主题
Koa 简单模拟
1. koa 的请求像一个洋葱模型
2. 实例代码
下面的代码输出：
1
2
3
end
4
5javascript
const Koa = require("koa");
const app = new Koa();
app.use(async (ctx, next) => {
console.log(ctx);
if (ctx.request.url === "/favicon.ico") {
ctx.status = 404;
return;
}
console.log("1");
await next();
console.log("5");
});
app.use(async (ctx, next) => {
console.log("2");
await next();
console.log("4");
});
app.use((ctx) => {
console.log("3");
console.log("end");
ctx.status = 200;
ctx.body = `Hello from koa`;
});
app.listen(3002, () => console.log("Koa app listening on 3002"));2. 如何模拟一个简单的 koa
不断的递归 next 函数来模拟
javascript
const http = require("http");
class Koa {
constructor() {
this.middleware = [];
this.server = http.createServer();
this.ctx = {};
}
listen(...args) {
this.server.on("request", this.request.bind(this)).listen(...args);
}
async request(req, res) {
this.ctx.request = req;
this.j = -1;
await this.next();
res.statusCode = this.ctx.status || 200;
res.end(this.ctx.body || "OK");
}
async next() {
this.j++;
if (this.j > this.middleware.length - 1) {
return;
}
await this.middleware[this.j](this.ctx, this.next.bind(this));
}
use(fn) {
this.middleware.push(fn);
}
}3. 如何模拟一个简单的 koa
通过调用高阶函数 compose, 然后里面通过 Promise 来巧妙的递归。
javascript
const http = require("http");
class Koa {
constructor() {
this.middleware = [];
this.server = http.createServer();
}
listen(...args) {
this.server.on("request", this.request.bind(this)).listen(...args);
}
async request(req, res) {
const ctx = {
request: req
};
const fn = this.compose(this.middleware);
try {
await fn(ctx);
res.statusCode = ctx.status || 200;
res.end(ctx.body || "OK");
} catch (e) {
res.statusCode = 200;
res.end(e.message || "error message");
}
}
compose(middleware) {
return (ctx) => {
let len = middleware.length;
const dispatch = (i) => {
if (i === len) return Promise.resolve();
const fn = middleware[i];
try {
return Promise.resolve(
fn(ctx, () => {
return dispatch(i + 1);
})
);
} catch (e) {
return Promise.reject(e);
}
};
return dispatch(0);
};
}
use(fn) {
this.middleware.push(fn);
}
}4. koa 如何监听报错
错误处理中间件在最顶端运行。
javascript
app.use(async (ctx, next) => {
try {
await next();
} catch (err) {
ctx.status = 400;
ctx.body = `Uh-oh: ${err.message}`;
console.log("Error handler:", err.message);
}
});5. koa 计算 http 的请求时间
javascript
app.use(async (ctx, next) => {
const t = Date.now();
await next();
console.log(Date.now() - t);
});