index.js

/**
 * External Links MarkdownIt plugin
 * Mark external, absolute links as necessary
 *
 * @param {import("markdown-it")} md
 * @param {object} options
 * @param {string?} options.domain domain to consider an internal link.
 * @param {string?} options.class add a class (preserves existing)
 */

module.exports = (md, options={}) => {

    // Finds external links
    let regex = /^https?:\/\//;

    if(!!options.domain) {
        // Sanitize domain - remove the protocol, if existing
        const domain = options.domain.replace(/^https?:\/\//, "");

        regex = RegExp(`^https?:\/\/(?!${domain}|localhost)`);
    }


    /**
     * @type {import("markdown-it/lib/parser_core").RuleCore}
     */
    function link_external({ tokens }) {
        // Iterate through tokens, looking for links
        for(let t of tokens) {
            // There are no tokens, or we got an empty one (skip)
            if(!t) continue;

            // It's a block, but any links will be inline (skip)
            if(t.type !== 'inline') continue;

            // It's inline, but there are no child tokens (skip)
            if(!t.children) continue;

            // There's children, but no links
            if(t.children.filter(c => c.type === "link_open").length === 0) {
                continue;
            }

            for(let c of t.children) {

                // It's not a link (skip)
                if(c.type !== "link_open") continue;

                let href = c.attrGet("href");

                // It's a link tag, but the url is missing (skip)
                if(!href) continue;

                // It's a link, but it's internal (skip)
                if(!regex.test(href)) continue;

                // We can start messing around now!

                // Note: `attrJoin` preserves existing values
                c.attrJoin("rel", "noopener noreferrer")
                c.attrSet("target", "_blank")
                if(!!options.class) c.attrJoin("class", options.class)
            }
        }
    }

    md.core.ruler.push("link_external", link_external);
};