CODEWITHSUNDEEP
javascriptnode.js
luckyging3r
luckyging3r

Reputation: 3165

When to use an anonymous function in Nodejs?

Recently I was debugging a bit of code that one of my coworkers wrote and found something along the lines of this within a conditional, inside of a larger function.

(function(foo){
    ...
    console.log(foo);
    ...
})();

So it looks like this very boiled down

function display(db, user, foo) {
    if (isValid(db, user) {
        (function(foo){
            console.log(foo);
        })();
    }
    ...
}

Originally it didn't have (); at the end so to my understanding it wasn't even being called but to my broader question, what's the difference between that and something like below? I've seen this syntax a few times but I don't understand how above would be useful for this purpose. My understanding would reason that this just adds unnecessary complexity. Someone please enlighten me!

function display(db, user, foo) {
    if (isValid(db, user) {
        // without abstract function
        console.log(foo);
    }
    ...
}

Thanks :-)

Upvotes: 1

Views: 143

Answers (2)

Eriks Klotins
Eriks Klotins

Reputation: 4180

Sometimes you use an anonymous function to limit the scope of your code. It may come handy if you do not want to pollute the global scope with your variables. Also, it makes dependency injection for testing easier.

Consider the following examples:

var a = 2, b = 3;
function print(x)
{
    console.log('x= ', x);
}
print(a+b);

In the above example, the a, b, and print now belongs to the window object. When your code grows it becomes difficult to keep track and avoid of name collisions. The problem becomes even more severe if you are using 3rd party libs etc.

In the example below, your code is wrapped in an anonymous function protecting the global scope. You can also pass parameters to the anonymous function 

(function(localParam){
    var a = 2, b = 3;
    function print(x)
    {
        console.log('x= ', x);
    }
    print(a+b);
    //also
    console.log(localParam); // = safe way to access and global variables
})(globalParam);

Upvotes: 1

Ian MacDonald
Ian MacDonald

Reputation: 14010

With such a simple example, there is no reason to use an anonymous function like this in node.js or any other JavaScript environment when the inner function could be called directly instead.

When conducting reviews, I often will assume that any console.log (or choose-your-language logging) is accidental. If the surrounding source doesn't do anything of value itself, it is likely only intended to support a (possibly-out-of-date) console.log during development and debugging.

Because JavaScript is single-threaded, constructing an anonymous function of this sort is strictly for scoping. In the case of the original source that didn't have the execution (i.e. it was missing the ()), this is actually a way to "hide" debug code from the runtime while leaving it in-place if a step-through debug ever needed it.

When debugging, it's as easy as adding the ('isValid succeeded') to have it execute all the source inside.

(function(foo){
    console.log(foo);
})( /* You're missing a parameter value here; will print 'undefined'. */ );

Upvotes: 1

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