Standard conventions for indicating a function argument is unused in JavaScript

Question

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?

Solution 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.

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.

Solution 2

Using destructuring assignment, one can do:

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

f(1, 2, 3);

Solution 3

With browsers supporting destructuring one can do:

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

One problem with this approach is that unused arguments of type undefined or null will throw.

For undefined one solution is to use default parameters:

function ({}={}, {}={}, value) {
  // console.log(value)
}

Sadly no such easily applied solution for null.

Solution 4

I would recommend this syntax:

function(_index, value) {...}

to not to shadow lodash variable and still have description of argument in case if it will be used.

VS Code is also highlight these names properly and these unused args won't be deleted after autofix code smells

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Comments

[Swaraj Giri]

From what i have seen so far, its still _ that's being used.

[Tushar]

If you're looking to remove unused variables use jslint

[Alec Mev]

Unfortunately, this throws if an unused argument is undefined or null.

[oluckyman]

@OlegsJeremejevs function ({}={}, {}={}, value) { ... } helps here, but it becomes messy...

[Alec Mev]

@oluckyman Nice workaround! "Don't want to name an unused arg? Just dumbbell it!"

[Hanming Zeng]

you can't do function (_, _, value) though

[T.J. Crowder]

@HanmingZeng - Indeed not. :-( You have to do things like (_0, _1, value) or (in my case) (unused0, unused1, value). I've also seen (_, __, value) but...yikes. :-) There was a strawman proposal to allow blank parameter names once upon a time but it never went anywhere.

[zrajm]

In Chromium it seems to work just fine to write a function declaration like function (_, _, a) { … } (of course inside the function the variable _ only contains the second passed argument) but there's no error and the function is created (and works) just fine.

[T.J. Crowder]

@zrajm - Only in loose mode. In strict mode, duplicate parameter names are disallowed. (It's best to always use strict mode except in rare situations where you can't. It's also the default for modules and the content of class constructs.)

[Doug Coburn]

I like the answer by @Joseph Marinier even better stackoverflow.com/questions/32197927/…

[Him Hah]

This seem more simpler to me.

[zypA13510]

Wow, I always thought that destructuring a primitive value, i.e. (function({}={}){})(0) or var {a}=0 will throw.