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I have been working on a project in JavaScript which requires a data structure (read only), to be shared between two functions.

var mySharedData = ['hours', 'minutes', 'seconds'];

Now I have two functions that need access to this (static) read only data structure.

var sampleFunction1 = function(userSuppliedData) {
    //map over user data applying mySharedData to it
};

var sampleFunction2 = function(userSuppliedData) {
    //reduce user data also accessing mySharedData
};

Since this is JavaScript and both functions are in the same scope I could just "cheat" and leverage the bad scoping of var and access the shared data in both functions but I don't feel like this is the proper way of doing it.

I also considered currying both functions and just passing the shared data as the first argument like so:

var mySharedData = ['hours', 'minutes', 'seconds'];

var sampleFunction1 = _.curry(function(sharedData, userSuppliedData) {
    //map over user data applying sharedData to it
})(mySharedData);

var sampleFunction2 = _.curry(function(sharedData, userSuppliedData) {
    //reduce user data also accessing sharedData
})(mySharedData);

What is the recommended way of sharing data between functions? Should this even be done in functional programming or am I making my functions impure with shared data structures?

share|improve this question
    
The shared data would be common to both closures –  Basile Starynkevitch Aug 22 at 17:00
    
So you suggest doing something like this? function() { //now I'm in a closure var mySharedData = ['hours', 'minutes', 'seconds']; var sampleFunction1 = function(userSuppliedData) { //map over user data applying mySharedData to it }; var sampleFunction2 = function(userSuppliedData) { //reduce user data also accessing mySharedData }; } –  KanskjeBen Aug 22 at 17:42
    
Every user function is implemented as a closure in Javascript –  Basile Starynkevitch Aug 22 at 17:51
2  
Why would you consider using the closure as cheating? –  Idan Arye Aug 22 at 18:41

1 Answer 1

up vote 1 down vote accepted

If mySharedData is private for both closures:

(function (context) {
    var mySharedData = ['hours', 'minutes', 'seconds'];

    context.sampleFunction1 = function (userSuppliedData) {
        //map over user data applying mySharedData to it
        // mySharedData....
    };

    context.sampleFunction2 = function (userSuppliedData) {
        //reduce user data also accessing mySharedData
        // mySharedData....
    };
})(this);

If mySharedData is public:

var mySharedData = ['hours', 'minutes', 'seconds'];

var sampleFunction1 = function (userSuppliedData) {
    //map over user data applying mySharedData to it
    // mySharedData...
};

var sampleFunction2 = function (userSuppliedData) {
    //reduce user data also accessing mySharedData
    // mySharedData...
};

If what you need is that mySharedData can not be edited:

Object.defineProperty(this, 'mySharedData', {
    get: function () {
        return ['hours', 'minutes', 'seconds'];
    }
});

console.log(mySharedData);
// ['hours', 'minutes', 'seconds']

mySharedData.push('foo');
mySharedData[1] = 'bar';

console.log(mySharedData);
// ['hours', 'minutes', 'seconds']
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