Generate version identifier for WordPress theme (Sage) using webpack

functions.php

<?php

/**
 * Theme version string.
 * Tries to read a "version.json" file in the "dist" folder - a JSON file with a "version" property.
 * If this file is not available it returns the WordPress version.
 *
 * @return string
 */
function theme_version()
{
    $version = get_bloginfo('version');
    $version_json_path = get_template_directory() . '/../dist/version.json';

    if (file_exists($version_json_path)) {
        $version_json = file_get_contents($version_json_path);

        if (!empty($version_json)) {
            $version_json_data = json_decode($version_json);

            if (!empty($version_json_data)) {
                if (!empty($version_json_data->version)) {
                    $version = $version_json_data->version;
                }
            }
        }
    }

    return $version;
}

/**
 * Theme assets
 */
add_action('wp_enqueue_scripts', function () {
    $version = theme_version();
    wp_enqueue_style('sage/main.css', asset_path('styles/main.css'), false, $version);
    wp_enqueue_script('sage/main.js', asset_path('scripts/main.js'), ['jquery'], $version, true);

    if (is_single() && comments_open() && get_option('thread_comments')) {
        wp_enqueue_script('comment-reply');
    }
}, 100);

install-package.sh

npm install --save-dev generate-json-webpack-plugin

webpack.config.js

const GenerateJsonPlugin = require('generate-json-webpack-plugin');

// Update your webpack config
const webpackConfig = {
  plugins: [
    new GenerateJsonPlugin('version.json', {
      version: Date.now(),
    }),
  ],
};

Assuming Sage theme and webpack build, but this could be adapted to other cases.