joplin/ReactNativeClient/lib/joplin-renderer/MdToHtml/rules/fence.ts

// Note: this is copied from https://github.com/markdown-it/markdown-it/blob/master/lib/renderer.js
// Markdown-it assigns a special meaning to code returned from highlight() when it starts with PRE or not.
// If it starts with PRE, the highlited code is returned as-is. If it does not, it is wrapped in <PRE><CODE>
// This is a bit of a hack and magic behaviour, and it prevents us from returning a DIV from the highlight
// function.
// So we modify the code below to allow highlight() to return an object that tells how to render
// the code.

function installRule(markdownIt:any) {
	// @ts-ignore: Keep the function signature as-is despite unusued arguments
	markdownIt.renderer.rules.fence = function(tokens:any[], idx:number, options:any, env:any, slf:any) {
		var token = tokens[idx],
			info = token.info ? markdownIt.utils.unescapeAll(token.info).trim() : '',
			langName = '',
			highlighted, i, tmpAttrs, tmpToken;

		if (info) {
			langName = info.split(/\s+/g)[0];
		}

		if (options.highlight) {
			highlighted = options.highlight(token.content, langName) || markdownIt.utils.escapeHtml(token.content);
		} else {
			highlighted = markdownIt.utils.escapeHtml(token.content);
		}

		const wrapCode = highlighted && highlighted.wrapCode !== false;
		highlighted = typeof highlighted !== 'string' ? highlighted.html : highlighted;

		if (highlighted.indexOf('<pre') === 0 || !wrapCode) {
			return `${highlighted}\n`;
		}

		// If language exists, inject class gently, without modifying original token.
		// May be, one day we will add .clone() for token and simplify this part, but
		// now we prefer to keep things local.
		if (info) {
			i        = token.attrIndex('class');
			tmpAttrs = token.attrs ? token.attrs.slice() : [];

			if (i < 0) {
				tmpAttrs.push(['class', options.langPrefix + langName]);
			} else {
				tmpAttrs[i][1] += ` ${options.langPrefix}${langName}`;
			}

			// Fake token just to render attributes
			tmpToken = {
				attrs: tmpAttrs,
			};

			return  `<pre><code${slf.renderAttrs(tmpToken)}>${
				highlighted
			}</code></pre>\n`;
		}


		return  `<pre><code${slf.renderAttrs(token)}>${
			highlighted
		}</code></pre>\n`;
	};
}

export default function() {
	return function(md:any) {
		installRule(md);
	};
}