带有 next() 调用的生成器函数

Question

我从 ecmascript 6 看到了新的未来 - 生成器函数，我对 .next() 函数的作用有点困惑。

在他们说的官方文档中，我引用了：A zero arguments function that returns an object with two properties:，根据他们的 website（<- link 更新了 Feb 17, 2015 4:57:46 PM 上的信息此处提供的文档)

所以，假设我们有这个生成器函数：

 function* first(){
    yield 1;
    yield 2;
 }
 var f= first();

当调用 f.next() 时将 return {value:1, done:false}。当你再次调用它时将 return {value:2, done:true}

但是，如果我们有这样的事情：

function* second() {
  var a = yield "HELLO";
  console.log("a = ", a);

  var b = yield a+ 1;
  console.log("b = ", b);

  return b
}
var f= second();

当您这样称呼它时：f.next() 您将收到 {value: "HELLO", done: false}

下一个调用是f.next(1)，它将1赋值给a，它会输出{value: 2, done: false}

下一次调用将是 f.next(1)，它将输出 {value: 1, done: true}

问题

如果在官方文档中声明它是一个零参数函数，你怎么可能用参数调用 .next()？
为什么在第 3 个结果中 value 属性等于 1 而在第二次调用时它等于 2？
为什么 b 是 1 而不是 2？

谢谢！

PS: Of course, there's another usage of generator functions ( to avoid callbacks ), but I'm asking about this particular case.

Answer 1

How it's possible you can call .next() with parameter if in the official documentations it's stated it's a zero parameter function ?

引用 draft version of ES-6,

Arguments may be passed to the next function but their interpretation and validity is dependent upon the target Iterator. The for-of statement and other common users of Iterators do not pass any arguments, so Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.

因此，将参数传递给 next 并不违反 ES6 规范。在这种情况下，传递给 next 的值将被分配给您要将 yield 表达式分配给的变量。

Why at 3rd result has value property equal to 1 and at the second call it's equal to 2?

Why b is 1 and not 2 ?

按发生顺序排列的操作列表

f.next()
```
yield "HELLO"
```
所以你得到 { value: 'HELLO', done: false }
f.next(1)
```
var a = 1;
console.log("a = ", a);
yield a + 1
```
这就是您在本次通话中获得 { value: 2, done: false } 的原因。
f.next(1)
```
var b = 1;
console.log("b = ", b);
return b
```
这就是为什么你在这里 { value: 1, done: true }。

Answer 2

使用其他值，它会变得更清晰。它按预期工作。

function* second() {
  var a = yield "HELLO";

  console.log("a = ", a);
  var b = yield a+ 1;

  console.log("b = ", b);
  return b
}
var f= second();

console.log('1:',f.next());
console.log('2:',f.next(5));
console.log('3:',f.next(7));

产出

///1st next()
//yeld returns iterator
{value: "HELLO", done: false}

//2nd next(5)
//the console log gets executed with var a = 5
a = 5
//yeld returns iterator
{value: 6, done: false}

//3rd next(7)
//the console log gets executed with var b = 7
b = 7
//it doesn't use the 'a' var at all, neither does it add +1 to the yeld allocation
//yeld returns iterator
{value: 7, done: true}

带有 next() 调用的生成器函数

Generator functions with next() call

javascript

ecmascript-6