a unit testable localStorage mock

fakeWindowLocalStorage.js

const fakeWindowLocalStorage = {
  localStorage: {
    data: {},
    length: 0,
    setItem(name, value) {
      this.data[name] = value.toString();
      this.length = Object.keys(this.data).length;
    },
    getItem(name) {
      return this.data[name];
    },
    removeItem(name) {
      delete this.data[name];
      this.length = Object.keys(this.data).length;
    },
  },
};