Rewrite links on HTML pages

export default {
	async fetch(request) {
		// Define the old hostname here.
		const OLD_URL = "oldsite.com";
		// Then add your new hostname that should replace the old one.
		const NEW_URL = "newsite.com";

		class AttributeRewriter {
			constructor(attributeName) {
				this.attributeName = attributeName;
			}
			element(element) {
				const attribute = element.getAttribute(this.attributeName);
				if (attribute) {
					element.setAttribute(
						this.attributeName,
						attribute.replace(OLD_URL, NEW_URL),
					);
				}
			}
		}

		const rewriter = new HTMLRewriter()
			.on("a", new AttributeRewriter("href"))
			.on("img", new AttributeRewriter("src"));

		const res = await fetch(request);
		if (!res.headers.has("Content-Type")) {
			return res;
		}
		const contentType = res.headers.get("Content-Type");
		if (typeof contentType !== "string") {
			return res;
		}

		// If the response is HTML, it can be transformed with
		// HTMLRewriter -- otherwise, it should pass through
		if (contentType.startsWith("text/html")) {
			return rewriter.transform(res);
		} else {
			return res;
		}
	},
};