从 forEachloop Javascript 中的异步函数返回值答案

【问题标题】：Return value from async function inside forEachloop Javascript从 forEachloop Javascript 中的异步函数返回值
【发布时间】：2020-12-05 07:59:49
【问题描述】：

所以我有一个内部有“n”个元素的对象，我必须进入每个元素并使用另一个函数检查状态，该函数的返回应该告诉我有多少元素是“真”但在 forEach 之外循环，问题是一切都在第二个函数的返回内部进行，但在外部打印 [object promise] 这是我的代码：

var count=0;
object_x =  "relations": [
        {
            "rel": "System.LinkTypes.Related",
            "url": "545",
            "attributes": {
                "isLocked": false,
                "name": "Related"
            }
        },
        {
            "rel": "System.LinkTypes.Related",
            "url": "494",
            "attributes": {
                "isLocked": false,
                "name": "Related"
            }
        },
        {
            "rel": "System.LinkTypes.Parent",
            "url": "508",
            "attributes": {
                "isLocked": false,
                "name": "Parent"
            }
        }
    ],
object_x.forEach((element) =>{ 
    var name = element['attributes];
    name = name['name]; 
    if(name=='Related'){
        let return_of_function_A = function_A(element['url']); //Async function: based on "url" number this function will return true or false
            return_of_function_A.then(function(result){
                if(result == true){
                    count++;
                }else;
            }); 
    }else;
});
console.log(count); //prints [object Promise]

我不是 javascript 专家，但我认为这可能与 return_of_function_A.then(function(result){ 有关 ...

【问题讨论】：

您的代码有问题。当您说a.function_A 应该返回真或假时，代码会将其视为一个承诺。它应该是一个确定为真或假的承诺吗？在这种情况下，您在count 上有一个竞争条件。
您能否编辑您的问题，使其包含mcve？
@IvanRubinson 我刚刚更新，这是一个承诺
你在哪里使用async/await？

标签： javascript asynchronous foreach async-await

【解决方案1】：

重要的想法是收集 Promise（可能由 function_A 返回），然后计算所有解决方案中的真实结果。

let promises = object_x.map(element => A.function_A(element))

// now promises is an array of promises returned by function_A given the parameters in object_x
// create a promise that resolves when all of those promises resolve
// Promise.all() does that, resolving to an array of the resolutions of the passed promises

Promise.all(promises).then(results => {
  // results is the array of bools you wish to count
  let count = results.filter(a => a).length
  console.log(count)
})

编辑 - 相同的想法，使用您的数据。

const function_A = url => {
  // simulate an async function on a "url" param
  // after a short delay, return true if the int value of url > 500
  return new Promise(resolve => {
    let large = parseInt(url) > 500
    setTimeout(resolve(large), 1000) // delay for 1000ms
  })
}

const object_x = {
  "relations": [{
      "rel": "System.LinkTypes.Related",
      "url": "545",
      "attributes": {
        "isLocked": false,
        "name": "Related"
      }
    },
    {
      "rel": "System.LinkTypes.Related",
      "url": "494",
      "attributes": {
        "isLocked": false,
        "name": "Related"
      }
    },
    {
      "rel": "System.LinkTypes.Parent",
      "url": "508",
      "attributes": {
        "isLocked": false,
        "name": "Parent"
      }
    }
  ]
}


// gather the relations who are "Related"
let relatedRelations = object_x.relations.filter(element => {
  return element.attributes.name === "Related"
})

// gather promises for those objects
let promises = relatedRelations.map(element => function_A(element.url))

// now promises is an array of promises returned by function_A given the parameters in object_x
// create a promise that resolves when all of those promises resolve
// Promise.all() does that, resolving to an array of the resolutions of the passed promises

Promise.all(promises).then(results => {
  // results is the array of bools you wish to count
  let count = results.filter(a => a).length
  // expected result : 1.just one of the name === "Related" urls is > 500
  console.log(count)
})

【讨论】：