顺序执行Promise.all

时间:2019-02-25 13:28:23

标签: javascript arrays node.js promise es6-promise

嗨,我需要一个接一个地执行promise,我如何使用promise来实现这一点。所有帮助都会很棒。以下是我当前正在使用的代码示例,但由于它是并行执行的,因此搜索将无法正常工作

public testData: any = (req, res) => {
    // This method is called first via API and then promise is triggerd 
    var body = req.body;

    // set up data eg 2 is repeated twice so insert 2, 5 only once into DB
    // Assuming we cant control the data and also maybe 3 maybe inside the DB
    let arrayOfData = [1,2,3,2,4,5,5];

    const promises = arrayOfData.map(this.searchAndInsert.bind(this));

    Promise.all(promises)
        .then((results) => {
            // we only get here if ALL promises fulfill
            console.log('Success', results);
            res.status(200).json({ "status": 1, "message": "Success data" });
        })
        .catch((err) => {
            // Will catch failure of first failed promise
            console.log('Failed:', err);
            res.status(200).json({ "status": 0, "message": "Failed data" });
        });
}

public searchAndInsert: any = (data) => {
    // There are database operations happening here like searching for other
    // entries in the JSON and inserting to DB
    console.log('Searching and updating', data);
    return new Promise((resolve, reject) => {
        // This is not an other function its just written her to make code readable
        if(dataExistsInDB(data) == true){
            resolve(data);
        } else {
            // This is not an other function its just written her to make code readable
            insertIntoDB(data).then() => resolve(data);
        }
    });
}

我在Google中抬头看了看,发现reduce将有所帮助,我将非常感谢您提供任何有关如何将其转换为reduce的帮助,或者您建议的任何方法(.map中的并发均无效)

1 个答案:

答案 0 :(得分:1)

很遗憾,该承诺不允许对他们的流程进行任何控制。这意味着->创建新的Promise后,它将按自己的意愿进行异步处理。

Promise.all不会更改它,它的唯一目的是检查您放入其中的所有诺言,并在所有诺言完成(或其中一个失败)后予以解决。

为了能够创建和控制异步流,最简单的方法是将Promise的创建包装到函数中并创建某种工厂方法。然后,无需先创建所有承诺,而仅在需要时创建一个承诺,等到解决它并以相同的方式继续执行即可。

async function doAllSequentually(fnPromiseArr) {
  for (let i=0; i < fnPromiseArr.length; i++) {
    const val = await fnPromiseArr[i]();
    console.log(val);
  }
}

function createFnPromise(val) {
  return () => new Promise(resolve => resolve(val));
}

const arr = [];
for (let j=0; j < 10; j++) {
  arr.push(createFnPromise(Math.random()));
}

doAllSequentually(arr).then(() => console.log('finished'));

PS:使用标准的Promise链也可以不进行异步/等待,但是它需要递归实现。