Cheerio + HTMLRewriter for Cloudflare Workers

HTMLRewriter.js

/**
 *   Cheerio Wrapper for HTMLRewriter on Cloudflare Workers®   
 *
 *   Usage of cheerio here only simulates stream-oriented parser! It is slow!
 */
import cheerio from 'cheerio'

function replace(content, options) {
    this[0].nodeValue = content
}

function hasAttribute(name) {
    return (this.getAttribute(name) !== undefined)
}

function getAttribute(name) {
    return this.attr(name)
}

function setAttribute(name, value) {
    this.attr(name, value)
}

export default class HTMLRewriter {
    constructor() {
        this.selectors = []
    }

    on(element, handler) {
        this.selectors.push([element, handler])
        return this
    }

    async transform(response) {
        const text = await response.text()
        const $ = cheerio.load(text)

        // Simulate stream-based parser
        this.walk($, $.root())

        return new Response($.root().html(), response)
    }

    walk($, node) {
        const $node = this.wrapElement($, node)

        // Select matching HTMLRewrite handlers
        const matchedHandlers = []
        for(const [selector, handler] of this.selectors) {
            if ($node.is(selector)) {
                matchedHandlers.push(handler)
            }
        }

        // Trigger HTMLRewrite handlers on Element
        for(const handler of matchedHandlers) {
            handler.element && handler.element($node)
        }

        // Walk all children
        const lastNode = $node.contents().length - 1
        const children = $node.contents().toArray()
        for (const [i, child] of children.entries()) {
            if (child.nodeType === 1) {
                this.walk($, child)
            } else {
                const lastInNode = (i === lastNode) || (children[i+1].nodeType !== child.nodeType)
                const $child = this.wrapOther($, child, lastInNode)
                
                for (let handler of matchedHandlers) {
                    if (child.nodeType === 3) {         // text node
                        handler.text && handler.text($child)
                    } else if (child.nodeType === 8) {  // comment node 
                        handler.comments && handler.comments($child)
                    }
                }
            }
        }
    }

    wrapElement($, node) {
        const $node = $(node)
        $node.tagName = node.name

        $node.hasAttribute = hasAttribute.bind($node)
        $node.getAttribute = getAttribute.bind($node)
        $node.setAttribute = setAttribute.bind($node)

        return $node
    }

    wrapOther($, node, lastInNode) {
        const $node = $(node)

        $node.text = node.nodeValue
        $node.replace = replace.bind($node)
        if (node.nodeType === 3) {
            $node.lastInTextNode = lastInNode
        }

        return $node
    }
}