Node.js: minify CSS files in ExpressJS without using a module

In ExpressJS we can minify CSS files on the fly without using a module.

Minifying a CSS file simply means removing a set of characters that are ignored by the CSS parser (e.g. new lines, comments, unnecessary spaces etc.).

'use strict';

const app = require('express')();
const fs = require('fs');

const readCSS = path => {
    return new Promise((resolve, reject) => {
        if(!/\.css$/.test(path)) {
            reject({error: 'Invalid file'});
        }
        if(!fs.existsSync(path)) {
            reject({error: 'File does not exist'});
        }

        fs.readFile(path, 'UTF-8', (err, contents) => {
            if(err) {
                reject({error: err});
            } else {
                resolve(contents.toString());
            }
        });
    });
};

const minifyCSS = async (req, res) => {
    try {
        let cssFile = await readCSS('.' + req.originalUrl);
        let buffer = cssFile.replace(/\s+/g, ' ').replace(/\n|\r\n/g, '')
                            .replace(/\/\*.*?\*\//g, '')
                            .replace(/;\s+/g, ';')
                  .replace(/:\s+/g, ':').
                  replace(/\s+\{/g, '{').
                  replace(/\{\s+/g, '{').
                  replace(/,\s+/g, ',').
                  replace(/\}\s+/g, '}').
                  replace(/;\}/g, '}');
        res.set('Content-Type', 'text/css');
        res.send(buffer);
    } catch(err) {
        res.sendStatus(404);
    }         
};

app.get('/public/css/style.min.css', minifyCSS);

app.listen(4000);

In order to maximize performance you can also specify cache headers when serving the CSS file by using the ExpressJS response.set() method.