Add Buttons to Rails ActionText toolbar

To add to Will's answer, you can avoid recreating the whole toolbar by using trix after initialize callback :

example, adding a 'Red' button that turns the text red :

var Trix = require("trix");

/* what the newly created button does */
Trix.config.textAttributes.red = {
    style: { color: "red" },
    parser: function(element) {
        return element.style.color === "red"
    },
    inheritable: true
}

/* insert the button visual in the default toolbar */
addEventListener("trix-initialize", function(event) {
    var buttonHTML = '<button type="button" data-trix-attribute="red">RED</button>'

    event.target.toolbarElement.
    querySelector(".trix-button-group").
    insertAdjacentHTML("beforeend", buttonHTML)
})

output:

I couldn't find any documentation about the 'correct' way to do this, but the following works:

I've made these modifications to my /app/javascript/application.js file, where ActiveText had placed the require statements for trix and actiontext.

First I got the Trix instance by returning the require into a Variable.

var Trix  = require("trix")
console.log("Config", Trix.config);

Then I specify the extra items I want Trix to recognise:

Trix.config.blockAttributes.heading2 = {
  tagName: 'h2'
}
Trix.config.blockAttributes.heading3 = {
  tagName: 'h3'
}
Trix.config.textAttributes.underline = {
  tagName: 'u'
}

And then I rebuilt the toolbar by just pasting in Trix's code and modifying the html, which is verbose, but simple to understand:

const {lang} = Trix.config;

Trix.config.toolbar = {
  getDefaultHTML() { return `\
<div class="trix-button-row">
  <span class="trix-button-group trix-button-group--text-tools" data-trix-button-group="text-tools">
    <button type="button" class="trix-button trix-button--icon trix-button--icon-bold" data-trix-attribute="bold" data-trix-key="b" title="${lang.bold}" tabindex="-1">${lang.bold}</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-italic" data-trix-attribute="italic" data-trix-key="i" title="${lang.italic}" tabindex="-1">${lang.italic}</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-strike" data-trix-attribute="strike" title="${lang.strike}" tabindex="-1">${lang.strike}</button>
    <button type="button" class="trix-button  " data-trix-attribute="underline" title="h3" tabindex="-1">u</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-link" data-trix-attribute="href" data-trix-action="link" data-trix-key="k" title="${lang.link}" tabindex="-1">${lang.link}</button>
  </span>
  <span class="trix-button-group trix-button-group--block-tools" data-trix-button-group="block-tools">
    <button type="button" class="trix-button  " data-trix-attribute="heading1" title="h1" tabindex="-1">h1</button>
    <button type="button" class="trix-button  " data-trix-attribute="heading2" title="h2" tabindex="-1">h2</button>
    <button type="button" class="trix-button  " data-trix-attribute="heading3" title="h3" tabindex="-1">h3</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-quote" data-trix-attribute="quote" title="${lang.quote}" tabindex="-1">${lang.quote}</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-code" data-trix-attribute="code" title="${lang.code}" tabindex="-1">${lang.code}</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-bullet-list" data-trix-attribute="bullet" title="${lang.bullets}" tabindex="-1">${lang.bullets}</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-number-list" data-trix-attribute="number" title="${lang.numbers}" tabindex="-1">${lang.numbers}</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-decrease-nesting-level" data-trix-action="decreaseNestingLevel" title="${lang.outdent}" tabindex="-1">${lang.outdent}</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-increase-nesting-level" data-trix-action="increaseNestingLevel" title="${lang.indent}" tabindex="-1">${lang.indent}</button>
  </span>
  <span class="trix-button-group trix-button-group--file-tools" data-trix-button-group="file-tools">
    <button type="button" class="trix-button trix-button--icon trix-button--icon-attach" data-trix-action="attachFiles" title="${lang.attachFiles}" tabindex="-1">${lang.attachFiles}</button>
  </span>
  <span class="trix-button-group-spacer"></span>
  <span class="trix-button-group trix-button-group--history-tools" data-trix-button-group="history-tools">
    <button type="button" class="trix-button trix-button--icon trix-button--icon-undo" data-trix-action="undo" data-trix-key="z" title="${lang.undo}" tabindex="-1">${lang.undo}</button>
    <button type="button" class="trix-button trix-button--icon trix-button--icon-redo" data-trix-action="redo" data-trix-key="shift+z" title="${lang.redo}" tabindex="-1">${lang.redo}</button>
  </span>
</div>
<div class="trix-dialogs" data-trix-dialogs>
  <div class="trix-dialog trix-dialog--link" data-trix-dialog="href" data-trix-dialog-attribute="href">
    <div class="trix-dialog__link-fields">
      <input type="url" name="href" class="trix-input trix-input--dialog" placeholder="${lang.urlPlaceholder}" aria-label="${lang.url}" required data-trix-input>
      <div class="trix-button-group">
        <input type="button" class="trix-button trix-button--dialog" value="${lang.link}" data-trix-method="setAttribute">
        <input type="button" class="trix-button trix-button--dialog" value="${lang.unlink}" data-trix-method="removeAttribute">
      </div>
    </div>
  </div>
</div>\
`; }
};

This is taken (and decafinated) from https://github.com/basecamp/trix/blob/master/src/trix/config/toolbar.coffee