How to push to array only reference to variable?

You have to create an instance of an object and push the object pointer onto the array. Right now the problem is that you are pushing undefined since you do not instantiate a Person object first. explicitly your code current looks like this:

var person1 = undefined;
var people = [];
people.push(person1);//pushing undefined unto the array
person1 = new Person();//now you have a pointer to an object

You have to first instantiate a Person then both your variable and array entry will point to the same thing

var person1 = new Person(); 
var people = [];
people.push(person1);//pushing a person reference onto the array
person1.someProperty = "lorem" 
people[0].someProperty = "ipsum"
//both person1.someProperty and people[0].someProperty are "ipsum"

Similarly you can instantiate the person directly in the array and create a variable referencing a object in the array.

var people = [new Person()]; //creates an array with a new person object.
people.push(new Person); //adds a new person to the array
var person1 = people[0];
var person2 = people[1];
person1.greet(person2);

There are no pointers in javascript. person1 is not going to be a pointer.

This variable is going to reference a value. When the variable is used, the current value is referenced. So, when this is used:

var person1;
people = [];
people.push(person1);

undefined shows up because that is what the current value of person1 was. Any future change to person1 will change the value of person1, but it will not affect any of the values in the person array.

It would be more appropriate to take a simpler approach here and just pass in the new value you create

people = [];
people.push(new Person("Mike",25,"London"));

first of all, you need to define people as an array var people = []

then push to array a default personnew person()for example, and then refer to person by index people[1] (for example), and then change that... to what you want

The only thing you can do is:

var person1 = new Person();
var people  = [];
people.push(person1);


people[0].name = "Mike";
people[0].age  = 25;
people[0].city = "London";