Bug: Generator in method call fails with augmented assignment (+=)

[swernerx]

Description

Generator expressions passed directly to method calls like .join() are not being converted to proper JavaScript syntax.

Python Input

result = fieldsep.join(repr(name) for name in names)
total = sum(x * 2 for x in values)
any_null = any(item is None for item in items)

Current Output (v1.3.3)

let result = fieldsep.join(repr(name) for name in names)  // ❌ INVALID!
let total = sum(x * 2 for x in values)  // ❌ INVALID!
let any_null = any(item is None for item in items)  // ❌ INVALID!

Expected Output

const result = fieldsep.join(names.map(name => repr(name)))
const total = sum(values.map(x => x * 2))
const any_null = any(items.map(item => item === null))

Or with IIFE generators (consistent with standalone generators):

const result = fieldsep.join((function*() { for (const name of names) yield repr(name) })())

Note

Standalone generator expressions ARE correctly converted to IIFE generators:

gen = (x for x in items)  # Works! → (function*() { ... })()

But when passed directly to a method call, they're not converted.

Affected Files

Found ~124 occurrences in NumPy migration.

Priority

🔴 Critical - Causes ~400 errors

[swernerx]

This was fixed in commit ff08efa. Generator expressions in .join() calls are now correctly converted:

result = fieldsep.join(repr(name) for name in names)

→

const result = [
  ...(function* () {
    for (const name of names) {
      yield repr(name)
    }
  })()
].join(fieldsep)

The fix hasn't been released to npm yet (still at v1.3.3).

[swernerx]

Found the specific edge case!

The bug occurs when using augmented assignment (+=) with a method call containing a generator expression:

# ✅ Works:
result = fieldsep.join(repr(name) for name in names)

# ❌ Fails (augmented assignment):
ret += fieldsep.join(repr(name) for name in names)

Current Output:

ret += fieldsep.join(repr(name) for name in names)  // Invalid!

Expected:

ret += [...(function*() { for (const name of names) yield repr(name) })()].join(fieldsep)

This pattern is used ~100+ times in NumPy for building strings incrementally.