_form.html

<%= f.InputTag("Title") %>
<%= f.TextArea("Body", {rows: 10}) %>

<div id="body-preview"></div>

<button class="btn btn-success" role="submit">Save</button>

application.js

require('expose-loader?$!expose-loader?jQuery!jquery');
require("bootstrap/dist/js/bootstrap.js");
let marked = require("marked/lib/marked.js");

marked.setOptions({
  renderer: new marked.Renderer(),
  gfm: true,
  tables: true,
  breaks: false,
  pedantic: false,
  sanitize: false,
  smartLists: true,
  smartypants: false
});

$(() => {

  $("#story-Body").keyup((e)=>{
    let text = $(e.target).val();
    $("#body-preview").html(marked(text));
  })

});

package.json

{
  "name": "coke",
  "version": "1.0.0",
  "description": "Thank you for choosing Buffalo for your web development needs.",
  "main": "webpack.config.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "babel-cli": "^6.24.1",
    "babel-core": "^6.25.0",
    "babel-loader": "^7.1.0",
    "babel-preset-env": "^1.5.2",
    "bootstrap": "^3.3.7",
    "copy-webpack-plugin": "^4.0.1",
    "css-loader": "^0.28.4",
    "expose-loader": "^0.7.3",
    "extract-text-webpack-plugin": "^2.1.2",
    "file-loader": "^0.11.2",
    "font-awesome": "^4.7.0",
    "jquery": "^3.2.1",
    "jquery-ujs": "^1.2.2",
    "marked": "^0.3.6",
    "node-sass": "^4.5.3",
    "npm-install-webpack-plugin": "^4.0.4",
    "path": "^0.12.7",
    "sass-loader": "^6.0.6",
    "style-loader": "^0.18.2",
    "uglifyjs-webpack-plugin": "^0.4.6",
    "url-loader": "^0.5.9",
    "webpack": "^2.3.3"
  }
}