Public and Private Methods in JavaScript

At Ning, here’s how we do public and private methods in JavaScript:

/**
 * A scrollable list that can display large numbers of contacts.
 *
 * @param contacts  contact objects
 */
xp.ContactSelector = function(args) {

    /** Container for public functions. */
    var self = {};

    /** Container for private functions. */
    var _ = {};

    /**
     * Initializes the object.
     */
    _.initialize = function() {
        if (!args.contacts) { 
            _.installSearchbox();
        }
    };

    /**
     * Adds a searchbox to the ContactSelector.
     */
    _.installSearchbox = function() {
        // This is a private method
    };

    /**
     * Returns the contacts that the user has selected
     *
     * @return  the selected contact objects
     */
    self.getSelectedContacts = function() {
        // This is a public method
    };
    
    . . . . . . . . . .
    
    _.initialize();
    return self;
};

Private methods begin with _., while public methods begin with self.. There are some interesting things about this setup:

• Public methods are accessible on the object created by this function. Private methods are not accessible.
• Public and private methods can be defined in any order, regardless of which calls which. This is because they are properties of self and _. If they were defined as standalone functions, then order would matter.
• You can inherit from another object by adding the following to initialize(): self = xp.AbstractContactSelector(args);

The first point is similar to Douglas Crockford’s technique described in Private Members in JavaScript.