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 &&
                !this._rescc.immutable)
        ) {
            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 toNumberOrZero(this._rescc['s-maxage']);
            }
        }

        // 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 toNumberOrZero(this._rescc['max-age']);
        }

        const defaultMinTtl = this._rescc.immutable ? this._immutableMinTtl : 0;

        const serverDate = 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 < serverDate) {
                return 0;
            }
            return Math.max(defaultMinTtl, (expires - serverDate) / 1000);
        }

        if (this._resHeaders['last-modified']) {
            const lastModified = Date.parse(this._resHeaders['last-modified']);
            if (isFinite(lastModified) && serverDate > lastModified) {
                return Math.max(
                    defaultMinTtl,
                    ((serverDate - lastModified) / 1000) * this._cacheHeuristic
                );
            }
        }

        return defaultMinTtl;
    }
Value of applicable max-age (or heuristic equivalent) in seconds. This counts since response's `Date`.

For an up-to-date value, see `timeToLive()`.

@return Number