javascript - 为什么 TS 提示函数体内的函数声明

Question

我从 TS 收到这个错误:

错误发生的原因很清楚:

function outer(){

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

       }
   }   
}

但我的问题是 - 为什么 TS 会提示 - 是否有一些技术原因我应该在转译到 ES5 时避免嵌套函数声明？

函数表达式会是更好的选择吗？为什么？

Answer 1

Would a function expression be a better choice

是的。以下是要走的路:

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

Why?

• ES 模块默认处于严格模式。
• 严格模式不允许在 block 中声明函数

它被禁止的原因包含在 original JavaScript specification 中.简短版本:实现之间的行为不一致。

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.

所以当严格模式(ES5)出现时，它就被禁止了。