Unexpected return values from CoffeeScript callbacks: implicit return leaks into promise chains

Implicit return behavior in compiled callbacks

CoffeeScript functions implicitly return the value of their last expression. When a callback's final statement is a loop, a comprehension, or an assignment, the compiled JavaScript returns a collected array or assigned value even though the author intended no return at all.

My concern is how this interacts with promise chains and event handlers in the compiled output. A non-undefined implicit return from a then callback becomes the resolution value of the next promise, and in some frameworks a truthy return from an event handler alters control flow. The documented escape hatch is a bare return with no value, but applying it consistently across a legacy codebase is error-prone.

There is also a version constraint: CoffeeScript 1.x and 2.x emit different JavaScript (ES5-style versus ES2015+ output), so the exact emitted code around the implicit return depends on the pinned compiler version.

1. Is a bare return the only reliable way to suppress implicit returns in callbacks, or are there expression forms that compile to undefined?
2. Does the implicit return behave differently in arrow functions emitted by CoffeeScript 2 compared to CoffeeScript 1 output?
3. Is there any compiler option or linting approach to flag unintended implicit returns before compilation?