问题:

Are there any standard ways of marking a function argument as unused in JavaScript, analogous to starting a method argument with an underscore in Ruby?

回答1:

Just so we have an example to work from, this is fairly common with jQuery's $.each where you're writing code that doesn't need the index, just the value, in the iteration callback ($.each is backward relative to Array#forEach):

$.each(objectOrArrayLikeThing, function(_, value) { }
    // Use value here
});

Using _ is the closest I've seen to a standard way to do that, yes, but I've also seen lots of others — giving it a name reflective of its purpose anyway (index), calling it unused, etc.

回答2:

With browsers supporting destructuring one can do:

function ({}, {}, value) {
  // ...value
}

Which is kind of neat in that it avoids the problem of multiple arguments having the same name and also won't create problems with libraries that assign methods to _ (lodash, underscore, etc.).