Asynchronous HTMLRewriter for Cloudflare Workers
Last year, we launched HTMLRewriter for Cloudflare Workers, which enables developers to make streaming changes to HTML on the edge. Unlike a traditional DOM parser that loads the entire HTML document into memory, we developed a streaming parser written in Rust. Today, we’re announcing support for asynchronous handlers in HTMLRewriter. Now you can perform asynchronous tasks based on the content of the HTML document: from prefetching fonts and image assets to fetching user-specific content from a CMS.
How can I use HTMLRewriter?
We designed HTMLRewriter to have a jQuery-like experience. First, you define a handler, then you assign it to a CSS selector; Workers does the rest for you. You can look at our new and improved documentation to see our supported list of selectors, which now include nth-child selectors. The example below changes the alternative text for every second image in a document.
async function editHtml(request) {
return new HTMLRewriter()
.on("img:nth-child(2)", new ElementHandler())
.transform(await fetch(request))
}
class ElementHandler {
element(e) {
e.setAttribute("alt", "A very interesting image")
}
}
Since these changes are applied using streams, we maintain a low TTFB (time to first byte) and users never know the HTML was transformed. If you’re interested in how we’re Continue reading
