为什么这段代码不会导致ReferenceError？ [英] Why does this code not result in a ReferenceError?

问题描述

if(true) {
  tmp = 'abc';
  console.log(tmp);//which should throw referenceError but not

  let tmp;
  console.log(tmp);

  tmp = 123;
  console.log(tmp);
}

此代码产生

abc
undefined
123

为什么第一个console.log（tmp）没有抛出错误？

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为什么它应抛出referenceError

Why does the first console.log(tmp) not throw an error?

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why it should throw a referenceError

在ECMAScript 2015中，让我们将变量提升到块的顶部。但是，在变量声明之前引用块中的变量会导致ReferenceError。该变量位于从块开始到处理声明的暂时死区中。

In ECMAScript 2015, let will hoist the variable to the top of the block. However, referencing the variable in the block before the variable declaration results in a ReferenceError. The variable is in a "temporal dead zone" from the start of the block until the declaration is processed.

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$ b我想，问题是bable设置。

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所以，也许这是一个巴贝尔的错误？
https://github.com/babel/babel.github.io / issues / 826

推荐答案

你是对的，在ES6中这确实会引发异常。它不适合你的原因有两个：

You are correct, in ES6 this does throw an exception. There's two reasons why it doesn't for you:

• node.js已经实现让 - 但它只能在严格模式下正常工作。您应该使用它。


• babel默认情况下似乎不会转换TDZ，因为它非常复杂并导致冗长的代码。但是，您可以使用 es6.blockScopingTDZ / es6.spec.blockScoping 选项启用它（但我不确定这是否仅适用于Babel 5以及Babel 6中发生的事情。）

• node.js already implemented let - but it works correctly only in strict mode. You should use it.
• babel does not appear to transpile the TDZ by default, as it is quite complicated and leads to lengthy code. You can however enable it with the es6.blockScopingTDZ/es6.spec.blockScoping option (but I'm not sure whether this worked in Babel 5 only and what happened in Babel 6 to them).

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