Updates:
Status: WorkingAsIntended
Owner: [email protected]
Comment #1 on issue 3491 by [email protected]: ReferenceError when using
variable in closure, in Object.keys(), in debugger
http://code.google.com/p/v8/issues/detail?id=3491
In your example, foo is not bound to the inner function since it does not
use it. Therefore, it is stack allocated.
The test case in d8 would be:
Debug = debug.Debug;
function f() {
var foo = 1;
Object.keys({a: 1, a: 2}).forEach(function(k) {
debugger;
});
}
function listener(event, exec_state, event_data, data) {
if (event == Debug.DebugEvent.Break) {
print(exec_state.frame(0).sourceLineText());
print(exec_state.frame(1).evaluate("foo").value());
}
}
Debug.setListener(listener);
f();
Running that with --print-scopes, we get
function f () { // (32, 134)
// scope has trivial outer context
// 1 stack slots
// local vars:
VAR foo; // local[0]
VAR this; // parameter[-1]
function (k) { // (96, 130)
// scope has trivial outer context
// local vars:
VAR this; // parameter[-1]
}
}
Now if I changed the inner function to use foo like this:
function f() {
var foo = 1;
Object.keys({a: 1, a: 2}).forEach(function(k) {
foo;
debugger;
});
}
the scope would look different:
function f () { // (32, 132)
// scope has trivial outer context
// 5 heap slots
// local vars:
VAR foo; // context[4], forced context allocation
VAR this; // parameter[-1]
function (k) { // (96, 128)
// local vars:
VAR this; // parameter[-1]
}
}
foo is now context allocated.
Note that foo is still accessible in the outer function, which is still on
the stack. Having the debug listener evaluate using the context from frame
1 works.
print(exec_state.frame(1).evaluate("foo").value());
You might argue that it works if we had something like
function f() {
var foo = 1;
Object.keys({a: 1, a: 2}).forEach(function(k) {
print(eval("foo"));
});
}
That's correct. But that's due to special handling of eval. Since we have
eval at that point, the compiler knows that "anything could happen" at that
point, and allocates every local variable to the context just to cater to
eval. But the debugger could break anywhere in the code. We are not going
to context-allocate everything just because the debugger might inspect the
value at any point.
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