javascript – 为什么在.then()链接到Promise时未定义值？

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?
})

为什么在.then()链接到doStuff()调用时数据未定义？

解决方法:

因为链接到Promise构造函数的.then()没有返回Promise或其他值.

请注意,.then()返回一个新的Promise对象.

解决方案是返回一个值或其他函数调用,它从.then()返回一个值或Promise.

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`
});