<pre class="shiki github-dark shaku"><code>class Middleware { constructor() { this.funcs = []; // Create a function queue to help with execution this.req = null; } /** * @param {MiddlewareFunc} func 参数是函数 */ use(func) { this.funcs.push(func); // Push the function into Queue } /** * @param {Request} req */ start(req) { this.req = req; // 把参数req赋值给this.reqs this._next(); // Start the chain } _next = (err) =&gt; { const toExecute = this.funcs.shift(); // take out the function to execute from the queues try { // 2个参数时候是normal call if (toExecute.length === 2) { //case 1: there is no error, execute the function with current request and next() if (!err) toExecute(this.req, this._next); //case 3: There is an error, then skip the execute function, call next else this._next(err); } //case4: 3个参数时候是 error call (i.e. an error handler) if (toExecute.length === 3) { toExecute(err, this.req, this._next); } } catch (e) { //case 2: Catch execution error when a function throw an error this._next(e); } };}/*-------------------- 用例测试 1 -------------------------*/const middleware = new Middleware();// (case1 : normal call and execution success)middleware.use((req, next) =&gt; { req.a = 1; next();});// (case1 : normal call and execution success)middleware.use((req, next) =&gt; { req.b = 2; next();});// (case1 : normal call and execution success)middleware.use((req, next) =&gt; { console.log(req);});middleware.start({}); // {a: 1, b: 2}/*-------------------- 用例测试 2 -------------------------*/const middleware = new Middleware();// throw an error at first function (case2: normal call but throw error --&gt; should Catch execution error when a function throws an error)middleware.use((req, next) =&gt; { req.a = 1; throw new Error('sth wrong'); // or next(new Error('sth wrong'));});//Since error occurs, this is skipped (case3: normal call but skipped execution, because next has err)middleware.use((req, next) =&gt; { req.b = 2;});//Since error occurs, this is skipped (case3: normal call but skipped execution, because next has err)middleware.use((req, next) =&gt; { console.log(req);});//Since error occurs, this is called (case4: error call))middleware.use((error, req, next) =&gt; { console.log(error); console.log(req);});middleware.start({}); // Error: sth wrong {a: 1}</code></pre>

Detail eng + chinese explanation

Have you ever used Express Middleware before? Middleware functions are functions with fixed interface that could be chained up like following two func…