Add linting for to_json

[allcentury]

Is your feature request related to a problem? Please describe.

classes that override to_json need to ensure an optional parameter is
available.

class MyClass
  def to_json
  end
end

While calling MyClass.new.to_json works, calling
JSON.generate(MyClass.new) results in an ArgumentError. That's because the ruby JSON api wants to send additional options (via a hash) to to_json, ie to_json(opts = {})

Describe the solution you'd like

I'd like a linter to catch this as it's not obvious from the ruby JSON docs.

Describe alternatives you've considered

[bquorning]

That's because the ruby JSON api wants to send additional options (via a hash) to to_json, ie to_json(opts = {})

Are you sure the argument is a Hash?

require 'json'
class MyClass
  def to_json(*args)
    p args
    ""
  end
end
JSON.generate(MyClass.new)
#=> [#<JSON::Ext::Generator::State:0x00007faacb155a40>]

[allcentury]

@bquorning thanks for calling that out - it currently returns a hash-like object:

https://ruby-doc.org/stdlib-2.0.0/libdoc/json/rdoc/JSON.html#method-i-generate

from the docs:

Generate a JSON document from the Ruby data structure obj and return it. state is

a JSON::State object,

or a Hash like object (responding to to_hash),

or an object convertible into a hash by a to_h method, that is used as or to configure a State object.

Using a default of {} felt sane, because the api of State acts like a hash:

[6] pry(#<MyClass>)> args
=> #<JSON::Ext::Generator::State:0x00007fc4959e2af8>
[7] pry(#<MyClass>)> args[:indent]
=> " "
[8] pry(#<MyClass>)> args.to_hash
=> {:indent=>" ", :space=>"", :space_before=>"", :object_nl=>"", :array_nl=>"", :allow_nan=>false, :ascii_only=>false, :max_nesting=>100, :depth=>0, :buffer_initial_length=>1024}

that said perhaps just an argument with no default is enough, ie:

def to_json(_opts)
end

What do you think?

[bquorning]

Weird then, that the signature for e.g. Date#to_json says it accepts *args.

I found this old commit on Ruby where the documentation for to_json signature on TrueClass, FalseClass and NilClass is changed from state=nil to *: ruby/ruby@e662770

Perhaps we should just recommend having * in the signature?