javascript - 如何解释下面的例子？吊装？

Question

我试图解释 javascript 提升的问题，但我无法解释 b 的情况。

b = 50不是修改了全局变量b吗？

是 block 级作用域的原因吗？

环境

Chrome 77

{
  a = 50
  function a() {}
}
console.log(a) //  50
console.log(b) // undefined
{
  console.log(b) // f b () {}
  function b() {}
  b = 50
  console.log(b) // 50
}
console.log(b) // ƒ b () {}

我以为 b 和 a 一样是 50。但它是一个函数。

Answer 1

这里发生了两件重要的事情

1. 托管发生在一个函数中 - 将任何东西放在 {} 中被视为一个 block ，而不是一个函数。
2. 函数声明被提升到变量之上 - 因此如果 var 和函数具有相同的名称，函数将获得优先权

console.log(x); // f() {} //hoisted with assignment since "x" is function
var x = 90;
function x() {}

console.log(a); // undefined // hoisting of child block-scope variable is never assigned not even if "a" is function
{
    console.log(a); // f(){}
    a = 50; //**while execution //since global scope "a" not assigned yet it takes the first assignment in this child-block
    console.log(a); // 50 // because value has been assigned to "a" already
    function a(){} // Ignored //function "a" was never hoisted over variable assignment
    console.log(a); // 50 // block scope has "a=50" attached to it
}
console.log(a); // 50 // global scope also has "a=50" attached to it


console.log(b) // undefined // hoisting of child block-scope variable is never assigned not even if "a" is function
{
  console.log(b) // f () {}
  function b() {} // While hoisting global scope and this block scope get "b" attached to their scope as a function
  b = 50 // var b is reassigned, but "b" attached to this block is only reassigned, since type is changed globally attached "b" is not reached
  console.log(b) // 50
}
console.log(b) // ƒ () {} // globally attached "b" is a function