Standard conventions for indicating a function argument is unused in JavaScript

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 and you're using this (which jQuery also sets to the value) for something else:

$.each(objectOrArrayLikeThing, (_, value) => {
    // Use `value` here
});

(Yes, $.each passes arguments to the callback backward compared to the JavaScript standard forEach.)

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.

If you need to ignore more than one parameter, you can't repeat the same identifier (it's disallowed in strict mode, which should be everyone's default and is the default in modules and class constructs), so you have do things like _0 and _1 or _ and __, etc.

Using destructuring assignment, one can do:

function f(...[, , third]) {
  console.log(third);
}

f(1, 2, 3);