localStorage/sessionStorage mock

storage.js

class Storage {
  constructor() {
    this.__valuesMap = new Map()
  }

  getItem(key) {
    return this.__valuesMap.has(key)
      ? String(this.__valuesMap.get(String(key)))
      : null
  }

  setItem(key, val) {
    this.__valuesMap.set(String(key), String(val))
  }

  removeItem(key) {
    this.__valuesMap.delete(key)
  }

  clear() {
    this.__valuesMap.clear()
  }

  key(i) {
    if (!arguments.length) {
      // this is a TypeError implemented on Chrome, Firefox throws Not enough arguments to Storage.key.
      throw new TypeError(
        "Failed to execute 'key' on 'Storage': 1 argument required, but only 0 present.",
      )
    }
    return Array.from(this.__valuesMap.keys())[i]
  }

  get length() {
    return this.__valuesMap.size
  }

  set length(val) {}
}

const getterSetter = instance => ({
  set: function(obj, prop, value) {
    if (Storage.prototype.hasOwnProperty(prop)) {
      instance[prop] = value
    } else {
      instance.setItem(prop, value)
    }
    return true
  },
  get: function(target, name) {
    if (Storage.prototype.hasOwnProperty(name) || name === '__valuesMap') {
      return instance[name]
    }
    if (instance.__valuesMap.has(name)) {
      return instance.getItem(name)
    }
  },
})

const localInstance = new Storage()
const sessionInstance = new Storage()
global.localStorage = new Proxy(localInstance, getterSetter(localInstance))
global.sessionStorage = new Proxy(
  sessionInstance,
  getterSetter(sessionInstance),
)