Basic Rollup plugin for nested handlebars. Quick 'n dirty, no configuration options, you have to add them yourself

rollup-plugin-handlebars-nested.js

import path from 'path';
import fs from 'fs';

import Handlebars from 'handlebars';

class ImportScanner extends Handlebars.Visitor {
	constructor() {
		super();
		this.partials = new Set();
	}

	PartialStatement(partial) {
		if (partial.name.type === 'SubExpression') {
			throw new Error('Dynamic partial resolution is not supported');
		}

		this.partials.add(partial.name.original);

		return super.PartialStatement(partial);
	}
}

export default function handlebarsNested() {
	// Template cache
	const cache = new Map();

	function getCompiledPartial(partial, dir) {
		const source = fs.readFileSync(path.join(__dirname, dir, `${partial}.hbs`), 'utf-8');

		const compiled = Handlebars.precompile(source);

		return [partial, compiled];
	}

	// Recursive function for getting nested partials
	function getPartials(source, dir, list = new Set()) {
		const tree = Handlebars.parse(source);

		const scanner = new ImportScanner();

		scanner.accept(tree);

		if (scanner.partials.size) {
			// We have found partials!
			for (const partial of scanner.partials) {
				// Add it to the list
				list.add(partial);

				// Get the source
				const src = fs.readFileSync(path.join(__dirname, dir, `${partial}.hbs`), 'utf-8');

				// And check if we have more nested partial in this one
				getPartials(src, dir, list);
			}
		}

		return list;
	}

	// Convert to ESM and register partial
	function toEsm(source, id) {
		const dir = path.dirname(id);
		const name = path.basename(id, '.hbs');

		// Get nested partials
		const partials = getPartials(source, dir);

		const children = [];

		for (const partial of partials) {
			children.push(getCompiledPartial(partial, dir));
		}

		// Create a tree
		const tree = Handlebars.parse(source);

		const { code, map } = Handlebars.precompile(tree, { srcName: id });

		// Import this (partial) template and nested templates
		const body = `
			import Handlebars from 'handlebars/runtime.js';

			const template = Handlebars.template(${code});

			Handlebars.registerPartial('${name}', template);

			${children.map(([p, s]) => `Handlebars.registerPartial('${p}', Handlebars.template(${s}));`).join('\n')}

			export default (data, options) => template(data, options);
		`;

		return {
			code: body,
			map,
		};
	}

	return {
		name: 'handlebars-nested',

		transform(source, id) {
			if (id.endsWith('.hbs')) {
				// Get from cache when avalaible
				if (cache.has(id)) {
					return cache.get(id);
				}

				const output = toEsm(source, id);

				cache.set(id, output);

				return output;
			}

			return null;
		},
	};
}

Source maps only work for the first imported template, not for the nested partials.