Should I always give a return value to my function?

Question

I write JavaScript code and I try to use its functional language nature.

In other functional languages (or even in Ruby), if I don't explicitly set the return value of a function, it will return the value of the last evaluated expression. JavaScript does not follow this pattern. (To be precise, JavaScript always returns a value as well. If nothing was set, then undefined.)

My question is the following: I have a function that doesn't need to (and does not) return a value. Does it make sense in a functional programming context to have a function with no explicit return value? Or did a fail somewhere if I found myself in this case?

For example, I have the following function. It checks periodically if location hash was changed, and if so, calls the given function.

LIB_hashManager = function(f, context) {
    var prev = '';
    var pollHash = function() {
        if (prev !== window.location.hash) {
            prev = window.location.hash;
            f.apply(context);
        }
    };
    window.setInterval(pollHash, 100);
};

Should I return here anything?

Update

Meanwhile it came to my mind, that if anytime in the future I'll need to extend the knowledge of LIB_hashManager, following the functional constructor pattern, I can simply add methods to an object and LIB_hashManager will return that produced object.

LIB_hashManager = function(f, context) {
    // inside logic
    // ...
};

And later I may write:

LIB_hashManager = function(f, context) {
    // inside logic
    // ...

    // return public methods
    return {
        // ...
    }
};

So doesn't make sense then to return an empty object in the first case?

Answer 1

A "pure" functional programming environment would have no side effects -- each function's work would be entirely in computing its return value; that's not really feasible in typical uses of Javascript, and so it's perfectly acceptable, when a function has done its work through side effects, to have it return nothing at all, i.e., be a "procedure" rather than a function.

Answer 2

My question is the following: I have a function that doesn't need to (and does not) return a value. Does it make sense in a functional programming context to have a function with no explicit return value? Or did a fail somewhere if I found myself in this case?

According to the academic description of a function: a function must give the same output given an input. A function that does not have any output is absolutely useless, because functions are not supposed to have any side effects.

However, since functional programming languages often need at least 1 side effect, the convention to return nothing is to return a unit or "()". Since this concept does not exist in Javascript, it shouldn't matter to you, since Javascript isn't strongly typed anyway.