Question

From jquery.ui.autocomplete.js:

_renderMenu: function( ul, items ) {
    var self = this;
    $.each( items, function( index, item ) {
        self._renderItem( ul, item );
    });
},

Answer 1

Underscore prefixes in javascript are mostly used to signify “private” functions, properties etc. I.e., functions and properties that are used internally and should not be accessed from outside the library/plugin.

And yes, as nnnnnn says, the important word there is “should”, as in:

• It can be accessed – Javascript won’t do anything to stop you – it’s just a naming convention.
• It would be ill-advised to actually do it. Mostly, the author will have good reason to indicate internal use. E.g. a property which may not behave as you think (for example cached values that won’t be available in all cases); a function may depend on specific internal state or have side effects on that state; etc.