Koa.js 核心函数 compose

See the Pen Koa.js compose by Willard (@ifyour) on CodePen.

// Koa.js 核心代码 compose
// 决定了 Koa.js 中间件的调用顺序，也就是洋葱模型
function compose(middleware) {
  return function(context, next) {
    // 最后调用的 middleware 索引
    let index = -1;
    return dispatch(0);
    function dispatch(i) {
      if (i <= index)
        return Promise.reject(new Error("next() called multiple times"));
      index = i;
      let fn = middleware[i];
      if (i === middleware.length) fn = next;
      if (!fn) return Promise.resolve(); // 结束递归条件
      try {
        return Promise.resolve(
          fn(context, function next() {
            return dispatch(i + 1); // 递归调用自身
          })
        );
      } catch (err) {
        return Promise.reject(err);
      }
    }
  };
}
async function a (ctx, next) {
  console.log(1);
  const hello = await Promise.resolve('Hello, node.js');
  console.log(hello);
  await next();
  console.log('a end');
}
async function b (ctx, next) {
  console.log(2);
  const hello = await Promise.resolve('Hello, node.js');
  console.log(hello);
  await next();
  console.log('b end');
}
//
// Demo：测试中间件调用
//
const myCtx = {};
compose([a, b])(myCtx);

执行结果：

1
"Hello, node.js"
2
"Hello, node.js"
"b end"
"a end"