Extend Methods in JavaScript

jQuery with deep extend.js

/*

  -> http://api.jquery.com/jQuery.extend/

  jQuery.extend( target [, object1] [, objectN] )

  * target An object that will receive the new properties if additional objects are passed in or that will extend the jQuery namespace if it is the sole argument.
  * object1An object containing additional properties to merge in.
  * objectNAdditional objects containing properties to merge in.

 */

jQuery.extend = jQuery.fn.extend = function() {
    var options, name, src, copy, copyIsArray, clone,
        target = arguments[0] || {},
        i = 1,
        length = arguments.length,
        deep = false;

    // 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 ( name in options ) {
                src = target[ name ];
                copy = options[ name ];

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

                // Recurse if we're merging plain objects or arrays
                if ( deep && copy && ( jQuery.isPlainObject(copy) || (copyIsArray = jQuery.isArray(copy)) ) ) {
                    if ( copyIsArray ) {
                        copyIsArray = false;
                        clone = src && jQuery.isArray(src) ? src : [];
                    } else {
                        clone = src && jQuery.isPlainObject(src) ? src : {};
                    }

                    // Never move original objects, clone them
                    target[ name ] = jQuery.extend( deep, clone, copy );

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

    // Return the modified object
    return target;
};

Underscore.js

/*

  -> http://underscorejs.org/#extend

  extend(destination, *sources)

  Copy all of the properties in the source objects over to the destination object, and return the destination object.
  It's in-order, so the last source will override properties of the same name in previous arguments. 

 */

  var slice = Array.prototype.slice, nativeForEach = Array.prototype.forEach, breaker = {};

  var each = function(obj, iterator, context) {
    if (obj == null) return;
    if (nativeForEach && obj.forEach === nativeForEach) {
      obj.forEach(iterator, context);
    } else if (obj.length === +obj.length) {
      for (var i = 0, l = obj.length; i < l; i++) {
        if (iterator.call(context, obj[i], i, obj) === breaker) return;
      }
    } else {
      for (var key in obj) {
        if (obj.hasOwnProperty(key)) {
          if (iterator.call(context, obj[key], key, obj) === breaker) return;
        }
      }
    }
  };

  var extend = function(obj) {
    each(slice.call(arguments, 1), function(source) {
      for (var prop in source) {
        obj[prop] = source[prop];
      }
    });
    return obj;
  };