Ta daa.

join.js

"use strict"; 

var _ = {
	join: (array, separator = ",") => {
		var str = "";
		for (var i = 0; i < array.length; i++) {
			str += separator + array[i];
		}
		return str.substring(2);
	}
}

const value = _.join(["hello", "goodbye"], " , ")

console.log(value);

const value2 = _.join(["dogs", "cats", "birds"], " + ")

console.log(value2);