Question

我试图弄清楚如何过滤掉通过Promise.all执行的promises并返回undefined。我在下面提供了一个例子。

我知道我可以在＆＃34;然后＆＃34;之后过滤掉它们。但是我想知道在它到达那一点之前是否可以删除它们。

＆＃13;

// External Resource Such as Redis
const getDataFromExternalResource = (key) => {
  const data = {
    "a": "{val: 'valA'}",
    "b": "{val: 'valB'}",
    "d": "{val: 'valD'}",
  }
  return new Promise((resolve, reject) => {
    resolve((data[key] ? data[key] : undefined))
  })
}

// Look up Keys that may or may not return 'undefined'
const lookups = ['a', 'b', 'c'];

Promise
.all(lookups.map(key => getDataFromExternalResource(key)))
.then(allData => console.log(allData)) // How can I get rid of the undefined values BEFORE it gets here?

＆＃13;

Answer 1

您可以在.then之后再插入一次Promise.all调用来过滤数组。没有其他办法，您无法在Promise.all中过滤。

＆＃13;

// External Resource Such as Redis
const getDataFromExternalResource = (key) => {
  const data = {
    "a": "{val: 'valA'}",
    "b": "{val: 'valB'}",
    "d": "{val: 'valD'}",
  }
  return new Promise((resolve, reject) => {
    resolve((data[key] ? data[key] : undefined))
  })
}

// Look up Keys that may or may not return 'undefined'
const lookups = ['a', 'b', 'c'];

Promise
.all(lookups.map(key => getDataFromExternalResource(key)))
// <== next line is the inserted one ==>
.then(allData => allData.filter(x => x !== undefined))
.then(allData => console.log(allData)) // How can I get rid of the undefined values BEFORE it gets here?

＆＃13;

删除返回未定义的承诺

1 个答案: