声明和定义的变量之间是否有任何区别 [英] Is there any difference between declared and defined variable

问题描述

我尝试逐一在控制台中编写以下行

I try writing the following lines in the console one by one

let x = y //throws error "Uncaught ReferenceError: y is not defined"
console.log(x) //throws error "ReferenceError: x is not defined"
let x = 3; //gives error "Uncaught SyntaxError: Identifier 'x' has already been declared"
x = 3 //ReferenceError: x is not defined

现在的问题是，如何变量未定义且同时声明。两者之间有什么区别吗？

Now problem is that how can be a variable not defined and has been declared at the same time. Is there any difference between both.

推荐答案

A 让或 const 变量只能声明一次 - 也就是说，当您在范围内有 let< variableName> 时，您已声明该范围内的< variableName> ，并且无法在该范围内再次声明。

A let or const variable can only be declared once - that is, when you have let <variableName> in a scope, you have declared <variableName> in that scope, and cannot declare it again in that scope.

来自先前关联的问题：

当有分配时，首先解析右侧;如果右侧抛出错误，它永远不会到达左侧，并且使用let声明的变量永远不会被正确初始化;它将永远停留在非军事区/临时死区

When there's assignment, the right-hand side is parsed first; if the right-hand side throws an error, it never gets to the left-hand side, and the variable declared with let never gets properly initialized; it'll stay in the demilitarized zone / temporal dead zone forever

你不能重新声明已经声明的变量，即使在初始化期间尝试的分配引发了错误。

You can't re-declare a variable that's already been declared, even though the attempted assignment during initialization threw an error.

但是在第4行，x = 3应该进行正确的分配，它应该删除来自TDZ的x。但那也失败了。我无法理解

But on line 4, x=3 should do a proper assignment and it should remove x from TDZ. But that also fails. I fail to understand that

变量初始化后（例如，让x 运行），可以将其分配给。但就像你无法在 c c> c>初始化之前分配变量一样，你也无法在以后的初始化时分配给变量没有成功完成：

After a variable has been initialized (for example, the let x runs), it can be assigned to. But just like you can't assign to a variable before its let initialization, you also can't assign to a variable later, when its initialization did not complete successfully:

x = 'foo';
let x = 'bar';

错误：

未捕获的ReferenceError：x未定义

Uncaught ReferenceError: x is not defined

这是同类的当您尝试时在控制台中发生的事情：

Which is the same sort of thing that happens in the console when you try:

let x = y
// Uncaught ReferenceError: y is not defined
// x has not been initialized, so the next line throws:
x = 'foo'
// Uncaught ReferenceError: x is not defined

x 仍未初始化，因此错误相同。

x still has not been initialized, so the error is the same.

遇到这种事情的是非常奇怪，但是 - 你只能在控制台中看到它。在普通脚本中，抛出的错误将阻止进一步执行，并且变量名称永远未被初始化的事实不值得担心。

Encountering this sort of thing is pretty odd, though - you only see it in the console. In normal scripts, a thrown error will prevent further execution, and the fact that a variable name remains uninitialized forever is not something to worry about.

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