Sending anonymous functions through socket.io?

Question

I want to create a client-side function that can receive and execute arbitrary commands using client-side variables. I will be sending these functions from my server by using socket.io to send a JSON object containing an anonymous function which will be my command. It looks something like the following:

//client side

socket.on('executecommand', function(data){
    var a = "foo";
    data.execute(a); //should produce "foo"
});

//server side

socket.emit('executecommand', {'execute': function(param){
    console.log(param);
}});

Yet, when I tried it out, the client side received an empty json object (data == {}), then threw an exception because data contained no method execute. What is going wrong here?

Answer 1

JSON doesn't support the inclusion of function definitions/expressions.

What you can do instead is to define a commands object with the functions you need and just pass a commandName:

// client-side

var commands = {
    log: function (param) {
        console.log(param);
    }
};

socket.on('executecommand', function(data){
    var a = 'foo';
    commands[data.commandName](a);
});

// server-side

socket.emit('executecommand', { commandName: 'log' });

You can also use fn.apply() to pass arguments and check the commandName matches a command with in:

// client-side
var commands = { /* ... */ };

socket.on('executecommand', function(data){
    if (data.commandName in commands) {
        commands[data.commandName].apply(null, data.arguments || []);
    } else {
        console.error('Unrecognized command', data.commandName);
    }
});

// server-side

socket.emit('executecommand', {
    commandName: 'log',
    arguments: [ 'foo' ]
});

Answer 2

You can't send literal JavaScript functions and expect it to work. You'll need to stringify the function first (i.e put it within a set of quotes), then eval the string on the client side.