Ruby hash equivalent of JavaScript's object initializer ES6 shorthand

Question

In JavaScript ES6 we can create objects where variable names become keys like this:

> let a = 'aaa'
'aaa'

> let b = 'bbb'
'bbb'

> { a, b }
{ a:"aaa", b:"bbb" }

Does Ruby have something equivalent for hashes?

Clarification: Obviously this question regards the shorthand notation. I'm looking for {a,b} not {a:a,b:b}.

Answer 1

No, there is no such shorthand notation.

Answer 2

Short answer no.

Longer answer

Shugo Maeda proposed a patch for this in 2015 (you can read the details about this here: https://bugs.ruby-lang.org/issues/11105).

At the time Matz wasn't into the idea, but might be willing to change his mind in the future.

In the mean time - you can make use of Shugo's patch and patch your own version of Ruby to have ES6 hash literals yourself!

To patch Ruby to add the hashes do the following:

1) Download the patch from here https://gist.github.com/thechrisoshow/1bb5708933d71e0e66a29c03cd31dcc3 (currently works with Ruby 2.5.0)

2) Use RVM to install a patched version of this Ruby. i.e.

rvm install 2.5.0 -n imphash --patch imphash.patch

Then you can use RVM to select the patched version of Ruby:

rvm use 2.5.0-imphash

(Imphash is short for implicit hash)

Answer 3

Although Ruby / Rails doesn't yet support an equivalent to the ES6 shorthand syntax for hashes, there are a few handy idioms already that often come in handy.

Ruby

Consider the following method:

def test_splat(a:, b:, c:)
  [a, b, c].inspect
end

test_splat(a: 4, b: 5, c: 6) yields "[4, 5, 6]"

Although if you already have a hash such as hash = { a: 1, b: 2, c: 3 }, you can simply call it like this:

test_splat(hash) which yields "[1, 2, 3]"

Further

If you have a sub_hash, you can use it alongside other kwargs using the kwarg splat operator **. For example if you have sub_hash = { a: 1, b: 2 }, calling:

test_splat(sub_hash) yields ArgumentError: missing keyword: c

and calling:

test_splat(sub_hash, c: 3) yields ArgumentError: wrong number of arguments (given 1, expected 0)

but using the splat operator **, you can do splat the sub_hash arg:

test_splat(**sub_hash, c: 3) which yields "[1, 2, 3]"

For more reading see https://www.justinweiss.com/articles/fun-with-keyword-arguments/

Rails

The above plus a few extra methods can come in handy for Rails users, particularly in controllers when params are passed in.

Suppose you have an ActionController::Parameters object with more attributes than you need and you want a subset. E.g: { a: 1, b: 2, d: 4 }. The slice method on Hash is very handy here.

First, permit your params:

permitted_params = params.permit(:a, :b, :d).to_h.symbolize_keys.

We add .to_h.symbolize_keys because ActionController::Parameters doesn't support symbolize_keys, and kwargs require the args' keys to be symbols, not strings. So the .to_h converts it to an ActiveSupport::HashWithIndifferentAccess, and the symbolize_keys converts the hash's keys from strings to symbols.

Now, calling:

test_splat(**permitted_params, c: 3) will yield ArgumentError: unknown keyword: d as expected since d isn't a kwarg for the test_splat method. Although using slice achieves what we want here:

test_splat(**permitted_params.slice(:a, :b), c: 3) yields "[1, 2, 3]"