index.js

var alphabet = "абвгдеёжзийклмнопрстуфхцчшщъыьэюя";
var max_num_of_symbols = 6;
var num_of_symbols = 0;
var randomString = "";

console.log("alphabet.length = ", alphabet.length);

while(num_of_symbols < max_num_of_symbols){
	var index = Math.floor(Math.random() * alphabet.length);
	randomString += alphabet[index]; // randomString = randomString + alphabet[index];
	num_of_symbols++;
}

console.log("randomString = ", randomString);