为什么在 while 表达式中使用 await 赋值不保留赋值? [英] Why does assignment using await within while expression not retain assigned value?

问题描述

给定 JavaScript 代码

Given the JavaScript code

function* gen(start = 0, stop = 5) {
   while (start < stop) yield ++start;
}

async function fn(g = gen()) {
  while (done = !await new Promise(resolve => 
                   setTimeout(resolve, 1000, g.next()))
                   .then(({value, done}) => {
                     // `value:undefined, done:true` following
                     // `value:5, done:false`
                     console.log(`value:${value}, done:${done}`); 
                     return done 
                   }
                   , err => Promise.reject(err)));
  return done; // what happened to the `true` value assigned?
}
// why is `res` `false`?
fn().then(res => console.log(`res:${res}`), err => console.error(err));

当done被赋值时，fn()返回的done的预期结果是truecode>true 从 .then() 在 while 表达式中返回.

the expected result of done returned from fn() is true when done is assigned the value true that is returned from .then() within while expression.

为什么使用await 在 while 声明 表达式未保留?

推荐答案

JavaScript 有三种"不同类型的作用域:局部、全局和块.

JavaScript has "three" different types of scope: local, global, and block.

全球

这种类型的变量在代码的所有地方都可以到达:

This type of variable can be reach in all places of the code:

var b = "bar";
   (function test1(a) {
         if (a == true) {
              var b = "bar";
         } else {
              var b = "foo";
         }
         return b;
    })(true);
    alert(b);

本地

这些变量只有在代码的一般范围内才能到达:

These variables can only be reached if the variable in the general scope of the code:

(function test2(a) {
     if (a == true) {
          var b = "bar";
     } else {
          var b = "foo";
     }
     return b;
})(true)

在这种情况下，一般作用域是函数.即使变量 b 是 if 语句的块作用域，函数仍然可以访问它.

In this case, the general scope is the function. Even though the variable b is block scoped by the if statement, the function can still reach it.

阻止

自定义块作用域变量对于 JavaScript 来说是非常新的，可以使用 let 关键字引入:

Self defined block scoped variables are pretty new to JavaScript and can be introduced using the let keyword:

(function test2(a) {
     if (a == true) {
          let b = "bar";
     } else {
          let b = "foo";
     }
     return b;
})(true)

这将导致错误，因为变量 b 现在块作用域为 if 状态.因此，在您的代码中，我们可以更改您的范围，以允许您更改 done 在异步循环的情况下，在 while 循环中是块范围的.

This will result in an error because the variable b is now block scoped to the if state. Thus in your code, we can change your scoping to allow you to change done which in a while loop is block scoped in the case of asynchronous loops.

function* gen(start = 0, stop = 5) {
   while (start < stop) yield ++start;
}

async function fn(g = gen()) {
  let doneVal;
  while (done = !await new Promise(resolve => 
                   setTimeout(resolve, 1000, g.next()))
                   .then(({value, done}) => {
                     // `value:undefined, done:true` following
                     // `value:5, done:false`
                     console.log(`value:${value}, done:${done}`); 
                     return done 
                   }
                   , err => Promise.reject(err))) {
       doneVal = done;
  }

  return doneVal; // what happened to the `true` value assigned?
}
// why is `res` `false`?
fn().then(res => console.log(`res:${res}`), err => console.error(err));

现在您将获得 res:true.在您的具体示例中，这样的代码存在问题:

Now you will get res:true. In your specific example, there is an issue with code like this:

var i;
var c = 0;
while (i = 90 && c < 10) {
    c++;
}
console.log(i, c);

i 为假而 c 等于 10

i is false while c is equal to 10

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