Extract an extendMap function as an attempt to create an optimization…

… boundary

packages/babel-plugin-transform-block-scoping/src/index.js

@@ -338,6 +338,12 @@ const loopVisitor = {
   },
 };
 
+function extendMap(map, obj) {
+  for (const key of Object.keys(obj)) {
+    map.set(key, obj[key]);
+  }
+}
+
 class BlockScoping {
   constructor(
     loopPath?: NodePath,
@@ -700,10 +706,7 @@ class BlockScoping {
       const init = this.loop.left || this.loop.init;
       if (isBlockScoped(init)) {
         declarators.push(init);
-        const names = t.getBindingIdentifiers(init);
-        for (const name of Object.keys(names)) {
-          this.outsideLetReferences.set(name, names[name]);
-        }
+        extendMap(this.outsideLetReferences, t.getBindingIdentifiers(init));
       }
     }
 
@@ -752,9 +755,7 @@ class BlockScoping {
       // declaration, rather than (for example) mistakenly including the
       // parameters of a function declaration. Fixes #4880.
       const keys = t.getBindingIdentifiers(declar, false, true);
-      for (const key of Object.keys(keys)) {
-        this.letReferences.set(key, keys[key]);
-      }
+      extendMap(this.letReferences, keys);
       this.hasLetReferences = true;
     }