Local Storage Wrapper

localStorageWrapper.js

var localStorageWrapper = {
    contains: function (key) {
        return this.keys().indexOf(key) !== -1;
    },

    getItem: function (key) {
        return localStorage.getItem(key);
    },

    keys: function () {
        var meta = JSON.parse(localStorage.getItem(this.metadataKey)) || { bytes: 0, chain: [] };
        return meta.chain.map(function (v) { return v.key; });
    },

    metadataKey: '__meta',

    removeItem: function (key) {
        var meta = JSON.parse(localStorage.getItem(this.metadataKey)) || { bytes: 0, chain: [] };

        for (var i = 0; i < meta.chain.length; i++) {
            if (meta.chain[i].key === key) {
                meta.bytes -= meta.chain[i].size;
                meta.chain.splice(i, 1);
                break;
            }
        }

        localStorage.removeItem(key);
        localStorage.setItem(this.metadataKey, JSON.stringify(meta));
    },

    setItem: function (key, val) {
        while (true) {
            var meta = JSON.parse(localStorage.getItem(this.metadataKey)) || { bytes: 0, chain: [] };
            var valSize = encodeURI(val).split(/%..|./).length - 1;

            if (this.keys().indexOf(key) !== -1) {
                this.removeItem(key);
                continue;
            }

            meta.bytes += valSize;
            meta.chain.push({
                key: key,
                size: valSize
            });

            try {
                localStorage.setItem(key, val);
                localStorage.setItem(this.metadataKey, JSON.stringify(meta));
                return;
            } catch (e) {
                if (meta.chain.length === 1) return;
                this.removeItem(meta.chain[0].key);
            }
        }
    },

    usage: function () {
        var meta = JSON.parse(localStorage.getItem(this.metadataKey)) || { bytes: 0, chain: [] };
        return meta.bytes;
    }
};