http-cache-semantics/index.js

'use strict';
// rfc7231 6.1
const statusCodeCacheableByDefault = [200, 203, 204, 206, 300, 301, 404, 405, 410, 414, 501];

// This implementation does not understand partial responses (206)
const understoodStatuses = [200, 204, 301, 302, 303, 404, 410, 501];

function parseCacheControl(header) {
    const cc = {};
    if (!header) return cc;

    // TODO: When there is more than one value present for a given directive (e.g., two Expires header fields, multiple Cache-Control: max-age directives),
    // the directive's value is considered invalid. Caches are encouraged to consider responses that have invalid freshness information to be stale
    const parts = header.trim().split(/\s*,\s*/); // TODO: lame parsing
    for(const part of parts) {
        const [k,v] = part.split(/\s*=\s*/, 2);
        cc[k] = (v === undefined) ? true : v.replace(/^"|"$/g, ''); // TODO: lame unquoting
    }

    return cc;
}

function CachePolicy(req, res, {shared} = {}) {
    if (!res || !res.headers) {
        throw Error("Response headers missing");
    }
    if (!req || !req.headers) {
        throw Error("Request headers missing");
    }

    this._responseTime = this.now();
    this._isShared = shared !== false;
    this._status = 'status' in res ? res.status : 200;
    this._resHeaders = res.headers;
    this._rescc = parseCacheControl(res.headers['cache-control']);
    this._method = 'method' in req ? req.method : 'GET';
    this._url = req.url;
    this._reqHeaders = req.headers;
    this._reqcc = parseCacheControl(req.headers['cache-control']);

    // When the Cache-Control header field is not present in a request, caches MUST consider the no-cache request pragma-directive
    // as having the same effect as if "Cache-Control: no-cache" were present (see Section 5.2.1).
    if (!res.headers['cache-control'] && /no-cache/.test(res.headers.pragma)) {
        this._rescc['no-cache'] = true;
    }
}

CachePolicy.prototype = {
    now() {
        return Date.now();
    },

    storable() {
        // The "no-store" request directive indicates that a cache MUST NOT store any part of either this request or any response to it.
        return !this._reqcc['no-store'] &&
            // A cache MUST NOT store a response to any request, unless:
            // The request method is understood by the cache and defined as being cacheable, and
            ('GET' === this._method || 'HEAD' === this._method || ('POST' === this._method && this._hasExplicitExpiration())) &&
            // the response status code is understood by the cache, and
            understoodStatuses.includes(this._status) &&
            // the "no-store" cache directive does not appear in request or response header fields, and
            !this._rescc['no-store'] &&
            // the "private" response directive does not appear in the response, if the cache is shared, and
            (!this._isShared || !this._rescc.private) &&
            // the Authorization header field does not appear in the request, if the cache is shared,
            (!this._isShared || !this._reqHeaders.authorization || this._allowsStoringAuthenticated()) &&
            // the response either:
            (
                // contains an Expires header field, or
                this._resHeaders.expires ||
                // contains a max-age response directive, or
                // contains a s-maxage response directive and the cache is shared, or
                // contains a public response directive.
                this._rescc.public || this._rescc['max-age'] || this._rescc['s-maxage'] ||
                // has a status code that is defined as cacheable by default
                statusCodeCacheableByDefault.includes(this._status)
            );
    },

    _hasExplicitExpiration() {
        // 4.2.1 Calculating Freshness Lifetime
        return (this._isShared && this._rescc['s-maxage']) ||
            this._rescc['max-age'] ||
            this._resHeaders.expires;
    },

    satisfiesWithoutRevalidation(req) {
        if (!req || !req.headers) {
            throw Error("Request headers missing");
        }

        // When presented with a request, a cache MUST NOT reuse a stored response, unless:
        // the presented request does not contain the no-cache pragma (Section 5.4), nor the no-cache cache directive,
        // unless the stored response is successfully validated (Section 4.3), and
        const requestCC = parseCacheControl(req.headers['cache-control']);
        if (requestCC['no-cache'] || /no-cache/.test(req.headers.pragma)) {
            return false;
        }

