semantic-network/lib/representation/checkHeaders.js

A utility library for manipulating a list of links that form a semantic interface to a network of resources.

import { parse } from 'cache-control-parser';
/**
 * The goal is to leave all heavy lifting to the browser (ie implement caching rules). The key issue
 * here is whether to return the in-memory resource or push through to the browser request (ie xhr).
 */
export class CheckHeaders {
    /**
     * The main issue is whether "time" is up and a potential refresh is required. This calculation is the
     * last-modified + max-age. The server provides this as an absolute date in the expires header.
     */
    static checkExpiresHeaderStrategy(headers, now) {
        const { expires = undefined, } = headers;
        /*
         * The goal is to leave all heavy lifting to the browser (ie implement caching rules). The key issue
         * here is whether to return the in-memory resource or push through to the browser request (ie xhr).
         *
           */
        if (expires) {
            return now > new Date(expires);
        }
        return false;
    }
    /**
     * Caters for the non-sensical situation where the Expires header is BEHIND the Date header.
     * @param headers
     */
    static checkExpiresBeforeDateHeaderStrategy(headers) {
        const { date = undefined, expires = undefined, } = headers;
        return !!(expires && date && new Date(date) > new Date(expires));
    }
    /**
     * Looks through the 'cache-control' headers and checks for max-age and no-cache.
     */
    static checkNoCacheHeaderStrategy(headers) {
        const { 'cache-control': cacheControl = undefined, } = headers;
        if (cacheControl) {
            const { 
            /* in seconds */
            'max-age': maxAge = undefined, 'no-cache': noCache = undefined, } = parse(cacheControl);
            if (maxAge === 0 || noCache) {
                return true;
            }
        }
        return false;
    }
    /**
     * Looks through the 'cache-control' headers and checks for expiry as the last served ('date') plus max-age being reached
     */
    static checkMaxAgeHeaderStrategy(headers, now) {
        const { 'cache-control': cacheControl = undefined, 
        // date will need to be exposed (eg as CORS headers—Access-Control-Expose-Headers: Date)
        'date': date = undefined, } = headers;
        if (date && cacheControl) {
            const { 
            /* in seconds */
            'max-age': maxAge = undefined, } = parse(cacheControl);
            if (maxAge) {
                const lastServed = new Date(date);
                lastServed.setTime(lastServed.getTime() + (maxAge || 0));
                return now > lastServed;
            }
        }
        return false;
    }
}
CheckHeaders.defaultStrategies = [
    CheckHeaders.checkExpiresBeforeDateHeaderStrategy,
    CheckHeaders.checkExpiresHeaderStrategy,
    CheckHeaders.checkNoCacheHeaderStrategy,
    CheckHeaders.checkMaxAgeHeaderStrategy,
];
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