Koa Introduction ​

Koa is the next-generation web framework based on Node.js, designed to be the successor of Express. It currently has two versions: 1.x and 2.x.

History ​

Express ​

Express was the first and most popular web framework, encapsulating Node.js's HTTP module. A simple example of using Express looks like this:

let express = require('express');
let app = express();

app.get('/', function (req, res) {
    res.send('Hello World!');
});

app.listen(3000, function () {
    console.log('Example app listening on port 3000!');
});

While Express has a straightforward API, it is based on ES5 syntax, which limits asynchronous code handling to callbacks. When asynchronous operations are nested, the code can become messy:

app.get('/test', function (req, res) {
    fs.readFile('/file1', function (err, data) {
        if (err) {
            res.status(500).send('read file1 error');
        }
        fs.readFile('/file2', function (err, data) {
            if (err) {
                res.status(500).send('read file2 error');
            }
            res.type('text/plain');
            res.send(data);
        });
    });
});

Although libraries like async can help manage asynchronous code, using callbacks can still be cumbersome.

Koa 1.x ​

With the introduction of ES6 in newer versions of Node.js, the Express team rewrote the next-generation web framework, Koa, using ES6 generators. Compared to Express, Koa 1.x leverages generators for asynchronous handling, making the code appear synchronous:

let koa = require('koa');
let app = koa();

app.use('/test', function *() {
    yield doReadFile1();
    let data = yield doReadFile2();
    this.body = data;
});

app.listen(3000);

Using generators simplifies asynchronous code compared to callbacks, but generators were not originally designed for asynchronous tasks. Promises are intended for that purpose, though their syntax can be complex. To further simplify asynchronous coding, JavaScript introduced the async and await keywords, which allow you to easily create asynchronous functions:

async function () {
    let data = await fs.read('/file1');
}

This is a standard way to write asynchronous code in JavaScript—clean and easy to use.

Koa 2.x ​

The Koa team continued to innovate and developed Koa 2, which entirely utilizes Promises and async for asynchronous operations. Koa 2.x code looks like this:

app.use(async (ctx, next) => {
    await next();
    let data = await doReadFile();
    ctx.response.type = 'text/plain';
    ctx.response.body = data;
});

Which Version to Choose? ​

To stay current with modern practices, this tutorial will use the latest version, Koa 2.x, for development!