        // The presented effective request URI and that of the stored response match, and
        return (!this._url || this._url === req.url) &&
            (this._reqHeaders.host === req.headers.host) &&
            // the request method associated with the stored response allows it to be used for the presented request, and
            (!req.method || this._method === req.method) &&
            // selecting header fields nominated by the stored response (if any) match those presented, and
            this._varyMatches(req) &&
            // the stored response is either:
            // fresh, or allowed to be served stale
            !this.stale() // TODO: allow stale
    },

    _allowsStoringAuthenticated() {
        //  following Cache-Control response directives (Section 5.2.2) have such an effect: must-revalidate, public, and s-maxage.
        return this._rescc['must-revalidate'] || this._rescc.public || this._rescc['s-maxage'];
    },

    _varyMatches(req) {
        if (!this._resHeaders.vary) {
            return true;
        }

        // A Vary header field-value of "*" always fails to match
        if (this._reqHeaders.vary === '*') {
            return false;
        }

        const fields = this._resHeaders.vary.trim().toLowerCase().split(/\s*,\s*/);
        for(const name of fields) {
            if (req.headers[name] !== this._reqHeaders[name]) return false;
        }
        return true;
    },

    /**
     * Value of the Date response header or current time if Date was demed invalid
     * @return timestamp
     */
    date() {
        const dateValue = Date.parse(this._resHeaders.date)
        const maxClockDrift = 8*3600*1000;
        if (Number.isNaN(dateValue) || dateValue < this._responseTime-maxClockDrift || dateValue > this._responseTime+maxClockDrift) {
            return this._responseTime;
        }
        return dateValue;
    },

    /**
     * Value of the Age header, in seconds, updated for the current time
     * @return Number
     */
    age() {
        let age = Math.max(0, (this._responseTime - this.date())/1000);
        if (this._resHeaders.age) {
            let ageValue = parseInt(this._resHeaders.age);
            if (isFinite(ageValue)) {
                if (ageValue > age) age = ageValue;
            }
        }

        const residentTime = (this.now() - this._responseTime)/1000;
        return age + residentTime;
    },

    maxAge() {
        if (!this.storable() || this._rescc['no-cache']) {
            return 0;
        }

        // Shared responses with cookies are cacheable according to the RFC, but IMHO it'd be unwise to do so by default
        // so this implementation requires explicit opt-in via public header
        if (this._isShared && (this._resHeaders['set-cookie'] && !this._rescc.public)) {
            return 0;
        }

        if (this._resHeaders.vary === '*') {
            return 0;
        }

        if (this._isShared) {
            if (this._rescc['proxy-revalidate']) {
                return 0;
            }
            // if a response includes the s-maxage directive, a shared cache recipient MUST ignore the Expires field.
            if (this._rescc['s-maxage']) {
                return parseInt(this._rescc['s-maxage'], 10);
            }
        }

        // If a response includes a Cache-Control field with the max-age directive, a recipient MUST ignore the Expires field.
        if (this._rescc['max-age']) {
            return parseInt(this._rescc['max-age'], 10);
        }

        const dateValue = this.date();
        if (this._resHeaders.expires) {
            const expires = Date.parse(this._resHeaders.expires);
            // A cache recipient MUST interpret invalid date formats, especially the value "0", as representing a time in the past (i.e., "already expired").
            if (Number.isNaN(expires) || expires < dateValue) {
                return 0;
            }
            return (expires - dateValue)/1000;
        }

        if (this._resHeaders['last-modified']) {
            const lastModified = Date.parse(this._resHeaders['last-modified']);
            if (isFinite(lastModified) && dateValue > lastModified) {
                return (dateValue - lastModified) * 0.00001; // In absence of other information cache for 1% of item's age
            }
        }
        return 0;
    },

    stale() {
        return this.maxAge() <= this.age();
    },
};

module.exports = CachePolicy;