process.send is conditionally defined in Node.js

For whatever reason, in Node.js, the function process.send is defined in some environments but not defined in others. For instance, when I fork a child process from a parent process in Node.js like so:

//parent process
var cp = require('child_process');
var k = cp.fork('./child.js',['arg1','arg2','arg3']);
k.send('fail'); //k.send is defined...
process.send("ok, let's try this..."); //process.send is NOT defined

inside the child process:

//child.js
process.send('message'); //process.send is defined, and will send a message to the parent process above

why is process.send conditionally defined in some Node.js processes but not others? Seems like a poor design decision by Node.js architects.

The only way I know how to get around this is:

if (typeof process.send === 'function') { process.send('what I want to send');
}

2 Answers

Child processes have a process.send method to communicate back with the process that spawned them, while the root process doesn't have any "parent" to communicate, so its not there. From the docs:

In the child the process object will have a send() method, and process will emit objects each time it receives a message on its channel.

To avoid having to "litter the code with conditionals", a temporary solution might be to just put a "noop" function in its place at the top of any "root" files you might be spawning processes from:

process.send = process.send || function () {};

Workaround for Sapper & Svelte usage:

Basically, this error can be received in flow when you provided SAPPER_EXPORT variable in your environment.

More details about issue: