Question

为什么以下代码会出现错误 TypeError

我正在阅读You-Dont-Know-JS一书中的javascript。它应该显示＆＃39; b＆＃39;如书中所示。

https://github.com/getify/You-Dont-Know-JS/blob/master/scope%20%26%20closures/ch4.md

<body>
 <script>
  foo(); // "b"
  var a = true;
  if (a) {
     function foo() { console.log( "b" ); }
   }
  else {
     function foo() { console.log( "b" ); }
   }

  </script>
</body>

Answer 1

简短版本：YDKJS似乎已经过时了。在ES6中，块内功能的行为发生了变化。在ES6中，这些函数的作用域是它们的封闭块，就好像它们是用{ "doc" : "test_doc", "docString" : "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" }声明的一样。在ES6之前，块内的功能甚至都不合法。

更长的版本：

除了在作用域的顶层以外，函数声明曾经是非法的。

在ES5中，语句包括var语句，if语句，return语句......但不包括函数声明。相反，有一个“源元素”，它是一个语句或一个函数声明，程序本身和函数体被定义为0个或多个源元素。因此，函数声明可以出现在程序的顶层，也可以出现在另一个函数的顶层，但不能出现在块内（如果块，块等）。

与ES6及更高版本相比，函数声明在技术上仍然不是声明，但它们可以出现在声明列表中，因为声明列表项是声明或声明和块内的函数被认为是词法（不是可提升的）声明。

词汇范围和javascript中的提升

1 个答案: