javascript - 如何预测 Promises 的异步性质

Question

我有一段代码示例，我无法预测代码的流程。

var x = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve([1, 2, 3]);
    }, 0);
  });
};

x().then((val) => {
  console.log(val);
}).catch((err) => {
  console.log(err.message);
});

console.log("hello");

for (var i = 0; i < 10; i++) {
  console.log(i);
}

var y = Promise.all([Promise.resolve(1), Promise.reject(Error("err"))]);

y.then((arr) => {
    console.log(arr);
  })
  .catch((err) => {
    console.log(err);
  });

Promise.resolve('ta-da!')
  .then((result) => {
    console.log('Step 2 received ' + result);
    return 'Greetings from step 2';
  })
  .then((result) => {
    console.log('Step 3 received ' + result);
  })
  .then((result) => {
    console.log('Step 4 received ' + result);
    return Promise.resolve('fulfilled value');
  })
  .then((result) => {
    console.log('Step 5 received ' + result);
    return Promise.resolve();
  })
  .then((result) => {
    console.log('Step 6 received ' + result);
  });

日志按以下顺序排列:

"hello"
0
1
2
3
4
5
6
7
8
9
"Step 2 received ta-da!"
"Step 3 received Greetings from step 2"
"err"
"Step 4 received undefined"
"Step 5 received fulfilled value"
"Step 6 received undefined"
[1, 2, 3]

for 循环正在按预期执行。setTimeout() 按预期工作， promise 在事件循环后实现。

其他 2 个 promise 发生冲突。我期待 promise 会立即同步实现，结果会是

"hello"
    0
    1
    2
    3
    4
    5
    6
    7
    8
    9
    "err"
    "Step 2 received ta-da!"
    "Step 3 received Greetings from step 2"
    "Step 4 received undefined"
    "Step 5 received fulfilled value"
    "Step 6 received undefined"
    [1, 2, 3].

即使 promise 是异步解决的。它们是如何发生冲突的。

附上截图。

Answer 1

您的代码中有三个独立的 promise 链:

1. 以x()开头的，因为超时需要最长的
2. 以y开头的那个(来自Promise.all(…))抛出错误的地方
3. 以 Promise.resolve('ta-da!') 开头的那个

他们不会互相等待 - 你没有告诉他们去做，他们也不会神奇地自己去做。
相反，它们的流程是任意交错的，就像标准异步函数的情况一样(想想两个具有不同周期的 setInterval)。

I was expecting the promises will be fulfulled immediately in syncronously

没有。 Promise 回调总是(可靠地)是异步的，即使 promise 已经确定。