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// define a function

function foo() {
  var bar = 1;
};

// or

var foo = function() {
  var bar = 1;
};

// invoke foo

foo();

// if:
//   foo === function() { var bar = 1; }
//
// then:
//   foo() is equivalent to (function() { var bar = 1; })();

// why do this:

(function() {
  var bar = 1;
})();

// instead of this?

var foo = function() {
  var bar = 1;
};

foo();

// because the latter leaves a reference to foo in the
// local scope, which you may not need