Header generator file stripped of all comments used in my "Asset fingerprinting and the preload header in 11ty" blog post

header-generator.js

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

export function generatePreloadHeaders() {
	console.log('🔗 Generating preload headers for CSS files...');
	const cssDir = path.join('./_site', 'css');

	if (!fs.existsSync(cssDir)) {
		console.log('CSS directory not found, skipping header generation');
		return;
	}

	const cssFiles = fs.readdirSync(cssDir)
		.filter(file => {
			return file.match(/^index-[a-f0-9]{10}\.css(\.br)?$/);
		});

	
	if (cssFiles.length === 0) {
		console.log('No fingerprinted CSS files found, skipping header generation');
		return;
	}
	
	cssFiles.sort((a, b) => {
		if (a.endsWith('.br') && !b.endsWith('.br')) return -1;
		if (!a.endsWith('.br') && b.endsWith('.br')) return 1;
		return 0;
	});
	
	const cssFile = cssFiles[0];
	const cssPath = `/css/${cssFile}`;

	console.log(`📄 Found CSS file: ${cssFile}`);
	
	try {
		const sourceHeadersPath = path.join('./public', '_headers');
		const targetHeadersPath = path.join('./_site', '_headers');

		if (!fs.existsSync(sourceHeadersPath)) {
			console.log('Source _headers file not found');
			return;
		}

		let headersContent = fs.readFileSync(sourceHeadersPath, 'utf8');

		const preloadHeader = `  Link: <${cssPath}>; rel=preload; as=style; nopush`;
		const lines = headersContent.split('\n');
		let globalSectionIndex = -1;
		let nextSectionIndex = -1;

		for (let i = 0; i < lines.length; i++) {
			if (lines[i].trim() === '/*') {
				globalSectionIndex = i;
				break;
			}
		}

		if (globalSectionIndex !== -1) {
			for (let i = globalSectionIndex + 1; i < lines.length; i++) {
				if (lines[i].trim() !== '' && !lines[i].startsWith('  ')) {
					nextSectionIndex = i;
					break;
				}
			}
		}

		if (globalSectionIndex !== -1) {
			let linkHeaderExists = false;
			for (let i = globalSectionIndex + 1; i < (nextSectionIndex === -1 ? lines.length : nextSectionIndex); i++) {
				if (lines[i].includes('Link:')) {
					lines[i] = preloadHeader;
					linkHeaderExists = true;
					break;
				}
			}

			if (!linkHeaderExists) {
				let lastHeaderIndex = globalSectionIndex;
				for (let i = globalSectionIndex + 1; i < (nextSectionIndex === -1 ? lines.length : nextSectionIndex); i++) {
					if (lines[i].trim() !== '' && lines[i].startsWith('  ')) {
						lastHeaderIndex = i;
					}
				}
				lines.splice(lastHeaderIndex + 1, 0, preloadHeader);
			}

			headersContent = lines.join('\n');
		} else {
			console.log('Could not find global section in _headers file');
			return;
		}
		fs.writeFileSync(targetHeadersPath, headersContent);
		console.log(`Generated preload header: Link: <${cssPath}>; rel=preload; as=style; nopush`);
	} catch (error) {
		console.error('Error generating preload headers:', error);
	}
}