Object Property Assignment

https://www.codecademy.com/paths/web-development/tracks/web-dev-js-arrays-loops-objects/modules/learn-javascript-objects/lessons/objects/exercises/add-property
Why we don’t use ‘let’ or ‘var’ keyword when we create object property?

Because we don’t want our object variable to be reassigned. We only allow changes in the key values but not the object as a whole.

I know but how object know that it is a new property?