Issue 3673 - Use separate function to chain promises

lib/abp2blocklist.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 50 matching lines @@
       setTimeout(call, 0);
   });
 }
 
 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)
   {
     if (domain != "")
@@ skipping 1214 matching lines @@
       .then(rules =>
       {
         ruleGroups[index] = rules;
         return next();
       });
     }
 
     return next();
   });
 };