JS Deep Dive 11 capter

Today, I'm going to organize what I've studied over the past few days.
There is a book called 'The Modern JS Deep Dive.'
This is one of the most famous books about JS in Korea, so I chose it.
 

so What I studied? 
It is about object literals.
First of all, before explaining the types of objects, we should understand what an 'object' is. 
It is a combination of property keys and values.
Ex) 
var p = {
    name: 'lee',
    age: 8
}
In this case, `name` is the key, and 'lee' is the value.

 Types of objects
There are two types:

        one is a reference type, and the other is a primitive type.
A primitive type is an immutable value, so we can't modify the value after assigning it to an object.

Ex)
const a = 10;
a = 20; 
On the surface, it seems like the value is changed, but it isn't.
Actually, the value is saved in memory, so even if you try to change something in a primitive type, the original value remains unchanged, and only the reference to the new value is updated.

And you can think of a reference type as being more flexible.
You can write a method as part of a reference type's object.
  Ex) 
       var p = {
       name: 'lee',
       sayHello: function() {
            console.log(`hello! my name is ${this.name}`);
        }
  }
As you can see, there is a function within the `p` object.

Rules for creating keys and values
And there are some rules for creating keys and values.

1. If you want to make a string value, you should use double quotes ("") or single quotes ('').
2. You can't use '-' in a key without quotes. For example, 'last-name' should be written as `lastName` or 'last-name'.
3. You can use words like `var`, `function`, etc. as keys, but it can cause issues, so it's better not to use them.
4. If you use the same key in an object, it's like a duplicate, so the last value will overwrite the original one.
Ex) 
var foo = {
    name: 'kim',
    name: 'lee'
}
console.log(foo); // {name :'lee'}

 how we can access properties??
You can access a value like an array using bracket notation. 
If the key is not a number, you should use quotes ('') to access the value.
Ex) 
var foo = {
    name: 'kim'
};
console.log(foo.name); // or console.log(foo['name']);
If the key is a number, we can access the value in two ways:
1. `foo[1]`
2. `foo['1']`
You don't need to use quotes ('') when accessing a numerical key.