Prevent sending data to stdin if spawn fails - node.js

Question

In my Node.js (v0.10.9) code I'm trying to detect 2 cases:
an external tool (dot) is installed - in that case I want to send some data to stdin of created process
the external tool is not installed - in that case I want to display warning and I don't want to send anything to process' stdin
My problem is that I don't know how to send data to child's stdin if and only if the process was spawned successfully (i.e. stdin is ready for writing).
Following code works fine if dot is installed, but otherwise it tries to send data to the child although the child wasn't spawned.
var childProcess = require('child_process');
var child = childProcess.spawn('dot');
child.on('error', function (err) {
console.error('Failed to start child process: ' + err.message);
});
child.stdin.on('error', function(err) {
console.error('Working with child.stdin failed: ' + err.message);
});
// I want to execute following lines only if child process was spawned correctly
child.stdin.write('data');
child.stdin.end();
I'd need something like this
child.on('successful_spawn', function () {
child.stdin.write('data');
child.stdin.end();
});

Answer 1

From the node.js docs: http://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options
Example of checking for failed exec:
var spawn = require('child_process').spawn,
child = spawn('bad_command');
child.stderr.setEncoding('utf8');
child.stderr.on('data', function (data) {
if (/^execvp\(\)/.test(data)) {
console.log('Failed to start child process.');
}
});

Answer 2

Have a look at core-worker:
https://www.npmjs.com/package/core-worker
This package makes it a lot easier to handle processes.
I think what you want to do is something like that (from the docs):
import { process } from "core-worker";
const simpleChat = process("node chat.js", "Chat ready");
setTimeout(() => simpleChat.kill(), 360000); // wait an hour and close the chat
simpleChat.ready(500)
.then(console.log.bind(console, "You are now able to send messages."))
.then(::simpleChat.death)
.then(console.log.bind(console, "Chat closed"))
.catch(() => /* handle err */);
So if the process is not started correctly, none of the .then statements are executed which is exactly what you want to do, right?