Brendan Eich wrote:
Morever, the opening left brace does not make a scope boundary between
formals and local vars. It doesn't even make a hoisting boundary for
functions:
js> function f(x){ function x(){}; return x; }
js> f(42)
function x(){}
I plead jetlag. Still not quite what I wanted. Try this:
js> function f(x){ function x(){}; return [arguments[0], x]; }
js> f(42)
[function x(){}, function x(){}]
If hoisting stopped at the { then one might expect, even with one scope
contour in which to bind 'x', that arguments[0] would be 42. Not so, but
of course with arguments a magic object whose element accessors alias
formal parameters, this still doesn't prove enough.
But I claim it does show backward compatibility constraints that we must
satisfy, while also minimizing the complexity of the user's cognitive
model, and of the spec. The sweet spot for me is to fold formal
parameters into the body, as far as default parameters and anything else
involving expression evaluation go.
Yes, we could complicate the spec with temporal dead zones for
parameters to the right, so that
function f(a, b=a, c=d, d=42) {...}
would either find an outer 'd' for the c parameter default value, or
throw on read from a temporally dead 'd' parameter. But given the
var-like nature of args in ES1-5:
function f(x) { var x; return x; } // identity function
function f(x) { arguments[i]=42; return x; } // 42 if i=0
the simplest, or least mentally burdensome in full (avoiding splitting
cases into compatible and new), way forward is for parameters to be
var-like bindings in the same scope as top level var and function (but
not let), and for default parameters to be evaluated as if in the body,
first thing after any implicit yield (if in a generator function), as
Jason suggested.
/be
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