Object extending - supports N... arguments

extend.js

function extend() {
  var args = [].slice.call(arguments);
  args[0] = (typeof args[0] === 'object') ? args[0] : {};
  for (var i=1; i<args.length; i++) {
    if (typeof args[i] === 'object') {
      for (var key in args[i]) {
        if (args[i].hasOwnProperty(key)) {
          args[0][key] = args[i][key];
        }
      }
    }
  }
  return args[0];
}

extend_test.js

describe('#extend', function() {
  it('should return the first object', function() {
    var testObj = {};
    assert.equal(extend(testObj), testObj);
  });

  it('should copy properties from the second object to the first', function() {
    var obj1 = { one: 1 };
    var obj2 = { two: 2 };
    extend(obj1, obj2);
    assert.equal(obj1.two, 2);
  });

  it('should not modify properties from the second object', function() {
    var obj1 = { one: 1 };
    var obj2 = { two: 2 };
    extend(obj1, obj2);
    assert.deepEqual({two: 2}, obj2);
  });

  it('should overwrite properties with the same name from right to left', function() {
    var obj1 = { one: 1 };
    var obj2 = { one: 2 };
    extend(obj1, obj2);
    assert.equal(obj1.one, 2);
  });

  it('should support n number of objects to extend from', function() {
    assert.deepEqual({
      name: 'Force Push',
      power: 100,
      available: true
    }, extend({name: 'force push', power: 50}, {power: 75}, {available: true, power: 100}, {name: 'Force Push'}));
  });
});