Using Javascript closures for managing asynchronous Ajax requests within for loops

Question

Using Javascript closures for managing asynchronous Ajax requests within for loops

Let's consider the arrays provided below.

var clients = ['a','b'];
var reports = ['x','y','z'];
var finalData = [];

Now, I aim to iterate through them in a specific manner as shown.

for(var i=0;i<reports.length;i++){
   var response = {report : reports[i]}
   for(var j=0;j<clients;j++){
       response.client = clients[i];
      $.ajax({
        url :url,
        success : function(data){
          response.data = data;
          finalData.push(response);
        })
     });
   }
}

However, due to asynchronous nature of ajax calls, it fails to work properly. To solve this issue, I intend to encapsulate this logic within the componentDidMount function of a react.js component and update the state with the finalData.

I attempted to use $.each instead of a traditional for loop and tried using .bind(this) to correctly update the state with finalData, but without success. I read that closures should be utilized for handling async calls effectively, however, I am unsure how to implement that here. The desired output is as follows:

finalData = [
        {client:a,report:x,data : 'xyz'},
        {client:b,report:x,data : 'xyz'},
        {client:a,report:y,data : 'xyz'},
        {client:b,report:y,data : 'xyz'},
        {client:a,report:z,data : 'xyz'},
        {client:b,report:z,data : 'xyz'}
     ]

javascript ajax reactjs asynchronous

Answer 1

Answer №1

To ensure the accuracy of your response variable, it is important to save the scope using closure. Consider implementing the following approach:

for(var i=0;i<reports.length;i++){
   var response = {report : reports[i]}
   for(var j=0;j<clients;j++){
      response.client = clients[i];

      (function(responce){
      $.ajax({
        url :url,
        success : function(data){
          response.data = data;
          finalData.push(response);
        })
      })(responce);

     });
   }
}

For a better understanding, let's simulate an ajax call using setTimeout in this example:

Imagine a scenario where we iterate a loop 5 times and aim to print the value of i after one second. Expected output: 0 1 2 3 4. However, due to the loop completing before the one-second delay, the actual output may be 5 printed five times instead.

for(var i=0;i<5;i++){
  setTimeout(function(){
    console.log(i);
    console.log("....");
  }, 1000) 
}

To resolve this issue, we need to encapsulate the value of i using closure, ensuring that the correct value is retrieved after the timeout:

for(var i=0;i<5;i++){
  (function(i){
    setTimeout(function(){
      console.log(i);
      console.log("....");
    }, 1000)
  })(i);
}

By applying this method, the code will print 0 1 2 3 4 as intended, which aligns with the solution provided in your example.

When updating the state, you have two options: either update it each time a value is received or check for completion before updating the state:

// Add this line at the beginning of the two for loops
var that = this;
finalData.push(response);
this.setState({data: finalData})

// Calculate 'total' by multiplying the number of reports by the number of clients
if(finalData.length == total){
  this.setState({data: finalData})
}

Answer 2

To ensure the accuracy of your response variable, it is important to save the scope using closure. Consider implementing the following approach:

for(var i=0;i<reports.length;i++){
   var response = {report : reports[i]}
   for(var j=0;j<clients;j++){
      response.client = clients[i];

      (function(responce){
      $.ajax({
        url :url,
        success : function(data){
          response.data = data;
          finalData.push(response);
        })
      })(responce);

     });
   }
}

For a better understanding, let's simulate an ajax call using setTimeout in this example:

Imagine a scenario where we iterate a loop 5 times and aim to print the value of i after one second. Expected output: 0 1 2 3 4. However, due to the loop completing before the one-second delay, the actual output may be 5 printed five times instead.

for(var i=0;i<5;i++){
  setTimeout(function(){
    console.log(i);
    console.log("....");
  }, 1000) 
}

To resolve this issue, we need to encapsulate the value of i using closure, ensuring that the correct value is retrieved after the timeout:

for(var i=0;i<5;i++){
  (function(i){
    setTimeout(function(){
      console.log(i);
      console.log("....");
    }, 1000)
  })(i);
}

By applying this method, the code will print 0 1 2 3 4 as intended, which aligns with the solution provided in your example.

When updating the state, you have two options: either update it each time a value is received or check for completion before updating the state:

// Add this line at the beginning of the two for loops
var that = this;
finalData.push(response);
this.setState({data: finalData})

// Calculate 'total' by multiplying the number of reports by the number of clients
if(finalData.length == total){
  this.setState({data: finalData})
}

Using Javascript closures for managing asynchronous Ajax requests within for loops

Answer №1

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