This PR looks reasonable.

Can you please make this additional change?

--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1413,7 +1413,7 @@ merge(Compressor.prototype, {
             && !self.uses_with
            ) {
             var in_use = [];
-            var in_use_ids = {}; // avoid expensive linear scans of in_use
+            var in_use_ids = Object.create(null); // avoid expensive linear scans of in_use
             var initializations = new Dictionary();
             // pass 1: find out which symbols are directly used in
             // this scope (not in nested scopes).

Lookups may be faster due to no proto chain, and It corrects a potential hasOwnProperty issue.

Would you like to move that same-line comment as well?

If we use Object.create(null) we can replace those id in in_use_ids with in_use_ids[id] which will provide speed-ups.

Side Notes:

• dropping of function arguments is introduced in fcc0229
• dropping of function names is from da407d4

Would you like to move that same-line comment as well?

The code's pretty dense there. I think the trailing comment is okay in that case.

If we use Object.create(null) we can replace those id in in_use_ids with in_use_ids[id] which will provide speed-ups.

I'm not sure it would be faster without some empirical evidence. It will likely be the same speed. There's a chance it could be slower since a value is being returned. Can we leave it as is since I know it works?

test/benchmark.js checks out for 20cbcf3 👍