为什么 const 在 JavaScript 的某些 for 循环中起作用? [英] Why does const work in some for-loops in JavaScript?

问题描述

我确实知道为什么 const 在 for 循环中不起作用.我们需要创建一个新的作用域并将一个值复制到其中.所以这不会飞.

I do know why const doesn't work in for-loops. We need to create a new scope and copy over a value into that. So this won't fly.

for(const i = 0; i < 5; i++) console.log(i);

然而这会.

for(let i = 0; i < 5; i++) console.log(i);

但是，我注意到它们在循环处理像这样的对象的属性时都可以工作.

However, I noticed that both of them work when looping though the properties of an object like this.

for(let property in thingy) console.log(property);
for(const property in thingy) console.log(property);

我不知道为什么.

推荐答案

for (const property in object) 之所以有效，是因为每次迭代都会得到一个新变量，该变量的作用域仅限于该迭代.您可以通过在循环内使用闭包轻松检查:

for (const property in object) works because with each iteration you get a new variable, which is scoped only to that iteration. You can easily check that by using a closure inside a loop:

for (const property in {a: 1, b: 2}) {
  setTimeout(() => {
    console.log(property);
  }, 100);
}

这会记录a 和b，但是如果您将const 更改为var，则会记录b 两次.

This logs a and b, but if you change const to var, it logs b twice.

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