Object-oriented – the recommended approach for helper functions in JavaScript

What is the recommended approach for helper functions? I would like to choose one technique, and run with it to create my new "class".

Here are the design options I have pondered:

Option 1: Helper function in outer scope, invoke with context of instance

function createPane (pane) {
    // logic to create pane
    var proto = Object.create(this.paneList);
    $.extend(paneProto, pane);
    return paneProto;
}

Panes.prototype.initialize = function (panes) {
    var _this = this;
    _.each(panes, function () {
        _this.panes.push(createPane.call(_this, this));
    });
}

• Pros: Simple syntax. createPane is not published on the instance.
• Cons: createPane is accessible in other scopes.

Option 2: Helper function in closure, invoke with context of instance

Panes.prototype.initialize = (function () {

    function createPane (pane) {
        // same logic as last createPane
    }

    return function (panes) {
        // same logic as before - calls createPane
    }

})();

• Pros: createPane is not published on the instance.
• Cons: Lower readability and testability; Testing of this helper must occur in the scope of initialize.

Option 3: Prepend _ to name to indicate a private method

Panes.prototype._createPane = function (pane) {
    // same logic as last createPane
}

Panes.prototype.initialize = function (panes) {
    // same logic as last, except calls this._createPane
}

• Pros: Implicit context of _createPane is the instance. Testability from the outside.
• Cons: Exposing helper function on the instance.

Option 4: Helper functions as arguments

Panes.prototype.initialize = (function (createPane) {

    return function (panes) {
        // same logic as before - calls createPane
    }

})(function createPane () {
   // same logic as last createPane
});

• Pros: createPane is not published on the instance. Helper functions lack access to each other.
• Cons: Lower readability and testability; Testing of this helper must occur in the scope of initialize.