JavaScript Breakdown of this code? Questions

[alvie666]

Hello everyone. I am relatively new to js and I have this bit of code I am trying to understand.
1) Why is the first line var for test_obj curly braces instead of brackets?
2) Why does the second line exist at all? Didn't we already declare it in line one? Why is it ".number"?
2) Why does line three follow with .number.number, and again, why isn't declaring this value on line one suffice?
Thanks for your help!

let test_obj = {};
test_obj.number = {};
test_obj.number.number = 8;
let new_number = test_obj.number;
new_number.number = new_number.number + 5;
console.log(test_obj.number.number);

 

[EkBass]

// Initializes test_obj. With [] it would be a array.
let test_obj = {};

// Adds a number property
test_obj.number = {};

// Adds a nested number property.
test_obj.number.number = 8;

// Creates a reference to the nested object
let new_number = test_obj.number;

// Updates nested number value.
new_number.number = new_number.number + 5;

// Output of the value
console.log(test_obj.number.number);

More fantastic help for JS adventures W3Schools.com

 

[alvie666]

// Initializes test_obj. With [] it would be a array.
let test_obj = {};

// Adds a number property
test_obj.number = {};

// Adds a nested number property.
test_obj.number.number = 8;

// Creates a reference to the nested object
let new_number = test_obj.number;

// Updates nested number value.
new_number.number = new_number.number + 5;

// Output of the value
console.log(test_obj.number.number);

More fantastic help for JS adventures W3Schools.com

Thank you! So initializing test_obj, using curly braces allows it to be undefined at first, then the property is set later?

 

[EkBass]

Yeah