A typescript koa 2 sass middleware

koa2-sass.ts

import * as fs from 'fs';
import * as Koa from 'koa';
import * as sass from 'node-sass';
import * as path from 'path';
import * as url from 'url';
import * as util from 'util';
const sassRenderAsync = util.promisify(sass.render);
const fileExistsAsync = util.promisify(fs.exists);

export default (options: ISassOptions) => {

    return async (context: Koa.Context, next: () => Promise<any>) => {
        if (!context.req.url || !/\.css/.test(context.req.url)) {
            return await next();
        }

        const scssFile = cssToLocalScssPath(context.req.url, options);
        if (!await fileExistsAsync(scssFile)) {
            context.throw(404, 'Cannot find file');
            return;
        }

        const sassOptions: sass.Options = { file: scssFile };
        const result = await sassRenderAsync(sassOptions);

        await next();
        context.res.writeHead(200, {
            'Cache-Control': 'max-age=0',
            'Content-Type': 'text/css',
        });
        context.body = result.css.toString('utf8');
    };
};

function cssToLocalScssPath(cssUrl: string, options: ISassOptions): string {
    const pathname = url.parse(cssUrl, true, true).pathname;
    if (!pathname) {
        throw new Error('No path could be found');
    }
    const relativePath = path.relative(options.publicPath, pathname);
    const file = path.join(options.src, relativePath).replace('.css', '.scss');

    return file;
}

export interface ISassOptions {
    publicPath: string;
    src: string;
}