JavaScript for循環不等待所有承諾執行

Question

我有異步調用和循環的問題。我現在面臨的是for循環在執行另一輪之前從不等待所有承諾完成。我的代碼框架這樣：

// removed code 

有沒有辦法做到這一點使得for循環將等待做了所有我的諾言執行另一輪之前執行？

Answer 1

你也可以做到這一點在調用它自身的功能：

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 

一種更好的方式與承諾的工作是then運營商。 這將是一個語法像：

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

Answer 2

我這裏寫了一個原型，希望幫助：

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

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

Answer 3

你想要的是連鎖的承諾。你可以做到這一點，像這樣：

//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 

}