Other commenters have already covered the semantic difference of the two variants above. I wanted to note a stylistic difference: Only the "assignment" variation can set a property of another object.

I often build javascript modules with a pattern like this:

(function(){
        var exports = {};

        function privateUtil() {
                ...
        }

        exports.publicUtil = function() {
                ...
        };

        return exports;
})();

With this pattern, your public functions will all use assignment, while your private functions use declaration.

(Note also that assignment should require a semicolon after the statement, while declaration prohibits it.)

Other commenters have already covered the semantic difference of the two variants above. I wanted to note a stylistic difference: Only the "assignment" variation can set a property of another object.

I often build JavaScript modules with a pattern like this:

(function(){
    var exports = {};

    function privateUtil() {
            ...
    }

    exports.publicUtil = function() {
            ...
    };

    return exports;
})();

With this pattern, your public functions will all use assignment, while your private functions use declaration.

(Note also that assignment should require a semicolon after the statement, while declaration prohibits it.)