Question

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JavaScript: Why the anonymous function wrapper?

Sometimes I see a structure like this in JS programs:

(function() {
 // blah blah
})();

Any ideas what’s this doing and what are its side-effects and benefits?

Answer 1

It’s to create a new scope (to avoid polluting the parent one). Surprisingly (to beginners familiar with C-like languages), JavaScript does not create a new scope when you use braces alone:

{
  var a = "foo";
}
// a still == "foo".

(function() {
  var b = "bar";
})();
// b doesn't exist.

However, functions always create a new scope. Thanks to JavaScript’s closures, existing variables from the parent scopes are automatically brought (closed) into the anonymous function. So in cases (like in the question) where the anonymous function is used nowhere else (since it’s never assigned), it basically acts like a C-style brace.