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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
The only way I know how to get around this is:
if (typeof process.send === 'function') { 
    process.send('what I want to send');
                If you mean something different than the question states, then please edit your question to correct it.
– jfriend00
                Jun 2, 2015 at 0:50
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 () {};
                this is super lame IMO. all processes should have a parent - process.send should do nothing if nobody is listening, just like every other pub/sub type API. but it's ok, we can just litter the code with conditionals, as I mention in the edit to the OP
– Alexander Mills
                Jun 2, 2015 at 0:52
                Yeah there is likely a reason.  For now you could just put process.send = process.send || function () {}; at the head of your 'root' file.
– Alex McMillan
                Jun 2, 2015 at 0:54
                thanks Mr. McMillan, you seem to get it. Your solution looks like no worse then any others. This is a nasty one. I hope there is a really good reason for this...doubtful.
– Alexander Mills
                Jun 2, 2015 at 0:59
                @AlexMills If you're "littering" your code with conditionals for this, something is probably wrong.  Parent/child processes typically share very little code in a well-designed system.
– Aaron Dufour
                Jun 2, 2015 at 4:07
                @AlexMills Sure. process.send uses a JSON-over-IPC-fd mechanism, which is completely specific to node. It doesn't make sense to use that to try to send messages to a non-node process. So why not default it to a no-op? Because making a meaningfully-named method a no-op is disastrously undiscoverable and confusing. And you can easily turn it into a no-op yourself as this answer makes clear, whereas the opposite isn't really true (how would you determine that it's a no-op when its only effect is in another process?).
– Aaron Dufour
                Jun 2, 2015 at 17:13
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: https://github.com/Zimtir/SENT-template/issues/114
        
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