disappearing a local variable

[masuhar]

how to reproduce

1. paste or upload the attached program
2. go to the beginning of line 166, where stack, this, maxNode and current are shown.
3. move the cursor to just left of the brace, where current disappears.

2026-02-24-kanon-bug.js

[masuhar]

Problem minimized.

2026-02-24-kanon-bug-minimized.js

[masuhar]

This looks to be an instrumenter's bug. The checkpoint at the end of the loop, which should include local variable current, does not.

[masuhar]

Kanon/src/js/code-instrumentation/ast-transforms.js

Lines 2350 to 2354 in da9b449

	[__$__.ASTTransforms.changedGraphStmt(),
	__$__.ASTTransforms.makeCheckpoint(start, variables),
	node,
	__$__.ASTTransforms.changedGraphStmt(),
	__$__.ASTTransforms.makeCheckpoint(end, variables)

This code fragment transforms a block of statements, where it inserts a checkpoint inserted at the bottom with variables. Since it is the list of variables at the beginning of the block, it does not contain local variables declared in the statements in the block.

[masuhar]

InsertCheckPoint() is adding local variables. Perhaps this is not done before the above checkpoint insertion.

Kanon/src/js/code-instrumentation/ast-transforms.js

Lines 2110 to 2131 in da9b449

	if (s.type === 'VariableDeclaration' && s.kind === 'var') {
	s.declarations.forEach(declarator => {
	__$__.ASTTransforms.varEnv.addVariable(declarator.id.name.slice(1, declarator.id.name.length), s.kind, false);
	});
	}
	});
	}
	node.params.forEach(param => {
	if(param instanceof Object) __$__.ASTTransforms.varEnv.addVariable(param.name, "var", true)
	});
	}
	if (node.type === 'BlockStatement') {
	__$__.ASTTransforms.varEnv.push(new __$__.Probe.BlockFlame());
	node.body.forEach(s => {
	if (s.type === 'VariableDeclaration' && s.kind !== 'var') {
	s.declarations.forEach(declarator => {
	__$__.ASTTransforms.varEnv.addVariable(declarator.id.name, s.kind, false);
	});

[masuhar]

• InsertCheckPoint() returns a visitor object, where
  • enter performs ...varEnv.addVariable(...) to register local variables
  • leave calls transformBlockStatement(start,end,variables,b,node,path), which inserts check points at the beginning and the end of each block
• how varEnv related to local variable lists that are used by transformBlockStatement?

[masuhar]

transformBlockStatement is called from leave(node,path,enterData) of a visitor returned by InsertCheckPoint().

[masuhar]

InsertCheckPoint visitor is created and called in instrument.js The visitor itself is called from walkAST in ast-walker.js

[masuhar]

on 2161 in ast-transform.js, it pops a local variable frame when the node is BlockStatement. After that, it extracts local variables on 2168. Maybe this should have been done before 2161.

[masuhar]

• the root cause is that the transformer inserts a block inside of a block. For example, a code if(true){ let x = 1; } is transformed to

if(true) {
  checkpoint
  {
     checkpoint
     let x = 1;
     checkpoint
   }
  checkpoint
}

• as a result, x cannot be seen at the last checkpoint.
• another cause is that the last checkpoint only collects variables available outside of the (outer) block. This can be fixed.

[masuhar]

find the code that makes nested blocks. It happens regardless the statements inside the block.
Input: if(true){ true; } Output:

let __loopLabels = ['main'], __loopCount = 1, __loopCounterObject = {}, __time_counter = 0, __call_count = {}, __newObjectIds = [], __newExpInfo = [], __stackForCallTree = [__$__.CallTree.rootNode];
__objs = [];
try {
    __$__.Checkpoint.checkpoint(__objs, __loopLabels.last(), __loopCount, __time_counter++, 5, { this: this }, __newExpInfo.last(), __stackForCallTree.last().getContextSensitiveID());
    if (true) {
        __$__.Context.ChangedGraph = true;
        __$__.Checkpoint.checkpoint(__objs, __loopLabels.last(), __loopCount, __time_counter++, 3, { this: this }, __newExpInfo.last(), __stackForCallTree.last().getContextSensitiveID());
        {
            __$__.Checkpoint.checkpoint(__objs, __loopLabels.last(), __loopCount, __time_counter++, 1, { this: this }, __newExpInfo.last(), __stackForCallTree.last().getContextSensitiveID());
            true;
            __$__.Context.ChangedGraph = true;
            __$__.Checkpoint.checkpoint(__objs, __loopLabels.last(), __loopCount, __time_counter++, 2, { this: this }, __newExpInfo.last(), __stackForCallTree.last().getContextSensitiveID());
        }
        __$__.Context.ChangedGraph = true;
        __$__.Checkpoint.checkpoint(__objs, __loopLabels.last(), __loopCount, __time_counter++, 4, { this: this }, __newExpInfo.last(), __stackForCallTree.last().getContextSensitiveID());
    }
    __$__.Context.ChangedGraph = true;
    __$__.Checkpoint.checkpoint(__objs, __loopLabels.last(), __loopCount, __time_counter++, 6, { this: this }, __newExpInfo.last(), __stackForCallTree.last().getContextSensitiveID());
} finally {
}