Rhino 1.8.0 Debugger does not show block‑scoped (let) variables in scope when paused inside nested block (shadowing declaration)

[pratik2294]

Summary

When pausing at a let declaration that shadows an outer let inside a nested block, the Rhino JavaScript Debugger does not display any of the block‑scoped variables that should be in scope. Additionally, the Watch window does not evaluate them. This makes it impossible to inspect variables like b (which should definitely be in scope) and x (which, at that specific pause location, should at least be reported as “uninitialized (TDZ)” or cause a ReferenceError—not disappear).

Screenshot of Rhino JS Debugger

Minimal script to reproduce issue.

let a = 'top level';
let i = 2;
while (--i > 0) {
    let b = 'inside while loop';
    let x = 1;
    if (b === 'inside while loop') {
        let x = 2; // <-- set breakpoint here (line 7)
    }
}

[aardvark179]

Rhino doesn’t handle anything about lexical scoped variables correctly. The debugger is likely the least of your problems here.

[pratik2294]

@aardvark179
Is this a limitation by design in Rhino’s current implementation, or is it a bug?
If Rhino doesn’t represent lexical scopes correctly internally, how does it still manage to run most JavaScript code as expected?
Is it just limited to the debugger?

Also replacing let b = 'inside while loop'; with var b = 'inside while loop'; , seems to solve the problem. But it should've worked with let b = 'inside while loop';

[aardvark179]

A huge amount of JavaScript code will work without the spec compliant scoping rules, partly because of a lot of that code was written before the better lexical scoping rules were added to the specification, and even code that does use these features often doesn't depend heavily on them.

There are a few things that really need to be put in place to get lexical scoping working correctly and efficiently. We have done some prototyping internally which I think has helped solidify some ideas in this area, but I'm not sure quite when we'll get to them.