javascript - resolve(thenable) 和 resolve ('non-thenable-object' ) 之间有什么区别？

Question

我试图理解 resolve(thenable) 和 resolve('non-thenable-object') 之间的区别。

在下面的示例中，使用 promise 而不是 thenable，因为 promise 也是 thenable 并且可能更容易理解。

Demo1: resolve(promise)

let resolvePromise = new Promise(resolve => {
  let resolvedPromise = Promise.resolve()
  resolve(resolvedPromise)
})
resolvePromise.then(() => {
  console.log('resolvePromise resolved')
})
let resolvedPromiseThen = Promise.resolve().then(res => {
  console.log('promise1')
})
resolvedPromiseThen
  .then(() => {
    console.log('promise2')
  })
  .then(() => {
    console.log('promise3')
  })

结果:

• promise 1
• promise 2
• resolvePromise 已解决
• promise 3

Demo2: resolve('non-thenable-object')

let resolvePromise = new Promise(resolve => {
  resolve('non-thenable-object')
})
resolvePromise.then(() => {
  console.log('resolvePromise resolved')
})
let resolvedPromiseThen = Promise.resolve().then(res => {
  console.log('promise1')
})
resolvedPromiseThen
  .then(() => {
    console.log('promise2')
  })
  .then(() => {
    console.log('promise3')
  })

结果:

• resolvePromise 已解决
• promise 1
• promise 2
• promise 3

所以，我检查了规范并找到了 Promise Resolve Functions.然后到了PromiseResolveThenableJob和 EnqueueJob .

所以，根据规范，我认为 demo1 就像

演示3:

let resolvePromise = new Promise(resolve => {
  let resolvedPromise = Promise.resolve()
 // resolve(resolvedPromise)
  // works like 
  Promise.resolve().then(() => {
    Promise.resolve(resolvedPromise).then(() => {
      resolve()
   })
  })
})
resolvePromise.then(() => {
  console.log('resolvePromise resolved')
})
let resolvedPromiseThen = Promise.resolve().then(res => {
  console.log('promise1')
})
resolvedPromiseThen
  .then(() => {
    console.log('promise2')
  })
  .then(() => {
    console.log('promise3')
  })

我认为是因为Promise Resolve Functions说:

13. Perform EnqueueJob("PromiseJobs", PromiseResolveThenableJob, « promise, resolution, thenAction »).

和PromiseResolveThenableJob说:

This Job uses the supplied thenable and its then method to resolve the given promise. This process must take place as a Job to ensure that the evaluation of the then method occurs after evaluation of any surrounding code has completed.

此外，我认为 demo2 的工作方式类似于

演示4:

//let resolvePromise = new Promise(resolve => {
  //resolve('str')
//})
//works like
let resolvePromise = Promise.resolve('str')

resolvePromise.then(() => {
  console.log('resolvePromise resolved')
})
let resolvedPromiseThen = Promise.resolve().then(res => {
  console.log('promise1')
})

resolvedPromiseThen
  .then(() => {
    console.log('promise2')
  })
  .then(() => {
    console.log('promise3')
  })

作为Promise Resolve Functions说:

If IsCallable(thenAction) is false, then Return FulfillPromise(promise, resolution).

虽然Demo1-Demo3和Demo2-Demo4的结果是一样的，但我还是不确定如果我是对的。所以，我来问一下

我的逻辑是否正确？如果不是，你如何解释不同的命令 在 resolve(thenable) 和 resolve(non-thenable) 之间？

Answer 1

是的，你的逻辑看起来是对的。

new Promise(resolve => resolve('non-thenable-object')) 等价于 Promise.resolve('non-thenable-object')所有目的。

不过，在您的 Demo3 中，我建议省略 Promise.resolve(resolvedPromise)。我不确定这是不是故意的，但是 Promise.resolve 确实有一个快捷方式，当它的参数已经是一个 promise 时，然后按原样返回 resolvedPromise .你宁愿写

new Promise((resolve, reject) => {
  let resolvedPromise = Promise.resolve();
  // resolve(resolvedPromise) works like 
  Promise.resolve().then(() => resolvedPromise.then(resolve, reject));
});