Use a helper to set a css class

I need to make reactive a class inside a const (exported from a module).

export const messageControls = '
    <div id="controls"">
        <i id="idcont" class="{{starred}}"></i>
    </div>
'

This class belongs to an HTML block who's inserted as innerHTML of a createElement.

var newElement = document.createElement('div');
newElement.id = i._id;
newElement.className = "single_message";
newElement.innerHTML = messageControls;               
document.getElementById('conversation_details').appendChild(newElement);

The {{helper}} below is not rendering anything :

starred: function () {
    return 'bob';
},

<i id="idcont" class="{{starred}}"></i> gives {{starred}} in plain text
<i id="idcont" class=" ' + {{starred}} + ' "></i> breaks all

Any idea?

Update - full Blaze template as requested

<template name="inbox">
    <div class="searchmessages">
        <input type="text" name="searchmessages" id="searchmessages" placeholder="&nbsp;&#xf002;&nbsp;&nbsp;any word / any date">
    </div>
    <div class="row">

        <div class="col-xs-4 l-O list_messages">
            <div id="gridreceived" class="gridmessages">
                {{#each incoming_all}}
                    <div id="{{_id}}" class="item {{readornot}}">
                        <div class="item-content">
                            <div class="task_inlist">
                                <div class="task_from">
                                    {{{from}}}
                                </div>
                                <div class="task_date">
                                    {{initialdate}}
                                </div>
                                <div class="task_subject">
                                    {{{subject}}}
                                </div>
                                <div class="task_content">
                                    {{{htmlbody}}}
                                </div>
                            </div>
                        </div>
                    </div>
                {{/each}}
            </div>
            <div class="grid_nomatch">{{grid_noresult}}</div>
        </div>

        <div id="conversation_details" class="col-xs-8" media="print">
            <!--
            here are each selected message details
            -->
        </div>
    </div>
</template>

Solution

You're trying to inject spacebars template markup directly into the DOM but meteor-blaze wants to use spacebars to build the DOM. It doesn't watch the DOM for arbitrary changes and then make template substitutions inside of it!

You can instead use Meteor's reactivity to automatically insert new items into the DOM for you based on changes to the underlying data. In your case it looks like you're trying to show the details of a message that's been clicked on. You probably have a template event handler already to catch the click. In that template handler you can set a Session variable which indicates which message is currently selected and then use that Session variable inside the helper that renders the message details.

For example:

<template name="inbox">
  <div class="searchmessages">
    <input type="text" name="searchmessages" id="searchmessages" placeholder="&nbsp;&#xf002;&nbsp;&nbsp;any word / any date">
  </div>
  <div class="row">

    <div class="col-xs-4 l-O list_messages">
      <div id="gridreceived" class="gridmessages">
        {{#each incoming_all}}
          <div id="{{_id}}" class="item {{readornot}}">
            // render summary of each message
          </div>
        {{/each}}
      </div>
    <div class="grid_nomatch">{{grid_noresult}}</div>

    {{#with selectedMessage}}
      <div id="conversation_details" class="col-xs-8" media="print">
        // render selected message details
      </div>
    {{/with}}
  </div>
</template>

Your js:

Template.inbox.events({
  'click .item'(ev) {
    Session.set('selectedMessageId',this._id);
  }
});

Template.inbox.helpers({
  selectedMessage() {
    return Messages.findOne(Session.get('selectedMessageId'));
  }
});

Now to your follow-up question about how to reactively style an element. Let's say your message* object has aisStarredboolean key. In the message detail section we've set the data context using{{#with currentMessage}}` so any key of the current message can be used directly in our spacebars template. Where you are displaying the message details you can do:

<div id="controls"">
    <i id="idcont" class="{{#if isStarred}}starred{{/if}}"></i>
</div>

Depending on whether or not the message is starred this will render as class="" or class="starred".