what does the symbol # inside a class javascript?

what does it means declare variables with the symbol "#" in javascript? I am learning about builder pattern and I found it with some examples. Also I saw that the variables are calls with this.#color.

# symbol is different to declare variable with let or var? what is the difference?

thanks

class Car {

    #color = null
    #spoiler = null
    #fuelType = null
    #productionDate = null

}

>Solution :

Private class members

class ClassWithPrivate {
  #privateField;
  #privateFieldWithInitializer = 42;

  #privateMethod() {
    // …
  }

  static #privateStaticField;
  static #privateStaticFieldWithInitializer = 42;

  static #privateStaticMethod() {
    // …
  }
}