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Define custom parameters inside javascript compare functions in JavaScript?

I have an array of objects that needs sorting by last name, however the object only has firstname and fullname parameters, so for last name I have to use fullname.replace((firstname + " "), ""). Example array below.
const names = [
{
firstname: "John",
fullname: "John Doe"
},
{
firstname: "Amber",
fullname: "Amber Smith"
},
{
firstname: "Michael",
fullname: "Michael Smith"
},
{
firstname: "Jessica",
fullname: "Jessica Kelly Charles"
}
]
While I can use the "replace" every time inside a sort compare function, I'd much rather have something like this:
names.sort(function(a, b) {
const lastname = fullname.replace((firstname+ " "), "");
if (a.lastname < b.lastname) {
return -1;
} else if (a.lastname > b.lastname) {
return 1;
}
return 0;
});
Obviously lastname comes up as undefined.
This has been rather hard to google for, and I think I'm missing some JavaScript basics here, but would greatly apprecite your help in helping me learn to write better code.

Your best bet is to modify the source of the array so it stores lastname upon collection.
If you can't do that:
Unless you do a prep pass through your array adding a lastname property, you'll have to compute it each and every time your sort callback is called, for both a and b.
names.sort((a, b) => {
const alast = a.fullname.replace(a.firstname + " "), "");
const blast = b.fullname.replace(b.firstname + " "), "");
return alast.localeCompare(blast);
});
(Note I used localeCompare, which is always a better choice for names and other natural language strings than < and >. For instance, ask the French whether ç should really come after z as it does with < and >. ;-) )
That will recompute the lastname for the same object repeatedly, though, since the same object may be passed to sort (as either a or b) repeatedly. If you think that may be a problem, you could do that prep pass I mentioned:
// Build a map of entry to last names
const lastnames = new Map(names.map(entry => {
const lastname = entry.fullname.replace(entry.firstname + " ", "");
return [entry, lastname];
}));
// sort
names.sort((a, b) => {
return lastnames.get(a).localeCompare(lastnames.get(b));
});

You are just declaring a new variable trying to access it via object's property. You should map your data in the first place like:
const names = [
{
firstname: "John",
fullname: "John Doe"
},
{
firstname: "Amber",
fullname: "Amber Smith"
},
{
firstname: "Michael",
fullname: "Michael Smith"
},
{
firstname: "Jessica",
fullname: "Jessica Kelly Charles"
}
].map(person => ({
...person,
lastName: person.fullName.replace(person.firstName + ' ', '')
}));
After mapping your data like that, you can use .lastName in the place you need.
names.sort(function(a, b) {
if (a.lastName < b.lastName) {
return -1;
} else if (a.lastName > b.lastName) {
return 1;
}
return 0;
});

printing the data of objects by storing it in variables

I have an object named data and on console of data, I did :
var data={
countrycode: "376",
dob: "2017-05-24",
email: "bsbsbs#gmail.com",
firstName: "ggsgsggsg",
gender: "male",
phoneNumbers: "88888888888888888",
surName: "hshshhshs"
}
let countrycode,iso2,number,firstName,surName,dob,gender,email;
countrycode=data.countrycode;
number=data.phoneNumbers;
firstName=data.firstName;
surName=data.surName;
email:data.email;
gender:data.gender;
dob:data.dob;
console.log('email is',email);
console.log('number is ',number);
console.log('gender is',gender);
console.log('surName is',surName);
console.log('dob is',dob);
console.log('firstName is',firstName);
What I see that as i console log the values , i get undefined for email,gender and dob and for rest of the others , i get the correct values .
What can be the reason this and why does this happen ?

You have to actually use = as an assignment operator in the example you gave, colons are for object property assignment.
email = data.email;
gender = data.gender;
dob = data.dob;

Use = for assignment, not : (which is reserved for assigning values to an Object's property).
It would be
email=data.email;
Not
email:data.email;
var data={
countrycode: "376",
dob: "2017-05-24",
email: "bsbsbs#gmail.com",
firstName: "ggsgsggsg",
gender: "male",
phoneNumbers: "88888888888888888",
surName: "hshshhshs"
}
let countrycode,iso2,number,firstName,surName,dob,gender,email;
countrycode=data.countrycode;
number=data.phoneNumbers;
firstName=data.firstName;
surName=data.surName;
email=data.email;
gender=data.gender;
dob=data.dob;
console.log('email is ',email);
console.log('number is ',number);
console.log('gender is ',gender);
console.log('surName is ',surName);
console.log('dob is ',dob);
console.log('firstName is ',firstName);

Javascript - how to capitalize key.value within an object?

