从forEachloop Javascript中的异步函数返回值

编程问答 2022-08-16

问题描述

所以我有一个对象内有'n'个元素，我必须进入每个元素内并用另一个函数检查状态，该函数的返回应该给我有多少元素为'true'，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，但外部打印[对象承诺]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",{
            "rel": "System.LinkTypes.Parent","url": "508","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方面的专家，但我认为这可能与return_of_function_A.then(function(result){ ...

有关

解决方法

重要的想法是收集承诺（大概由function_A返回），然后在所有解决方案中计算真实结果。

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-同样的想法，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",{
      "rel": "System.LinkTypes.Parent","url": "508","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)
})

async-awaitasynchronousforeachjavascript