Nothing is new here, under the straightforward semantics Jason
described. JS today:
var FOO = 42;
function bar(x, y) {
if (y === void 0) y = FOO + x;
...
}
ES6 as proposed:
var FOO = 42;
function bar(x, y = FOO + x) {
...
}
What's the problem?
The other cases you show are all required to work today, including those
with local var FOO and var y. We can't break compatibility.
Default parameters only enter into the picture if you think default
parameter syntax should trigger new semantics (new scope contour(s) for
parameter binding, new hoisting boundary in between parameters and body
bindings, etc.).
New semantics for new syntax are certainly possible, but anything more
complicated needs strong justification.
If you really want the Pythonic default parameter rules (evaluation in
definition context) then you need to address the bad case Jason showed:
function foo(arg = []) {
arg.push('strange');
return arg;
}
foo(); // ['strange']
foo(); // ['strange', 'strange']
This is a side channel and a big footgun. ES6 default parameters as
proposed avoid it.
/be
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