Method join

[Kacper-Leman]

Soultion to tests

[Radoslaw-Czerniawski]

Needs some refactoring

[Radoslaw-Czerniawski]

You may use the separator parameter and get rid of this declaration along with if, if you add a default value in the parameter declaration

[].__proto__.join2 = function(separator = ',') {

[Radoslaw-Czerniawski]

I'd say arr variable is unnecessary, from the context of this implementation it's implied this refers to an array

[Radoslaw-Czerniawski]

If you need an index regular for loop with let i = 0; i < this.length; i++ would be better

[Radoslaw-Czerniawski]

element.toString(); -> regarding this part, I personally am not sure whether all data types have .toString method, it would be safer to user something like String(element) or interpolate ${element}

result += this[i] === null || this[i] === undefined ? '' : String(this[i])

[Radoslaw-Czerniawski]

If you go with the previously suggested changes this here could be shortened to:

if (i < this.length - 1) {
  result += String(separator)
}

[Kacper-Leman]

I refactor this code like you said and yea is shorter and more readable :D

[DorotaLeniecDev]

Good job one suggestion from my side, for more readability, you can apply it
Approving 💪

[DorotaLeniecDev]

In my opinion this would be even more readable:

      if (this[i] !== undefined && this[i] !== null) {
        result += String(this[i]);
      }