TS为什么在函数体内抱怨函数声明 [英] Why TS complains with function declarations inside function body

问题描述

我在TS中遇到此错误：

很清楚为什么会发生错误：

  function external（）{
 
 if（true ）{
 function inner（）{//嵌套函数声明
 
} 
} 
} 
  

但是我的问题是-为什么TS会抱怨这一点-在转换为ES5时应该避免嵌套函数声明的某些技术原因吗？

函数表达式会是更好的选择，为什么？

解决方案

< blockquote>

将函数表达式作为更好的选择

是。以下是处理方法：

  function external（）{
 if（true）{
 const inner = function（）{// OK 
} 
} 
} 
  

为什么？

• ES模块处于严格模式默认。


• 严格模式不允许在块中声明函数

原因被禁止在原始JavaScript规范中。简短版本：实施之间的行为不一致。

注意已知有几种广泛使用的ECMAScript实现支持使用FunctionDeclaration作为语句。但是，在应用于此类FunctionDeclaration的语义中，实现之间存在重大且不可调和的变化。由于存在这些无法协调的差异，因此将FunctionDeclaration作为Statement的使用会导致代码无法在实现之间可靠地移植。建议ECMAScript实现禁止使用FunctionDeclaration或在遇到这种情况时发出警告。 ECMAScript的未来版本可能会定义用于在Statement上下文中声明函数的其他可移植方法。

因此，严格模式应运而生（ES5）

I have this error from TS:

It's pretty clear why the error occurs:

function outer(){

   if (true) {
        function inner(){    // nested function declaration

       }
   }   
}

but my question is - why does TS complain about that - is there some technical reason I should avoid nested function declarations when transpiling to ES5?

Would a function expression be a better choice, and why?

解决方案

Would a function expression be a better choice

Yes. The following is the way to go:

function outer() {
  if (true) {
    const inner = function() { // OK
    }
  }
}

Why?

• ES modules are in strict mode by default.
• strict mode does not allow function declarations in blocks

Reason why it was disallowed is covered in the original JavaScript specification. Short version: The behaviour was inconsistent between implementations.

NOTE Several widely used implementations of ECMAScript are known to support the use of FunctionDeclaration as a Statement. However there are significant and irreconcilable variations among the implementations in the semantics applied to such FunctionDeclarations. Because of these irreconcilable differences, the use of a FunctionDeclaration as a Statement results in code that is not reliably portable among implementations. It is recommended that ECMAScript implementations either disallow this usage of FunctionDeclaration or issue a warning when such a usage is encountered. Future editions of ECMAScript may define alternative portable means for declaring functions in a Statement context.

So when strict mode came into being (ES5) it made it disallowed.

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