):
这是多么const的作品（或不工作）：
Creates a constant1that can be global or local to the function in which it is declared. Constants follow the same scope rules as variables [.. and cannot share a name] with a function or a variable in the same scope.
Firefox [..] throws a TypeError if you redeclare2[which is different than re-assigning] a constant. None of the major browsers produce any notices or errors2,3if you assignanother value to a constant[..] but the reassignment is unsuccessful (only) in Firefox and Chrome (at least since version 20).
创建一个常量1，它可以是声明它的函数的全局或局部的。常量遵循与变量相同的范围规则 [.. 并且不能与同一范围内的函数或变量共享名称]。
如果您重新声明2[不同于重新分配] 常量，Firefox [..] 会抛出 TypeError 。如果您为常量[..]分配另一个值，则任何主要浏览器都不会产生任何通知或错误2,3但重新分配在 Firefox 和 Chrome 中（仅）不成功（至少从版本 20 开始）。
Note that constis notpart of the ECMAScript 5 specification and the JavaScript 1.5 semantics will be re-definedin ECMAScript 6.
请注意，const它不是ECMAScript 5 规范的一部分，JavaScript 1.5 语义将在 ECMAScript 6 中重新定义。
Behavior willvary across browser implementations with respect to support and re-declaration/re-assignments semantics.
在支持和重新声明/重新分配语义方面，行为将因浏览器实现而异。
1In IE 9, using const a = 2results in
1在 IE 9 中，使用const a = 2结果
"Syntax error"
“语法错误”
2In FF 14, const a = 2; var a = 3; a = 4; a, when evaluated as a single program, results in
2在 FF 14 中const a = 2; var a = 3; a = 4; a，当作为单个程序进行评估时，结果为
TypeError: redeclaration of const a
类型错误：重新声明 const a
which is differentthan executing each line one-at-a-time in the REPL. I suspectthis is because varis hoisted abovethe constand because a const "cannot share a name with a function or variable in the same scope".
这与在 REPL 中一次执行每一行不同。我怀疑这是因为它var被挂在了上面，const并且因为一个常量“不能与同一范围内的函数或变量共享一个名称”。
3In Chrome 21, const a = 2; var a = 3; a = 4; aevaluates to 2 with no warning or message.
3在 Chrome 21 中，const a = 2; var a = 3; a = 4; a评估为 2，没有警告或消息。
回答by Mercury
constscope is defined as 'block scoped'(the scope of which, is restricted to the block in which it is declared).
const范围被定义为“块范围”（其范围仅限于声明它的块）。
Constants are block-scoped, much like variables defined using the let statement. The value of a constant cannot change through re-assignment, and it can't be redeclared.
常量是块作用域的，很像使用 let 语句定义的变量。常量的值不能通过重新赋值而改变，也不能重新声明。
Regarding your specific issue:
First as comments said constis relevant in ES6. I don't know about you but i get (typing your line 2: var a = 3;): SyntaxError: Identifier 'a' has already been declared
so your example is not quite possible.
关于您的具体问题：首先，正如评论所说const，在 ES6 中是相关的。我不了解你，但我得到（输入你的第 2 行var a = 3;：）：SyntaxError：标识符 'a' 已经被声明，所以你的例子不太可能。
