CODEWITHSUNDEEP
javascriptclosures
Paul S.
Paul S.

Reputation: 1573

Why wrap a function definition in an immediate function?

Is there any advantage of wrapping a function definition in an immediate anonymous function?

Here is an example from the jsfeat library:

var get_channel = (function () {
    return function(type) {
        return (type & 0xFF);
    }
})();

Or is it better to just do the following?

var get_channel = function(type) {
    return (type & 0xFF);
};

It seems that in this case there are no advantages in favour of the first version:

  1. It's longer and harder to read,
  2. It takes up more memory, because the closure keeps a reference to the outer activation object, but there is no useful data in it,
  3. Sometimes it would be slower (at least in theory), because it takes longer to access global variables if the engine has to go through a longer scope chain.

Upvotes: 2

Views: 118

Answers (1)

Paul S.
Paul S.

Reputation: 66324

There are some advantages, but they aren't visible in the example you've provided. The advantages are that you can

  1. Create a cleaner function by having data captured in scope, making function logic not obstructed or obfuscated by the data. This is especially helpful if the data needs to be constructed but is expected to be the same every time thereafter, or if you want to have data shared across multiple invocations which does not cause conflicts in the global namespace.
  2. Choose what function to return depending on the environment or some calculation. This saves you have to run the same calculations over and over again.

For example, you could make a cross-browser XHR function like this

var XHR = (function () {
        if (window.XMLHttpRequest)
            return function () {
                return new XMLHttpRequest();
            };
        else
            return function () {
                return new ActiveXObject("Microsoft.XMLHTTP");
            };
    }());

Now future calls of XHR don't need to work out the if logic, which would always be the same anyway.

Upvotes: 2

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