Rails - How to add CSRF Protection to forms created in javascript?

I'm using backbone.js and it works great. but the forms I'm creating as a javascript template lacks the rails csrf protection token. How do I add it to templates I'm creating in javascript?

Best way I solved this, inside the form:

<%= hidden_field_tag :authenticity_token, form_authenticity_token %>

Update:

It looks like the form_authenticity_token is private for controllers in the newer rails versions.

If that's the case for you, what I suggest is: declare a variable in a controller like: @form_token = form_authenticity_token and use it in the view you are looking for.

If you have <%= csrf_meta_tag %> in your layout somewhere and that is accessible to you from the js, then you can access it using $('meta[name="csrf-token"]')

See http://eunikorn.blogspot.com/2011/07/working-with-backbonejs-in-harmony-with.html for an idea on how to hack in csrf support into each backbone request

You can prepend the csrf token to every form that uses 'post' or 'delete'. Here it is in coffeescript:

$ -> 
  for f in $("form")
    if f.method == 'post' or f.method == 'delete'
      $(f).prepend("<input type='hidden' name='authenticity_token' value='" + token + "'>")

Make sure you have <%= csrf_meta_tags %> in your layout. It should already be in the standard 'application' layout, but add it if you're using a different layout.

I have a form inside a Vue component in a Rails 6 app.

To my surprise, it was sufficient to include a hidden input with the name authenticity_token within the Vue template and on page load, Rails filled out the value with a CSRF protection token.

E.g.

<template>
  <div id="app">
    <form
      action="/submit"
      method="post"
      @submit.prevent="onSubmit"
    >
      <input
        type="hidden"
        name="authenticity_token"
        value=""
      >
      <!-- rest of form -->
    </form>
  </div>
</template>

Which gets rendered as:

<div id="app">
  <form action="/submit" method="post">
    <input type="hidden" name="authenticity_token" value="zl9PJiE...">
    ...
  </form>
</div>

As for Rails 4.2.2 you are not allowed to use

<%= hidden_field_tag :authenticity_token, form_authenticity_token %>

from your .js.erb assets file.

However You can create the form inside the .js.erb file and in the view containing the form .html.erb file use the hidden_field_tag helper to generate the token element. As this element is going to be generated outside the form you can use jquery to append this element to the form.

Case of study: SweetAlert (first version, version too seems to have solved this problem)

show.js.erb

$('.js-button-apply-offer').click(function(e) {
var urlOffer = $(this).attr('data-url-offer');
var modalParams = {
    type: 'warning',
    title: 'add file',
    text: '<p>Need to add a file before continuing</p>' // This is a hack for Sweet alert, solved in SweetAlert2 Consider upgrade
    +"<form action='"+urlOffer+"' id='formCustomCV' method='post' enctype='multipart/form-data' data-remote='true'>"
    + "<input type='file' name='custom_cv' id='fileToUploadAlert' accept='application/pdf'>\n"
    +"</form>",
    html: true,
    showCancelButton: true,
    confirmButtonColor: '#DD6B55',
    confirmButtonText: 'Send',
    cancelButtonText: 'Cancel',
    closeOnConfirm: false
  }
swal(modalParams,
function(){
  var form_token = $('#form_token');
  $('#formCustomCV').append(form_token).submit(); //update to submit using ajax
});

show.html.erb

<%= button_tag t('offers.offer.apply'),
  class: 'center-block btn btn-success js-button-apply-offer',
  id: "js-button-apply-offer",
  data: {
    url_offer: apply_talents_offer_path(@offer),
  } 
%>
<%= hidden_field_tag :authenticity_token, form_authenticity_token, id: :form_token %>