Although the current proposal has a requirement that only trailing formal parameters may have default values specified, this shouldn’t be necessary when named parameters are used:

The rationale here is that you can call a function that has named parameters with default values just like a function that doesn’t. So, a function:

function foo(x, y=2, z=3) {
 return [x, y, z];
}

could be called by: foo(1, 4) would return [1, 4, 3]

If a named parameter were in the first argument:

function foo(x=1, y, z) {
 return [x, y, z];
}

what would a call to foo(1,2) mean? (As far as I can tell, it would just effectively require all calls to treat all parameters using the named parameter syntax.) Python has the same restriction if you use the default value syntax (but not for the **kwargs syntax).