Ruby method to convert geojson to topojson in Rails API

droughtshapes_controller.rb

class DroughtshapesController < ApplicationController
  include ApplicationHelper

  def index
    if params[:s]
      state_name = params[:s].split.map {|i| i.capitalize}.join(" ")
      state = State.find_by(name: state_name)
      if state
        shapes = state.drought_shapes
        geojson_shapes = to_geojson(shapes)
        if params[:geo] == "true"
          render json: geojson_shapes # responds to requests with geojson
        else
          render json: convert_to_topojson(geojson_shapes) # responds to requests with topojson
        end
      else
        render json: "not found - check query string and try again"
      end
    end
  end
end

topojson_helper.rb

module TopojsonHelper
    
    # given a geojson "feature collection", return a topojson object
    def convert_to_topojson(geojson)
      collection = geojson.to_json #convert a ruby hash to JSON object literal, ex. => becomes :

      source = open('./lib/assets/topojson.js').read # load javascript library to memory
      context = ExecJS.compile(source) # create ExecJS "context"
      
      # string interpolate feature collection and pass it to topojson.topology
      # evaluate within context of topojson library, and returns ruby object
      topojson = context.eval("topojson.topology({collection: #{collection}})").to_json 
    return topojson
  end
  
end

project repo: https://github.com/radcliff/drought-monitor-api/tree/execjs