Node.js – How to emit an event and pass a function as a parameter with socket.io

node.jssocket.io

I am using socket.io and I am trying to emit an event from my server and pass an object with a function as a parameter. Here is my code:

  socket.emit('customEvent', {
    name : "Test".
    myFunc : function() {
        //some logic here
    }
  });

and then in the client (my app in the browser) I am able to access 'name' property but when I try to access 'myFunc' but I get 'undefined' for it. Here is my code

socket.on('customEvent', function(data){
    data.myFunc();
});

What is the correct approach for this (if it is possible at all)?

Best Answer

The data is transmitted as JSON, so it can't contain functions. Maybe you're looking for what's called 'acknowledgments' in socket.io's documentation?

// server
socket.on('customEvent', function (data, fn) {
    fn(data.x)
})

// client
socket.emit('customEvent', { x: 1 }, function(){
    // ...
})

Standard Method (no library)

The arguments are stored in process.argv

Here are the node docs on handling command line args:

process.argv is an array containing the command line arguments. The first element will be 'node', the second element will be the name of the JavaScript file. The next elements will be any additional command line arguments.

// print process.argv
process.argv.forEach(function (val, index, array) {
  console.log(index + ': ' + val);
});

This will generate:

$ node process-2.js one two=three four
0: node
1: /Users/mjr/work/node/process-2.js
2: one
3: two=three
4: four

Javascript – the purpose of Node.js module.exports and how do you use it

module.exports is the object that's actually returned as the result of a require call.

The exports variable is initially set to that same object (i.e. it's a shorthand "alias"), so in the module code you would usually write something like this:

let myFunc1 = function() { ... };
let myFunc2 = function() { ... };
exports.myFunc1 = myFunc1;
exports.myFunc2 = myFunc2;

to export (or "expose") the internally scoped functions myFunc1 and myFunc2.

And in the calling code you would use:

const m = require('./mymodule');
m.myFunc1();

where the last line shows how the result of require is (usually) just a plain object whose properties may be accessed.

NB: if you overwrite exports then it will no longer refer to module.exports. So if you wish to assign a new object (or a function reference) to exports then you should also assign that new object to module.exports

It's worth noting that the name added to the exports object does not have to be the same as the module's internally scoped name for the value that you're adding, so you could have:

let myVeryLongInternalName = function() { ... };
exports.shortName = myVeryLongInternalName;
// add other objects, functions, as required

followed by:

const m = require('./mymodule');
m.shortName(); // invokes module.myVeryLongInternalName

Best Answer

Related Solutions

Javascript – How to pass command line arguments to a Node.js program

Standard Method (no library)

Javascript – the purpose of Node.js module.exports and how do you use it

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