javascript - 让 let 替换匿名闭包以获得私有(private)变量吗？

Question

如果您想在 JavaScript 中使用某种私有(private)变量，您可以将代码放在匿名闭包中，对吗？现在既然包含了 let，闭包的这个特定用例就消失了吗？或者它仍然相关吗？

顶级示例:

// global variable a
var a = 6;

// using let
{
  // new variable a that only lives inside this block
  let a = 5;
  console.log(a); // => 5
}

console.log(a); // => 6

// using a closure
(function() {
  // again a new variable a that only lives in this closure
  var a = 3;
  console.log(a); // => 3
})();

console.log(a); // => 6

Answer 1

有一种东西叫Hoisting在 Javascript 中，它甚至在初始化之前就“提升”了上面的变量。

// global variable a
var a = 6;

// using let
{
  // new variable a that only lives inside this block
  let a = 5;
  console.log(a); // => 5
}

console.log(a); // => 6

// using a closure
(function() {
  // again a new variable a that only lives in this closure
  var a = 3;
  console.log(a); // => 3
})();

console.log(a); // => 6

所以这段代码变为:

// The global variable 'a' is hoisted on the top of current scope which is Window as of now
var a;

// Initialization takes place as usual
a = 6;


// This is a block
{

  // This local variable 'a' is hoisted on the top of the current scope which is the 'block'
  let a;

  a = 5;
  console.log(a); // => 5
}

console.log(a); // => 6

// Using an IIFE
(function() {

  // This local variable 'a' is hoisted on the top of the current scope which is the block of IIFE
  var a;

  a = 3;
  console.log(a); // => 3
})();

console.log(a); // => 6

我们曾经使用 Pre ES6 IIFEs使变量不会污染全局范围，但在 ES6 之后我们通常使用 let 和 const 因为它们提供了 block-scope .