Nodejs fork callback executed repeated times

Question

I'm working on Express 4 and I have moved one heavy task to a child process. Everything is working fine except that the callback in the parent process is somehow executed more than once. That, is causing the error "can't set headers after they are sent" Why is the first thing happening? Parent process:

var cp = require('child_process').fork('./child.js');
cp.send(data);
cp.on('message', function(data){
    console.log('status: '+data.status);
    return res.status(200).json(data);
});

Forked process:

process.on('message', function(data){
    /*process the data*/
    process.send({status: 200});
});

Result:

/*first time*/
status: 200
/*second time*/
status: 200
status: 200
/*third time*/
status: 200
status: 200
status: 200
/*random time*/
status: 200
status: 200
status: 200
status: 200

Pier-Luc Gendreau · Accepted Answer · 2022-07-08 13:53:16Z

1

You are creating a new listener every time, which is why the callback is called n+1 times.

Try using .once() instead of .on()

Since most libraries using events in Node.js inherit from EventEmitter, you will find related documentation on that page.

edited Jul 8, 2022 at 13:53

answered Dec 26, 2015 at 22:00

Pier-Luc Gendreau

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3 Comments

user3945438 Over a year ago

Great, .once() is not listed in the offical docs, it worked perfectly.

Amir Alam Over a year ago

well if once is not listed in official docs, how are supposed to find more functions that are not listed? by trying typing keywords randomly?

Pier-Luc Gendreau Over a year ago

once has been in the docs since Node 0.10 so that comment is extremely outdated.

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