Optional argument can appear before non-optional parameters

This code produces 2 warnings:

function f(a=1,b) {}
f(undefined, 3);

JSC_OPTIONAL_ARG_AT_END: optional arguments must be at the end at line 1 character 0
function f(a=1,b) {}
^
JSC_WRONG_ARGUMENT_COUNT: Function f: called with 2 argument(s). Function requires at least 0 argument(s) and no more than 1 argument(s). at line 2 character 0
f(undefined, 3);
^

https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540compilation_level%2520ADVANCED_OPTIMIZATIONS%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%2540formatting%2520pretty_print%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A%250Afunction%2520f(a%253D1%252Cb)%2520%257B%257D%250Af(undefined%252C%25203)%253B

And it cannot be suppressed with @suppress {functionParams}.

It is legal ES6 for a parameter with a default value to be followed by a required one, it should not be considered optional.

[dimvar]

In this case, state is a required parameter whose type includes undefined. That’s fine.

When the compiler sees an unannotated function such as function f(a=1,b) {}, it needs to decide whether a is required or optional. I think that in most cases the user wants it to be optional, so we would give spurious warnings when it was meant to be required. If we considered it to be required, there would be more spurious warnings when f is called without any arguments. It’s a tradeoff. If the user wants something that is not the default compiler assumption, they can just manually annotate their code.

But I do agree that the current situation of early warning is somewhat unsatisfactory, and we should just consider the trailing parameters optional and not warn.

[dimvar]

Right.