localStorage-compatible store backed by cookies. Spans across multiple cookies if data is too big for one cookie.

cookie-storage.js

/**
 * Simple storage interface which uses cookies. Mostly localStorage-compatible, but converts to JSON automatically.
 * There are plenty of packages which do this, but none that I could find split the data across multiple cookies if it
 * is too big for one cookie.
 */
window.cookieStorage = {
	setItem: function(key, value) {
		function splitString(string, size) {
			var re = new RegExp('.{1,' + size + '}', 'g');
			return string.match(re);
		};

		cookieStorage.removeItem(key);

		let expiryDate = new Date(2037, 0, 1, 0, 0, 0, 0).toUTCString();
		if (typeof value !== 'string') value = JSON.stringify(value);

		// Max size of cookies is around 4095 bytes, but need padding for base-64 encoding:
		// https://en.wikipedia.org/wiki/Base64#Output_padding
		let parts = splitString(value, 3000); 

		for (var i = 0; i < parts.length; i++)
			document.cookie = `${encodeURIComponent(key)}-${i}=${encodeURIComponent(parts[i])};path=/; secure=true; expires=${expiryDate}`;
	},

	getItem: function(key) {
		function getCookie(name) {
			var i, x, y, cookies = document.cookie.split(';');
			for (i = 0; i < cookies.length; i++) {
				x = cookies[i].substr(0, cookies[i].indexOf('='));
				y = cookies[i].substr(cookies[i].indexOf('=') + 1);
				x = x.replace(/^\s+|\s+$/g, '');
				if (x == name)
					return unescape(y);
			}
		};

		function tryParse(value) {
			let parsed;
			try {
				parsed = JSON.parse(value);
			} catch (e) {
				parsed = value;
			}
			return parsed;
		};

		var jsonStr = '';
		for (var i = 0; i < 20; i++) {
			var cookie = getCookie(`${encodeURIComponent(key)}-${i}`);
			if (typeof cookie == 'undefined') break;
			jsonStr += cookie;
		}

		return tryParse(jsonStr);
	},

	removeItem: function(key) {
		for (var i = 0; i < 20; i++)
			document.cookie = `${encodeURIComponent(key)}-${i}=; expires=Thu, 01 Jan 1970 00:00:00 UTC; path=/;`;
	},
};