Do you realize that if the optional arguments were not included in Function#length, you'd just be saying "They are not reflected in the function’s length property. C'mon, man" instead ?
Also, I don't understand Whoop-dee-freakin’-doo.
Whoop-dee-freakin’-doo
As for Whoop-dee-freakin’-doo, no, it don't think it means anything in the context. That's not an argument, and the following sentence isn't either. Maybe I don't understand because english isn't my native language, tho.
Let's say we have these functions:
function getElem(arr, index=0) { return arr[index]; }
var curried = curry(getElem);
// Hypothetical, "better" semantics
var arr = ["a", "b", "c"];
console.log(curried(arr)); // partially-applied function, *not* "a"
console.log(curried(arr, 0)); // "a"
console.log(curried(arr, 1)); // "b"
// Actual, "worse" semantics
console.log(curried(arr)); // "a"
console.log(curried(arr, 0)); // Error: attempts to run `"a"(0)`, but "a" is not a function
console.log(curried(arr, 1)); // Error: attempts to run `"a"(1)`, but "a" is not a function
I would argue that currying is actually more general than default arguments, so if it's a choice between one or the other, they should have added currying to the language instead of default arguments.
They're not quite comparable, but you can think of currying as supplying default arguments "dynamically" (at the call site), whereas regular default arguments are supplied "statically" (at the definition site). For example:
// Give "y" a default value for everyone
var defaultMultiply = function (x, y=2) { return x * y; };
console.log(defaultMultiply(5, 7)); // 35
console.log(defaultMultiply(5)); // 10
// Don't commit to any defaults yet
var curryMultiply = curry(function(x, y) { return x * y; });
console.log(curryMultiply(5, 7)); // 35
// Give "x" a default value, to act like defaultMultiply
var double = curryMultiply(2);
console.log(double(5)); // 10
// Give "x" a different default, which we can't do with defaultMultiply
var triple = curryMultiply(3);
console.log(triple(5)); // 15