Web Storage APIって何

declare var sessionStorage: Storage;
declare var localStorage: Storage;

interface Storage {
    /**
     * Returns the number of key/value pairs.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Storage/length)
     */
    readonly length: number;
    /**
     * Removes all key/value pairs, if there are any.
     *
     * Dispatches a storage event on Window objects holding an equivalent Storage object.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Storage/clear)
     */
    clear(): void;
    /**
     * Returns the current value associated with the given key, or null if the given key does not exist.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Storage/getItem)
     */
    getItem(key: string): string | null;
    /**
     * Returns the name of the nth key, or null if n is greater than or equal to the number of key/value pairs.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Storage/key)
     */
    key(index: number): string | null;
    /**
     * Removes the key/value pair with the given key, if a key/value pair with the given key exists.
     *
     * Dispatches a storage event on Window objects holding an equivalent Storage object.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Storage/removeItem)
     */
    removeItem(key: string): void;
    /**
     * Sets the value of the pair identified by key to value, creating a new key/value pair if none existed for key previously.
     *
     * Throws a "QuotaExceededError" DOMException exception if the new value couldn't be set. (Setting could fail if, e.g., the user has disabled storage for the site, or if the quota has been exceeded.)
     *
     * Dispatches a storage event on Window objects holding an equivalent Storage object.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/Storage/setItem)
     */
    setItem(key: string, value: string): void;
    [name: string]: any;
}

declare var Storage: {
    prototype: Storage;
    new(): Storage;
};

interface WindowSessionStorage {
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/sessionStorage) */
    readonly sessionStorage: Storage;
}

interface WindowLocalStorage {
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/Window/localStorage) */
    readonly localStorage: Storage;
}

interface Window extends EventTarget, AnimationFrameProvider, GlobalEventHandlers, WindowEventHandlers, WindowLocalStorage, WindowOrWorkerGlobalScope, WindowSessionStorage {

declare var sessionStorage: Storage;
declare var localStorage: Storage;

addEventListener("storage", (event) => {});

/**
 * A StorageEvent is sent to a window when a storage area it has access to is changed within the context of another document.
 *
 * [MDN Reference](https://developer.mozilla.org/docs/Web/API/StorageEvent)
 */
interface StorageEvent extends Event {
    /**
     * Returns the key of the storage item being changed.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/StorageEvent/key)
     */
    readonly key: string | null;
    /**
     * Returns the new value of the key of the storage item whose value is being changed.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/StorageEvent/newValue)
     */
    readonly newValue: string | null;
    /**
     * Returns the old value of the key of the storage item whose value is being changed.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/StorageEvent/oldValue)
     */
    readonly oldValue: string | null;
    /**
     * Returns the Storage object that was affected.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/StorageEvent/storageArea)
     */
    readonly storageArea: Storage | null;
    /**
     * Returns the URL of the document whose storage item changed.
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/StorageEvent/url)
     */
    readonly url: string;
    /**
     * @deprecated
     *
     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/StorageEvent/initStorageEvent)
     */
    initStorageEvent(type: string, bubbles?: boolean, cancelable?: boolean, key?: string | null, oldValue?: string | null, newValue?: string | null, url?: string | URL, storageArea?: Storage | null): void;
}