Is there a benefit to using a return statement that returns nothing?

I'm refactoring a large javascript document that I picked up from an open source project. A number of functions use inconsistent return statements. Here's a simple example of what I mean:

var func = function(param) {
    if (!param) {
        return;
    }
    // do stuff
    return true;
}

Sometimes the functions return boolean, sometimes strings or other things. Usually they are inconsistently paired with a simple return; statement inside of a conditional.

The problem is that the code is complex. It is a parser that uses a multitude of unique RegEx matches, creates and destroys DOM nodes on the fly, etc. Preliminary testing shows that, in the above example, I could change the return; statement to become return false;, but I'm concerned that I may not realize that it had a negative impact (i.e. some feature stopped working) on the script until much later.

So my questions: Is there a benefit to using a blank return statement? Could this have been intentionally coded this way or was it just lazy? Can I change them all to return false;, or return null; or do I need to dig through every call and find out what they are doing with the results of those functions?

Using return without a value will return the value undefined.

If the value is evaluated as a boolean, undefined will work as false, but if the value for example is compared to false, you will get a different behaviour:

var x; // x is undefined
alert(x); // shows "undefined"
alert(!x); // shows "true"
alert(x==false); // shows "false"

So, while the code should logically return true or false, not true or undefined, you can't just change return; to return false; without checking how the return value is used.

"Blank return" statements can be used to transfer the control back to the calling function (or stop executing a function for some reason - ex: validations etc). In most cases I use blank return statement is when I'm doing some kind of a validation. However, I make it a point to set some indicator as to why the execution of the function is stopped. For example, set the "innerText" property on a DIV element with the error message.

In the code above, it looks like it is a validation. The function returns a "true" if everything went well. It looks like the calling function parses the return value, and if it is "true", next step of statements (in the calling function) are executed.

It is a good practice to return "false" instead of a blank return in the above example. That way you make it all uniform and make life easy for other programmers.

You could fix such inconsistencies; however, make sure you test all the changes thoroughly. It is a good practice to test each change you make to the code, however small it may be.

What MIGHT be lost here (not direct with your example) is that you can then have a tri-state object:

var myfunc = function(testparam) {
    if (typeof testparam === 'undefined') return;
    if (testparam) {
        return true;
    }
    else {
        return false;
    }
};

var thefirst = myfunc(true)
var thesecond = myfunc(false);
var thelast = myfunc();
alert("type:" + typeof thefirst+" value:"+thefirst);
alert("type:" + typeof thesecond+" value:"+thesecond);  
alert("type:" + typeof thelast+" value:"+thelast); 

these return:

> type:boolean:true 
> type:boolean:false
> type:undefined:undefined

note: null would return false in this example myfunc(null);

There is no difference at all between return; and return undefined;. The result of calling both functions is to receive the value undefined.

(There's a very small specification-level difference between a function body that terminates with return vs. just falling off the end of the code, but it's nothing that can be detected in code.¹ Calling a function where execution falls off the end of the code also results in the value undefined.)

"use strict";

// Implicit return of `undefined`
function x() {
    return;
}
// Explicit return of `undefined`
function y() {
    return undefined;
}
// Execution falls off the end
function z() {
}
console.log(typeof x() === "undefined"); // true
console.log(typeof y() === "undefined"); // true
console.log(typeof z() === "undefined"); // true

Unless, of course, something has shadowed undefined. Which is still sadly possible (though not, gladly, at global scope). In that very edgy edge case, there's a difference:

"use strict";
(function() {
    const undefined = 42;
//  ^^^^^^^^^^^^^^^---- shadowing `undefined`
    // Implicit return of `undefined`
    function x() {
        return;
    }
    // Explicit return of `undefined`
    function y() {
        return undefined;
    }
    // Execution falls off the end
    function z() {
    }
    console.log(typeof x() === "undefined"); // true, `x` returns the canonical `undefined`
    console.log(typeof y() === "undefined"); // false, `y` returns 42
    console.log(typeof z() === "undefined"); // true, `z` (effectively) returns the canonical `undefined`
})();

¹ Using return is an abrupt completion that [[Call]] converts to a normal completion w/value. Falling off the end of the code is a normal completion (spec) (that [[Call]] ensures supplies undefined for the value). But again, this is a specification level difference, not something that's observable in code.