Expressive HTTP middleware for node.js to make web applications and APIs more enjoyable to write. Koa's middleware stack flows in a stack-like manner, allowing you to perform actions downstream then filter and manipulate the response upstream. Koa's use of generators also greatly increases the readability and robustness of your application.
Only methods that are common to nearly all HTTP servers are integrated directly into Koa's small ~550 SLOC codebase. This includes things like content negotiation, normalization of node inconsistencies, redirection, and a few others.
Koa is not bundled with any middleware.
Koa requires node v4.0.0 or higher for (partial) ES2015 support.
const Koa = require('koa');
const app = new Koa();
// logger
app.use((ctx, next) => {
const start = new Date;
return next().then(() => {
const ms = new Date - start;
console.log(`${ctx.method} ${ctx.url} - ${ms}`);
});
});
// response
app.use(ctx => {
ctx.body = 'Hello World';
});
app.listen(3000); Example with async functions (Babel required)
const Koa = require('koa');
const app = new Koa();
// logger
app.use(async (ctx, next) => {
const start = new Date;
await next();
const ms = new Date - start;
console.log(`${ctx.method} ${ctx.url} - ${ms}`);
});
// response
app.use(ctx => {
ctx.body = 'Hello World';
});
app.listen(3000);
To use generator functions, you must use a wrapper such as co that is no longer supplied with Koa.
const Koa = require('koa');
const app = new Koa();
const co = require('co');
// logger
app.use(co.wrap(function *(ctx, next){
const start = new Date;
yield next();
const ms = new Date - start;
console.log(`${ctx.method} ${ctx.url} - ${ms}`);
}));
// response
app.use(ctx => {
ctx.body = 'Hello World';
});
app.listen(3000); Example with old signature
If you want to use old signature or be compatible with old middleware, you must use koa-convert to convert legacy generator middleware to promise middleware.
const Koa = require('koa');
const app = new Koa();
const convert = require('koa-convert')
// logger
app.use(convert(function *(next){
const start = new Date;
yield next;
const ms = new Date - start;
console.log(`${this.method} ${this.url} - ${ms}`);
}));
// response
app.use(ctx => {
ctx.body = 'Hello World';
});
app.listen(3000);
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