This widget is also present in new installations in the Demo Notes.

Create a Code note of type JS frontend and give it a `#widget` label.

    /*
     * This defines a custom widget which displays number of words and characters in a current text note.
     * To be activated for a given note, add label 'wordCount' to the note, you can also make it inheritable and thus activate it for the whole subtree.
     *
     * See it in action in "Books" and its subtree.
     */
    const TPL = `<div style="padding: 10px; border-top: 1px solid var(--main-border-color); contain: none;">
        <strong>Word count: </strong>
        <span class="word-count"></span>

        &nbsp;

        <strong>Character count: </strong>
        <span class="character-count"></span>
    </div`;

    class WordCountWidget extends api.NoteContextAwareWidget {
        get position() { return 100; } // higher value means position towards the bottom/right

        get parentWidget() { return 'center-pane'; }

        doRender() {
            this.$widget = $(TPL);
            this.$wordCount = this.$widget.find('.word-count');
            this.$characterCount = this.$widget.find('.character-count');
            return this.$widget;
        }

        async refreshWithNote(note) {
            if (note.type !== 'text' || !note.hasLabel('wordCount')) {
                // show widget only on text notes and when marked with 'wordCount' label
                this.toggleInt(false); // hide

                return;
            }

            this.toggleInt(true); // display

            const {content} = await note.getNoteComplement();

            const text = $(content).text(); // get plain text only

            const counts = this.getCounts(text);

            this.$wordCount.text(counts.words);
            this.$characterCount.text(counts.characters);
        }

        getCounts(text) {
            const chunks = text
                .split(/[\s-+:,/\\]+/)
                .filter(chunk => chunk !== '');

            let words;

            if (chunks.length === 1 && chunks[0] === '') {
                words = 0;
            }
            else {
                words = chunks.length;
            }

            const characters = chunks.join('').length;

            return {words, characters};
        }

        async entitiesReloadedEvent({loadResults}) {
            if (loadResults.isNoteContentReloaded(this.noteId)) {
                this.refresh();
            }
        }
    }

    module.exports = new WordCountWidget();

After you make changes it is necessary to restart Trilium so that the layout can be rebuilt.

At the bottom of the note you can see the resulting widget: