Configurable cache heuristic

README.md

@@ -35,11 +35,14 @@ const response = {
 
 const options = {
     shared: true,
+    cacheHeuristic: 0.1,
 };
 ```
 
 If `options.shared` is true (default), then response is evaluated from perspective of a shared cache (i.e. `private` is not cacheable and `s-maxage` is respected). If `options.shared` is false, then response is evaluated from perspective of a single-user cache (i.e. `private` is cacheable and `s-maxage` is ignored).
 
+`options.cacheHeuristic` is a fraction of response's age that is used as a fallback cache duration. The default is 0.1 (10%), e.g. if a file hasn't been modified for 100 days, it'll be cached for 100*0.1 = 10 days.
+
 ### `satisfiesWithoutRevalidation(request)`
 
 If it returns `true`, then the given `request` matches the response this cache policy has been created with, and the existing response can be used without contacting the server.

index.js

@@ -20,7 +20,7 @@ function parseCacheControl(header) {
     return cc;
 }
 
-function CachePolicy(req, res, {shared} = {}) {
+function CachePolicy(req, res, {shared, cacheHeuristic} = {}) {
     if (!res || !res.headers) {
         throw Error("Response headers missing");
     }
@@ -30,6 +30,8 @@ function CachePolicy(req, res, {shared} = {}) {
 
     this._responseTime = this.now();
     this._isShared = shared !== false;
+    this._cacheHeuristic = undefined !== cacheHeuristic ? cacheHeuristic : 0.1; // 10% matches IE
+
     this._status = 'status' in res ? res.status : 200;
     this._resHeaders = res.headers;
     this._rescc = parseCacheControl(res.headers['cache-control']);
@@ -204,7 +206,7 @@ CachePolicy.prototype = {
         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 (dateValue - lastModified)/1000 * this._cacheHeuristic;
             }
         }
         return 0;