创建一个在变量未定义时解决的承诺

如何解决创建一个在变量未定义时解决的承诺

我正在尝试创建一个Promise，当变量不是 undefined 时解决。

代码示例

https://codesandbox.io/s/quizzical-feynman-ktvox?file=/src/index.js

let fetchedPromise = new Promise((resolve,reject) => {
  const check = ()=>{
    setTimeout(() =>{
    console.log("checking")
    if (dataFetched) resolve(dataFetched);
    else check()
    },100);
  }
  check()
});

const waitForDataFromAnotherComponent = async () => {
  let result = await fetchedPromise;
  console.log("Result: ",result);
};

const assignData = () => {
  setTimeout(() => {
    dataFetched = 1000;
    console.log(dataFetched);
  },5000)
};

waitForDataFromAnotherComponent();
assignData();

这行得通，但我发现它效率低下，因为它容易发生调用栈，并且似乎是错误的。

我尝试过的其他无效解决方案：

//-------------SOLUTION 1
let fetchedPromise = new Promise((resolve,reject) => {
  const check = ()=>{
    if (dataFetched) resolve(dataFetched);
    else check()
  }
  check()
});

//--------------------SOLUTION 2
let fetchedPromise = new Promise((resolve,reject) => {
   if (dataFetched) resolve(dataFetched);
});

场景

我需要像解决方案3这样的函数，该函数不依赖setTimeout

解决方法

使用Javascript代理解决

基本上，我将代理对象分配给侦听更改的dataFetched。 由于必须包含resolve()

，因此我重新创建了监听功能

let dataFetched
let x = {
  aListener: function (val) {},set a(val) {
    dataFetched = val;
    this.aListener(val);
  },get a() {
    return dataFetched;
  },registerListener: function (listener) {
    this.aListener = listener;
  }
};

let fetchedPromise = new Promise((resolve,reject) => {
  x.registerListener(function (val) {
    console.log("yeyyyyyyyyyyyyyyy");
    if (dataFetched) resolve(dataFetched);
  });
});

const waitForDataFromAnotherComponent = async () => {
  let result = await fetchedPromise;
  console.log("Result: ",result);
};
const assignData = async () => {
  await new Promise((resolve,reject) =>
    setTimeout(() => {
      x.a = 1000;
      console.log(dataFetched);
      resolve(dataFetched);
    },1000)
  );
};

waitForDataFromAnotherComponent();
assignData();

编辑

实际上，可以将诺言的resolve()功能外部化，但存在here所说的一些缺点

示例

let dataFetched
var promiseResolve,promiseReject;

let x = {
  aListener: function (val) {
    if (dataFetched) promiseResolve(dataFetched);
},reject) => {
 promiseResolve = resolve;
  promiseReject = reject;
});