Iterator references wrong string

Question

I am trying to map a string to a function like so in Node.js.

function foo(data) {
    //foo body
}

function bar(data) {
    //bar body
}

var FUNCTION_MAP = {
    "foo": foo,
    "bar": bar
}


for (var event in FUNCTION_MAP) {
    socket.on(event, function(data) {
        FUNCTION_MAP[event](data);
    });
}

Some hand-testing shows that event will always be "bar", so the bar function is always called regardless of what event is invoked, i.e. whether the client calls socket.emit("foo", {}) or socket.emit("bar", {}), the bar function is always called on the server side. Even creating a new string object doesn't work.

for (var event in FUNCTION_MAP) {
    socket.on(event, function(data) {
        FUNCTION_MAP[new String(event)](data);
    });
}

Why does event retain the value "bar" after the loop terminates? What can I do to create a string object that has its own value and doesn't reference the event variable?