What are “downward funargs”?

Downward funargs are local functions that are not returned or otherwise leave their declaration scope. They only can be passed downwards to other functions from the current scope.

Two examples. This is a downward funarg:

function () {
    var a = 42;
    var f = function () { return a + 1; }
    foo(f); // `foo` is a function declared somewhere else.
}

While this is not:

function () {
    var a = 42;
    var f = function () { return a + 1; }
    return f;
}