kutaisan-react-native-mmkv-storage/src/keygen.js

This library aims to provide a fast & reliable solution for you data storage needs in react-native apps. It uses [MMKV](https://github.com/Tencent/MMKV) by Tencent under the hood on Android and iOS both that is used by their WeChat app(more than 1 Billion

const lowercase = "abcdefghijklmnopqrstuvwxyz";
const uppercase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const numbers = "0123456789";

const all = lowercase + uppercase + numbers;

export default function generatePassword() {
  let password = "";

  password += pick(password, lowercase, 1, 5);
  password += pick(password, uppercase, 1, 5);
  password += pick(password, all, 10);

  return shuffle(password);
}

function pick(exclusions, string, min, max) {
  var n,
    chars = "";

  if (max === undefined) {
    n = min;
  } else {
    n = min + Math.floor(Math.random() * (max - min + 1));
  }

  var i = 0;
  while (i < n) {
    const character = string.charAt(Math.floor(Math.random() * string.length));
    if (exclusions.indexOf(character) < 0 && chars.indexOf(character) < 0) {
      chars += character;
      i++;
    }
  }

  return chars;
}

function shuffle(string) {
  var array = string.split("");
  var tmp,
    current,
    top = array.length;

  if (top)
    while (--top) {
      current = Math.floor(Math.random() * (top + 1));
      tmp = array[current];
      array[current] = array[top];
      array[top] = tmp;
    }

  return array.join("");
}