ahbeng/NUSMods

import { size } from 'lodash';

let usableLocalStorage: Storage;

// Adapted from https://gist.github.com/juliocesar/926500
// Exported for unit tests
export function createLocalStorageShim(): Storage {
  const privData: Record<string, string> = {};

  const storage = {
    privData,

    get length() {
      return size(storage.privData);
    },

    key(index: number) {
      return Object.keys(storage.privData)[index];
    },

    clear() {
      storage.privData = {};
    },

    setItem(key: string, val: string) {
      storage.privData[key] = val;
    },

    getItem(key: string) {
      return storage.privData[key] || null;
    },

    removeItem: (key: string) => delete storage.privData[key],
  };

  return storage;
}

export function canUseBrowserLocalStorage() {
  try {
    // Ensure that accessing localStorage doesn't throw
    // Next line throws on Chrome with cookies disabled
    const storage = window.localStorage;

    // Ensure that localStorage isn't null
    // Resolves https://sentry.io/share/issue/d65da46a7e19406aaee298fb89a635d6/
    if (!storage) throw new Error();

    // Ensure that if setItem throws, it's not because of private browsing
    // If storage is empty AND setItem throws, we're probably in iOS <=10 private browsing
    if (storage.length === 0) {
      storage.setItem('____writetest', '1');
      storage.removeItem('____writetest');
    }

    // Only return true AFTER we know it can be used
    return true;
  } catch (e) {
    return false;
  }
}

// Returns localStorage if it can be used, if not then it returns a shim
export default function getLocalStorage() {
  // If we've performed all our checks before, just assume results will be the same
  // Key assumption: writability of localStorage doesn't change while page is loaded
  if (usableLocalStorage) return usableLocalStorage;

  // Sets usableLocalStorage on the first execution
  if (canUseBrowserLocalStorage()) {
    usableLocalStorage = window.localStorage;
  } else {
    usableLocalStorage = createLocalStorageShim();
  }

  return usableLocalStorage;
}