用cookie模拟localStorage

 function setCookie(key, value = '', days) {
        if (!key) return
        let date = new Date()
        date.setTime(date.getTime() + days * 24 * 60 * 60 * 1000); // 设置过期时间为 days 天之后
        document.cookie = key + "=" + decodeURIComponent(value) + ";expires=" + date.toGMTString();
    }

    function getCookie(key) {
        let result = ''
        let cookie = document.cookie + ";"
        let queryString = key + "="
        let start = cookie.indexOf(queryString)
        if (start > -1) {
            start += queryString.length
            let end = cookie.indexOf(";", start)
            result = (cookie.substring(start, end))
        }
        // return getCookies()[key]
        return result
    }

    function getCookies() {
        let result = {}
        if (!document.cookie) return result
        document.cookie.split(';').forEach((item) => {
            const [key, value] = item.split('=')
            result[key.trim()] = value.trim()
        })
        return result
    }

    window.localStorage = window.localStorage || new LocalStorage()

    function LocalStorage() {
        const cookies = getCookies()
        this.length = Object.keys(cookies).length
        for (let [key, value] of Object.entries(cookies)) {
            this[key] = value
        }
    }

    LocalStorage.prototype = function () {
        function getItem(key) {
            return getCookie(key);
        }

        function setItem(key, value, days) {
            this[key] = value
            this.length++
            setCookie(key, value, days)
        }

        function removeItem(key) {
            this.length--
            delete this[key]
            setItem(key, '', -1);
        }

        function clear() {
            const cookies = getCookies();
            for (let key of Object.keys(cookies)) {
                delete this[key]
                removeItem(key);
            }
            this.length = 0
        }

        return {
            constructor: LocalStorage,
            getItem,
            setItem,
            removeItem,
            clear,
        }
    }()