Add pre-publish conditions to the block editor

EDIT Sept 2021:

An updated version of the answer that uses hooks.

This version tracks changes in the editor much better. It also uses import statement instead of importing directly from the wp global. This approach when used with the @wordpress/scripts package will correctly add dependencies for this file when being enqueued. Accessing the wp global will still work but you will have to be sure you’re managing your script dependencies manually.

Thanks to everyone in the comments!

import { useState, useEffect } from '@wordpress/element';
import { registerPlugin } from '@wordpress/plugins';
import { PluginPrePublishPanel } from '@wordpress/edit-post';
import { useSelect, useDispatch } from '@wordpress/data';
import { count } from '@wordpress/wordcount';
import { serialize } from '@wordpress/blocks';

const PrePublishCheckList = () => {
    // Manage the messaging in state.
    const [wordCountMessage, setWordCountMessage] = useState('');
    const [catsMessage, setCatsMessage] = useState('');
    const [tagsMessage, setTagsMessage] = useState('');
    const [featuredImageMessage, setFeaturedImageMessage] = useState('');

    // The useSelect hook is better for retrieving data from the store.
    const { blocks, cats, tags, featuredImageID } = useSelect((select) => {
        return {
            blocks: select('core/block-editor').getBlocks(),
            cats: select('core/editor').getEditedPostAttribute('categories'),
            tags: select('core/editor').getEditedPostAttribute('tags'),
            featuredImageID:
                select('core/editor').getEditedPostAttribute('featured_media'),
        };
    });

    // The useDispatch hook is better for dispatching actions.
    const { lockPostSaving, unlockPostSaving } = useDispatch('core/editor');

    // Put all the logic in the useEffect hook.
    useEffect(() => {
        let lockPost = false;
        // Get the WordCount
        const wordCount = count(serialize(blocks), 'words');
        if (wordCount < 500) {
            lockPost = true;
            setWordCountMessage(`${wordCount} - Minimum of 500 required.`);
        } else {
            setWordCountMessage(`${wordCount}`);
        }

        // Get the category count
        if (!cats.length || (cats.length === 1 && cats[0] === 1)) {
            lockPost = true;
            setCatsMessage('Missing');
            // Check that the cat is not Uncategorized - this assumes that the ID of Uncategorized is 1, which it would be for most installs.
            if (cats.length === 1 && cats[0] === 1) {
                setCatsMessage('Cannot use Uncategorized');
            }
        } else {
            setCatsMessage('Set');
        }

        // Get the tags
        if (tags.length < 3 || tags.length > 5) {
            lockPost = true;
            setTagsMessage('Required 3 - 5 tags');
        } else {
            setTagsMessage('Set');
        }
        // Get the featured image
        if (featuredImageID === 0) {
            lockPost = true;
            setFeaturedImageMessage('Not Set');
        } else {
            setFeaturedImageMessage(' Set');
        }

        if (lockPost === true) {
            lockPostSaving();
        } else {
            unlockPostSaving();
        }
    }, [blocks, cats, tags, featuredImageID]);

    return (
        <PluginPrePublishPanel title={'Publish Checklist'}>
            <p>
                <b>Word Count:</b> {wordCountMessage}
            </p>
            <p>
                <b>Categories:</b> {catsMessage}
            </p>
            <p>
                <b>Tags:</b> {tagsMessage}
            </p>
            <p>
                <b>Featured Image:</b> {featuredImageMessage}
            </p>
        </PluginPrePublishPanel>
    );
};

registerPlugin('pre-publish-checklist', { render: PrePublishCheckList });

Old Version:

const { registerPlugin } = wp.plugins;
const { PluginPrePublishPanel } = wp.editPost;
const { select, dispatch } = wp.data;
const { count } = wp.wordcount;
const { serialize } = wp.blocks;
const { PanelBody } = wp.components;

const PrePublishCheckList = () => {
    let lockPost = false;

    // Get the WordCount
    const blocks = select( 'core/block-editor' ).getBlocks();
    const wordCount = count( serialize( blocks ), 'words' );
    let wordCountMessage = `${wordCount}`;
    if ( wordCount < 500 ) {
        lockPost = true;
        wordCountMessage += ` - Minimum of 500 required.`;
    }

    // Get the cats
    const cats = select( 'core/editor' ).getEditedPostAttribute( 'categories' );
    let catsMessage="Set";
    if ( ! cats.length ) {
        lockPost = true;
        catsMessage="Missing";
    } else {
        // Check that the cat is not uncategorized - this assumes that the ID of Uncategorized is 1, which it would be for most installs.
        if ( cats.length === 1 && cats[0] === 1 ) {
            lockPost = true;
            catsMessage="Cannot use Uncategorized";
        }
    }

    // Get the tags
    const tags = select( 'core/editor' ).getEditedPostAttribute( 'tags' );
    let tagsMessage="Set";
    if ( tags.length < 3 || tags.length > 5 ) {
        lockPost = true;
        tagsMessage="Required 3 - 5 tags";
    }

    // Get the featured image
    const featuredImageID = select( 'core/editor' ).getEditedPostAttribute( 'featured_media' );
    let featuredImage="Set";

    if ( featuredImageID === 0 ) {
        lockPost = true;
        featuredImage="Not Set";
    }

    // Do we need to lock the post?
    if ( lockPost === true ) {
        dispatch( 'core/editor' ).lockPostSaving();
    } else {
        dispatch( 'core/editor' ).unlockPostSaving();
    }
    return (
        <PluginPrePublishPanel title={ 'Publish Checklist' }>
            <p><b>Word Count:</b> { wordCountMessage }</p>
            <p><b>Categories:</b> { catsMessage }</p>
            <p><b>Tags:</b> { tagsMessage }</p>
            <p><b>Featured Image:</b> { featuredImage }</p>
        </PluginPrePublishPanel>
    )
};

registerPlugin( 'pre-publish-checklist', { render: PrePublishCheckList } );

Display:

The solution above addresses the requirements listed in the question. One thing that can be expanded on is the category checking, I am making some assumptions about the category ID.

I have kept all of the checks in the same component for the sake of brevity and readability here. I would recommend moving each portion into a separate component and potentially making them Higher Order Components ( i.e withWordCount ).

I have inline comments that explain what is being done but am happy to explain further if there are any questions.

EDIT: Here’s how I’m enqueuing the script

function enqueue_block_editor_assets() {
    wp_enqueue_script(
        'my-custom-script', // Handle.
        plugin_dir_url( __FILE__ ) . '/build/index.js',
        array( 'wp-blocks', 'wp-i18n', 'wp-element', 'wp-editor', 'wp-edit-post', 'word-count' ) // Dependencies, defined above.
    );
}
add_action( 'enqueue_block_editor_assets', 'enqueue_block_editor_assets' );

Adding some more details about the build process. I am using @wordpress/scripts and running the following scripts:

"scripts": {
    "build": "wp-scripts build",
    "start": "wp-scripts start"
  },

Edit 2:

You can get the attachment data via:

wp.data.select('core').getMedia( ID )