Javascript for循环不等待所有承诺执行

Question

我在异步调用和 for 循环方面遇到了一些问题。我现在面临的是 for 循环在执行另一轮之前永远不会等待所有承诺完成。我的代码框架如下：

 // removed code 

有什么方法可以让 for 循环在执行另一轮之前等待我的所有承诺执行完毕？

Answer 1

我在这里写了一个原型，希望对你有所帮助：

var donePromise = 0;
let n = 10;
for(let count = 0; count < n; count++){
    DoAPromisse(function() {
        donePromise++;

        if (donePromise == n) {
            //all promisesses done
        }
    });
}

Answer 2

你也可以在一个调用它自己的函数中做到这一点：

function ramdomMerch (index, maxIndex) {
  count = index;
  count++;
  if (count > maxIndex) {
    exit(); //This will end the loop as soon as it hits the maxIndex
  }


  // random select merchant
  let randomMerchantName = new Promise((resolve, reject) => {
      merchantName = branchlist[branchIndex].merchantName;
      resolve(merchantName);
  });


            let promiseMerchantKey = new Promise((resolve, reject) => {
              randomMerchantName.then((merchantName) => {
                firebase.database().ref('merchants').orderByChild('merchantName').equalTo(merchantName).once('value', function(snapshot) {
                  var merchantData = snapshot.val();
                    if (merchantData){
                      console.log('merchant exists');
                      snapshot.forEach(function(childSnapshot) {
                        var item = childSnapshot.val();
                        item.key = childSnapshot.key;
                        resolve(item.key);
                      });
                    }else{
                      var merchantKey = firebase.database().ref('merchants').push({
                        merchantName : merchantName
                      }).getKey();

                      resolve(merchantKey);
                    }
                    ramdomMerch(count, maxIndex); // Here we simply call after finish the next "loop"
                });
              });
            });
}

randomMerch(1, 10); //We start it like an loop with the start and endIndex

使用 Promise 的更好方法是 then 运算符。 它的语法如下：

let promiseMerchantKey = new Promise(( ... 
promiseMerchantKey.then(function(value) {
   // Sucess, call next promise or next loop if it was the last
  }, function(reason) {
  // error
});

Answer 3

你想要的是链接承诺。你可以这样做：

//Just so we have a promise to start with for the chain.
let promiseMerchantKey = Promose.resolve();
for(let count = 0; count < 10; count++){

// random select category from list

// random select merchant
let randomMerchantName = promiseMerchantKey.then(()=>{
    return new Promise((resolve, reject) => {
        merchantName = branchlist[branchIndex].merchantName;
        resolve(merchantName);
    })
});

// create merchant
// check if merchant exists before add
promiseMerchantKey = randomMerchantName.then((marchantName)=>{
    return new Promise((resolve, reject) => {
        firebase.database().ref('merchants').orderByChild('merchantName').equalTo(merchantName).once('value', function(snapshot) {
                  var merchantData = snapshot.val();
                    if (merchantData){
                      console.log('merchant exists');
                      snapshot.forEach(function(childSnapshot) {
                        var item = childSnapshot.val();
                        item.key = childSnapshot.key;
                        resolve(item.key);
                      });
                    }else{
                      var merchantKey = firebase.database().ref('merchants').push({
                        merchantName : merchantName
                      }).getKey();

                      resolve(merchantKey);
                    }
                });
              });
});


// all other promises to insert record into different child

}