Merge Two Objects

merge

/*
Write a function that will recursively merge two objects with the following conditions:
1.) If a[field] is an array, and b[field] is defined and is not an array, add b[field] to the array
2.) If a[field] is an array an b[field] exists but is undefined or null, set a[field] to an empty array
3.) If a[filed] is an array and b[field] is an array, set a[field] to b[field]
4.) If a[field] exists and b[field] exists but is undefined, delete a[field]
5.) If b[field] is a non-complex type (number, string, boolean, et cetera), copy to a[field]
*/

var a = {
    first_name: 'Bob',
    last_name: 'Jones',
    
    email: 'bob@gmail.com',
    
    address: {
        line_1: '1234 Main St',
        line_2: 'Apt 413',
        city: 'Los Angeles',
        state: 'CA',
        zip: '90048'
    },
    
    logins: [
        { date: '10/22/2012', ip: '192.168.0.1' },
        { date: '10/21/2012', ip: '192.168.0.1' }
    ],
    
    photos: [
        'IMG-1985.jpg',
        'IMG-1987.jpg'
    ]
}

var b = {
    last_name: 'Jones',
    active: true,

    address: {
        line_1: '2143 South Main St',
        line_2: undefined
    },
    
    logins: { date: '10/23/2012', ip: '192.168.0.1' },
    
    photos: undefined
}

var result = {
    first_name: 'Bob',
    last_name: 'Jones',
    
    active: true,
    email: 'bob@gmail.com',
    
    address: {
        line_1: '2143 South Main St',
        city: 'Los Angeles',
        state: 'CA',
        zip: '90048'
    },
    
    logins: [
        { date: '10/22/2012', ip: '192.168.0.1' },
        { date: '10/21/2012', ip: '192.168.0.1' },
        { date: '10/23/2012', ip: '192.168.0.1' }
    ],
    
    photos: []
}

//merges b into a
var merge = function(a, b) {
	for (var field in b) {
		if (a.hasOwnProperty(field)) {
			if (a[field] instanceof Array) {
				//If a[field] is an array, and b[field] is defined and is not an array, add b[field] to the array
				if (typeof b[field] != "undefined" && !(b[field] instanceof Array)) {
					a[field].push(b[field])
				} else if (typeof b[field] === "undefined" || b[field] == null) { //If a[field] is an array an b[field] exists but is undefined or null, set a[field] to an empty array
					a[field] = new Array();
				} else if (b[field] instanceof Array) { //If a[field] is an array and b[field] is an array, set a[field] to b[field]
					a[field] = b[field];
				}   
			} else if (b[field] == undefined) { //If a[field] exists and b[field] exists but is undefined, delete a[field]
				delete a[field];
			} else if (!(b[field] instanceof Object)) { //If b[field] is a non-complex type (number, string, boolean, et cetera), copy to a[field]
				a[field] = b[field];
			} else if (b[field] != null && b[field] instanceof Object) {
				merge(a[field], b[field]);
			}
		} else {
			a[field] = b[field];
		}
	}
}