Issue 3673 - Use separate function to chain promises

lib/abp2blocklist.js

Index: lib/abp2blocklist.js
===================================================================
--- a/lib/abp2blocklist.js
+++ b/lib/abp2blocklist.js
@@ -63,42 +63,48 @@
 }
 
 function async(callees, mapFunction)
 {
   if (!(Symbol.iterator in callees))
     callees = [callees];
 
   let lastPause = Date.now();
-  let index = 0;
 
   let promise = Promise.resolve();
 
-  for (let next of callees)
+  let chain = (next, index) =>
   {
-    let currentIndex = index;
-
     promise = promise.then(() =>
     {
       if (mapFunction)
-        next = mapFunction(next, currentIndex);
+        next = mapFunction(next, index);
 
       // If it has been 100ms or longer since the last call, take a pause. This
       // keeps the browser from freezing up.
       let now = Date.now();
       if (now - lastPause >= 100)
       {
         lastPause = now;
         return callLater(next);
       }
 
       return next();
     });
+  };
 
-    index++;
+  // We must iterate manually here, because JSHydra does not generate correct
+  // code for let..of over non-array iterables.
+  let it = callees[Symbol.iterator]();
+  for (let next = it.next(), i = 0; !next.done; next = it.next(), i++)
+  {
+    // This must be a function call, because the value of i is evaluated
+    // lazily. let inside a loop does automatically get a closure, but JSHydra
+    // translates let to var so we lose that benefit.
+    chain(next.value, i);
   }
 
   return promise;
 }
 
 function parseDomains(domains, included, excluded)
 {
   for (let domain in domains)