How do I capitalize the first letter of each contact's first name?
String.prototype.capitalize = function (string) { return string.charAt(0).toUpperCase() + string.slice(1);}
var contactList = {};
contactList.bobSmith = {
firstName: "bob",
lastName: "smith",
location: "new york"
};
contactList.johnDoe = {
firstName: "john",
lastName: "doe",
location: "san francisco"
};
var contact2 = contactList["johnDoe"].firstName;
contact1.capitalize();
contact2.capitalize();
console.log(contact1 + " " + contact2);
I get an error message that says "Uncaught TypeError: Cannot read property 'charAt' of undefined".

function () { return this.charAt(0).toUpperCase() + this.slice(1);}

Your String.prototype.capitalize function requires an argument string which you're not passing to it when called. I'd recommend avoiding adding methods to the String.prototype and just use a stand-alone function instead...
function capitalise(str) {
return str.slice(0,1).toUpperCase() + str.slice(1);
}
contact2 = capitalise(contact2);
//=> "John"
...or if you're looking to capitalise the value in the contactList object, then just...
contactList.johnDoe.firstName = capitalise(contact2);
Hope that helped. :)

I wouldn't make it a habit of popping methods into prototype. It's frowned upon because future versions of JavaScript could come out and put a method with the same name in the prototype and you would be screwed.
I didn't write it this way but you could put the capitalize function on to the contactList object and you would basically achieve the same thing your trying to do with the prototype.
var contactList = {};
contactList.names =[
{
firstName: "bob",
lastName: "smith",
location: "new york"
},
{
firstName: "john",
lastName: "doe",
location: "san francisco"
}];
function capitalizeName(name) {
var strCap = name[0].toUpperCase();
var tailTxt = name.substr(1, name.length);
fullTxt = strCap + tailTxt;
return fullTxt;
)
$.each( contactList.names, function(i){
capitalizeName(contactList.names[i].firstName);
});

The property() method of function class right using

Example in the documentation:
App.Person = Ember.Object.extend({
// these will be supplied by `create`
firstName: null,
lastName: null,
fullName: function() {
return this.get('firstName') + ' ' + this.get('lastName');
}.property('firstName', 'lastName')
});
var ironMan = App.Person.create({
firstName: "Tony",
lastName: "Stark"
});
ironMan.get('fullName') // "Tony Stark"
I can't understand why I must specify dependencies like .property('firstName', 'lastName') if I had indicated what needs to return: return this.get('firstName') + ' ' + this.get('lastName') and the example above can work without it (just using property()).
Thanks.

It's for caching.
From docs:
By default the function backing the computed property will only be
called once and the result will be cached. You can specify various
properties that your computed property is dependent on. This will
force the cached result to be recomputed if the dependencies are
modified.

custom javascript objects as properties of other custom javascript objects

I want to create some custom javascript objects some of which have properties which are other objects. I'm not sure what the syntax would be to do this. My pseudo code is below with the idea that I have a person object and an order object. The order object has a property that I want to be of the person object type. Is this possible in javascript and if so, can someone give me a basic example? Thanks.
var person {
name: "John",
gender: "M",
age: 3
}
var order {
customer: person, /*the property name is customer but of type person - is this possible?*/
total: 100
}

Consider constructors:
function Person( name, gender, age ) {
this.name = name;
this.gender = gender;
this.age = age;
}
function Order( customer, total ) {
this.customer = customer;
this.total = total;
}
Usage:
var person1 = new Person( 'John', 'M', 3 );
var order1 = new Order( person1, 100 );
The constructors act as classes. You invoke them via new to create new instances (persons and orders).

You were almost correct; all you need is to include some '='
var person = {
name: "John",
gender: "M",
age: 3
}
var order = {
customer: person, /*the property name is customer but of type person - is this possible?*/
total: 100
}

You could do this:
var person = {
name: "John",
gender: "M",
age: 3
};
var order = {
customer: person,
total: 100
};
This also passes in JSLint. You were missing '=' signs.

You code is nearly fine (missing the = to assign the anonymous object to the variable):
var person = {
name: "John",
gender: "M",
age: 3
};
var order = {
customer: person, /*the property name is customer but of type person - is this possible?*/
total: 100
};
http://jsfiddle.net/D7u3x/

Sure, thats pretty much it. Your syntax is a little off, but only slightly.
Here is your example: jsfiddle