TryGhost/Ghost

class LinkReplacer {
    /**
     * Replaces the links in the provided HTML
     * @param {string} html
     * @param {(url: URL, originalPath: string): Promise<URL|string|false>} replaceLink
     * @param {object} options
     * @param {string} [options.base] If you want to replace relative links, this will replace them to an absolute link and call the replaceLink method too
     * @returns {Promise<string>}
    */
    async replace(html, replaceLink, options = {}) {
        const {tokenize} = require('html5parser');
        const entities = require('entities');

        try {
            const tokens = tokenize(html); // IToken[]
            const replacements = [];

            let inAnchor = false;
            let inHref = false;

            // interface IToken {
            //   start: number;
            //   end: number;
            //   value: string;
            //   type: TokenKind;
            // }

            // const enum TokenKind {
            // 0  Literal,
            // 1  OpenTag, // trim leading '<'
            // 2  OpenTagEnd, // trim tailing '>', only could be '/' or ''
            // 3  CloseTag, // trim leading '</' and tailing '>'
            // 4  Whitespace, // the whitespace between attributes
            // 5  AttrValueEq,
            // 6  AttrValueNq,
            // 7  AttrValueSq,
            // 8  AttrValueDq,
            // }

            for (const token of tokens) {
                if (token.type === 1 && token.value === 'a') {
                    inAnchor = true;
                }

                if (inAnchor) {
                    if (token.type === 2) {
                        inAnchor = false;
                        inHref = false;
                    }

                    if (token.type === 6 && token.value === 'href') {
                        inHref = true;
                    }

                    if (inHref && token.type === 8) {
                        const path = entities.decode(token.value.substring(1, token.value.length - 1));
                        let url;
                        try {
                            url = new URL(path, options.base);
                        } catch (e) {
                            // Ignore invalid URLs
                        }
                        if (url) {
                            url = await replaceLink(url, path);
                            const str = url.toString();
                            replacements.push({url: str, start: token.start + 1, end: token.end - 1});
                        }

                        inHref = false;
                    }
                }
            }

            let offsetAdjustment = 0;

            replacements.forEach(({url, start, end}) => {
                const originalLength = end - start;
                const replacementLength = url.length;

                html = html.slice(0, start + offsetAdjustment) + url + html.slice(end + offsetAdjustment);

                offsetAdjustment += replacementLength - originalLength;
            });

            return html;
        } catch (e) {
            // do nothing in case of error,
            // we don't want to break the content for the sake of member attribution
            return html;
        }
    }
}

module.exports = new LinkReplacer();