How can I merge 2 javascript objects, populating the properties in one if they don't exist in the other?

Question

If I have a javascript object/assoc. array defined like this:

function somefunction(options) {

    var defaults = {
        prop1: 'foo',
        prop2: 'bar'
    };

    //Do stuff here

}

and I want to use this as the default values for the function. So when the function gets called I want to populate the options variable with the values in defaults, but only if they don't exist in options.

So lets say that this was called

somefunction({ prop1: 'fish' });

How can I make it so that options gets merged with defaults such that I get this

{
    prop1: 'fish',
    prop2: 'bar'
}

Answer 1

After re-reading the question, I realized you're probably looking for something more like this:

var a = { 'foo': 'bar', 'baz': 'bat' };
var b = { 'foo': 'quux' };
for (var prop in a) {
    if (prop in b) { continue; }
    b[prop] = a[prop];
}

Answer 2

You could look at either jQuery's or Prototypes extend functionality.

It looks like this: (taken directly from jQuery)

jQuery.extend = jQuery.fn.extend = function() {
    // copy reference to target object
    var target = arguments[0] || {}, i = 1, length = arguments.length, deep = false, options;

    // Handle a deep copy situation
    if ( typeof target === "boolean" ) {
        deep = target;
        target = arguments[1] || {};
        // skip the boolean and the target
        i = 2;
    }

    // Handle case when target is a string or something (possible in deep copy)
    if ( typeof target !== "object" && !jQuery.isFunction(target) )
        target = {};

    // extend jQuery itself if only one argument is passed
    if ( length == i ) {
        target = this;
        --i;
    }

    for ( ; i < length; i++ )
        // Only deal with non-null/undefined values
        if ( (options = arguments[ i ]) != null )
            // Extend the base object
            for ( var name in options ) {
                var src = target[ name ], copy = options[ name ];

                // Prevent never-ending loop
                if ( target === copy )
                    continue;

                // Recurse if we're merging object values
                if ( deep && copy && typeof copy === "object" && !copy.nodeType )
                    target[ name ] = jQuery.extend( deep, 
                        // Never move original objects, clone them
                        src || ( copy.length != null ? [ ] : { } )
                    , copy );

                // Don't bring in undefined values
                else if ( copy !== undefined )
                    target[ name ] = copy;

            }

    // Return the modified object
    return target;
};