为什么 .then() 处的值未定义链接到 Promise? [英] Why is value undefined at .then() chained to Promise?

问题描述

给定

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 data undefined 在 .then() 链接到 doStuff() 调用?

Why is data undefined at .then() chained to doStuff() call?

推荐答案

因为没有 Promise 或其他值是 returned from .then() 链接到 Promise 构造函数.

Because no Promise or other value is returned from .then() chained to Promise constructor.

注意 .then() 返回一个新的 Promise 对象.

Note that .then() returns a new Promise object.

解决方案是从.then()<return一个值或其他函数调用returnsa value或Promise/代码>.

The solution is to return a value or other function call which returns a value or Promise from .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`
});

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