Make function calls in for loop synchronous

Question

I get an array of strings in the request. Each string contains a command to be executed on the native shell.

var process = require('child_process'); 

function execCommand(req,res,callback){
 var params = req.params.list              //list is an array in the   request
 var result = '';
 var command = '';  
 for (var i = 0; i < params.length; i++) {
  command = params[i];
  cmd = process.exec(command);                        
  cmd.stdout.on('data', function(data){                       
   //push the shell output in variable result
  });
  cmd.on('close', function(code) {                            
   //convert the variable result to a valid JSON
  });
 }
 res.send(result);
};

The result of all the commands get mixed up in the result variable. How to make the function calls in the for loop synchronous?

Answer 1

Use execSync instead of exec!

 for (var i = 0; i < params.length; i++) {
    command = params[i];
    result += process.execSync(command).toString();                        
 }

As others have pointed out, this might not be a very good idea as it will block the event loop and any requests that are being made. It is acceptable if the processes being executed will take a very short period of time to finish, but not otherwise.

Here is a simple pattern to control flow of asynchronous functions in a for loop... every time a process returns, params_finished increments by one. Once the number of finished processes equals the total number of processes the response is sent.

 for (var i = 0, total_params=params.length, params_finished=0; i < total_params; i++) {
    command = params[i];
    cmd = process.exec(command);                        
    cmd.stdout.on('data', function(data){                       
        //push the shell output in variable result
    });
    cmd.on('close', function(code) {                            
        //convert the variable result to a valid JSON
        params_finished++
        if(params_finished == total_params){
            res.send(result);
        }
    });
 }