Parameters with default value should be allowed anywhere

[jrieken]

Have code like this

function foo(one = 1, two, three) {

}

which is valid ES6 syntax (http://www.ecma-international.org/ecma-262/6.0/index.html#sec-functions-and-classes). TypeScript flags such a signature as error because it treats default values like optional parameters.

[JsonFreeman]

@jrieken Correct me if I'm wrong, but I think you can only elide arguments at the end of the argument list, right? You can't do this for example:

foo(, 2, 3);

[jrieken]

unsure about that, but you can call foo(undefined, 2, 3)

[mhegazy]

The initialization is considered a marker for optionality, and ? is prohibited. no non-optional members are allowed after an optional one. doing this would be a large breaking change, unless we only treat them as optional if they fall at the end, which would be a convoluted rule.

Unless we have a proposal for how to make this work, i would be inclined to mark it as won't fix.

[JsonFreeman]

@jrieken Yes, I just wanted to make sure that was the only way to pass undefined.

@mhegazy I discussed this with @ahejlsberg and we think it is reasonable to only treat them as optional at the end (the one that you are saying is a convoluted rule).

[RyanCavanaugh]

doing this would be a large breaking change

How is making a prior error a non-error a breaking change?

[JsonFreeman]

@RyanCavanaugh That is a good question.

[JsonFreeman]

I do not think it is a breaking change.