为什么在.then()链接到Promise时未定义值？

Question

特定

function doStuff(n /* `n` is expected to be a positive number */) {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      resolve(n * 10)
    }, Math.floor(Math.random() * 1000))
  })
  .then(function(result) {
    if (result > 100) {
      console.log(result + " is greater than 100")
    } else {
      console.log(result + " is not greater than 100");
    }
  })
}

doStuff(9)
.then(function(data) {
  console.log(data) // `undefined`,  why?
})

为什么要被拴data undefined在一起打电话？.then()doStuff()

Answer 1

因为从链接到构造函数没有Promise或其他值.return.then()Promise

请注意,.then()返回一个新Promise对象.

解决方案是对值或来自return的值或其他函数调用.returnPromise.then()

function doStuff(n /* `n` is expected to be a positive number */) {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      resolve(n * 10)
    }, Math.floor(Math.random() * 1000))
  })
  .then(function(result) {
    if (result > 100) {
      console.log(result + " is greater than 100")
    } else {
      console.log(result + " is not greater than 100");
    }
    // `return` `result` or other value here
    // to avoid `undefined` at chained `.then()`
    return result
  })
}

doStuff(9)
.then(function(data) {
  console.log("data is: " + data) // `data` is not `undefined`
});

Answer 2

问题正面临着：

return 
(new Promise(..)) //the promise we want to return
.then(()=>undefined) // the promise were actually returning, which resolves to undefined

您可能已经注意到，然后返回一个新的承诺。这有一个很好的理由，它使promise链接变得容易，例如：

getUser()//an asynchronous action
 .then(user=>login(user))//then if we get the user,promise to log in
 .then(token=>console.log("logged in,token is "+token) //then if we logged in, log it
 .catch(error=>"login failed");//catch all errors from above

但这也造成了我们面临的小陷阱。解决方案可能是返回原始的Promise，而不是.then（）自动返回的新Promise，因为这被解析为undefined，因为里面的函数没有显式返回任何东西：

//what were doing:
Promise.resolve(n*10)//the original promise resolves to n*10
.then(a=>undefined)//the then gets n*10 passed as a, but returns undefined
.then(b=>console.log(b));//b will be undefined  :0 

//what we want:
var promise=Promise.resolve(n*10);
promise.then(a=>undefined);//a is n*10, this resolves to undefined
promise.then(b=>console.log(b));//but this still logs n*10, as its the original promise  :) 

如您所见，要返回原始的Promise，我们只需将其存储在变量中，然后为其分配.then处理程序，并仍然引用原始的Promise，我们可以将其分配给其他处理程序（或return）。

实际上：

function doStuff(n /* `n` is expected to be a number */) {

    //create a new promise and store it

    var promise=new Promise(function(resolve, reject) {
        setTimeout(function() {
           resolve(n * 10)
        },1000);
    });

    //add a then handler to this promise  

    promise.then(result=>console.log(result + " is "+result<100?"not":""+" greater than 100"));

    //return the original one

    return promise;

}

doStuff(9).then(function(data) {
  console.log(data) //not undefined, as original promise
})

Answer 3

doStuff 正在返回Promise. 但是，您的最后一个then函数没有返回任何值，因此data作为undefined.

在 Promise 中，下一个then函数的参数值是前一个函数的返回值then。

function doStuff(n /* `n` is expected to be a positive number */) {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      resolve(n * 10)
    }, Math.floor(Math.random() * 1000))
  })
  .then(function(result) {
    if (result > 100) {
      console.log(result + " is greater than 100")
    } else {
      console.log(result + " is not greater than 100");
    }
    return result;
  })
}

doStuff(9)
.then(function(data) {
  console.log(data) // `90`
})