JavaScript How to make a node wait before returning in node.js/Node-RED?

Question

experienced programmer, inexperienced javascript programmer.

I am working with Node-RED and in one of my nodes, I am calling an external program with 'child_process' which sends data back to my node via stdout.

I am using console.info(stdout) to log the child process data, so I know that it is working appropriately. This is the code within the function node:

/* required modules */
var cp = context.global.child_process;

/* variables */
var packet = {};
var cmd = '~/packet/packet.py ';

/* construct the packet object */
packet.networkTime = (msg.payload[3] << 24) + (msg.payload[2] << 16) + (msg.payload[1] << 8) + msg.payload[0];
packet.sampleCount = msg.payload[4];
packet.uncompressedWidth = msg.payload[5];
packet.compressedWidth = msg.payload[6];

packet.sample0 = (msg.payload[8] << 8) + msg.payload[7];

var compressedData = msg.payload.slice(9);

/* change buffer type to array */
packet.compressedData = [];
compressedData.forEach(
    function(element){
        packet.compressedData.push(element);
    }    
);

/* stringify the object */
var packet_str = "'" + JSON.stringify(packet) + "'";

/* this section will execute the command */
cp.exec(
    cmd + packet_str,
    function(error, stdout, stderr){
        console.info(stdout);
        return JSON.parse(stdout);
    }
);

//return data_msg;

I tried setting data_msg to {} and using JSON.parse(stdout) to place into that object, but the 'return data_msg' always executes before the value is returned. I simply want to wait for my external process to return data before returning this node.

Thank you for your assistance,

hownowbrowncow · Accepted Answer · 2016-02-25 14:14:38Z

Node.js has asynchronous execution. Put some things in functions and handle sequential execution in callbacks or use promises. Below is my use of promises that I use to ensure a scipt is finished executing before using it's stdout data within it's calling function.

Upon stderr the promise is rejected and an error message is propagated. Upon stdout data is stored in result. When the process finishes the data is returned as the promise is resolved.

var spawn = require('child_process').spawn;

anExecutable = function(cmd, packet_str){
  return new Promise(function(resolve, reject){

    var aProcess = spawn(cmd, packet_str);

    aProcess.stdout.on('data', (data) =>{
      var result = JSON.parse(data);
    });
    aProcess.on('error', (error) => {
      reject(error);
    });
    aProcess.on('exit', (code) => {
      resolve(result);
    });
  });
}

Call like this:

anExecutable(cmd, packet_str).then(function(data_msg){
  return data_msg;
}).catch(function(error){
  return error;
});

Let me know if you have any Q's :D

eosterberg · Accepted Answer · 2016-02-24 20:41:11Z

0

Maybe you're looking for execSync?

answered Feb 24, 2016 at 20:41

eosterberg

1,46211 silver badges11 bronze badges

1 Comment

slightlynybbled Over a year ago

I'm not ignoring you, I have just gotten distracted by a different issue. Thank you for the reference, I will try it out.

Collectives™ on Stack Overflow

JavaScript How to make a node wait before returning in node.js/Node-RED?

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related