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A Koa-style middleware chain

Write compose(middlewares): it returns a function (ctx) => Promise that runs the middlewares in order. Each one gets ctx and next; await next() hands control to the next one and waits until the rest of the chain finishes, and the code after it runs on the way back (the "onion").

If a middleware doesn't call next(), the rest doesn't run. An error in any middleware (sync or async) rejects the promise and can be caught with try/catch around await next() higher up the chain. Calling next() twice in one middleware is an error: next() called multiple times.

function compose(middlewares) {
  return async function (ctx) {
    // ...
  };
}

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Tests (6)

const log = [];
const app = compose([
  async (ctx, next) => { log.push('a1'); await next(); log.push('a2'); },
  async (ctx, next) => { log.push('b1'); await next(); log.push('b2'); },
  async () => { log.push('c'); },
]);
await app({});
expect(log).toEqual(['a1', 'b1', 'c', 'b2', 'a2']);

const app = compose([
  async (ctx, next) => { ctx.user = 'ann'; await next(); },
  async (ctx) => { ctx.body = 'hello ' + ctx.user; },
]);
const ctx = {};
await app(ctx);
expect(ctx.body).toBe('hello ann');

const third = fn();
const app = compose([
  async (ctx, next) => { ctx.status = 401; },
  async () => third(),
]);
const ctx = {};
await app(ctx);
expect(ctx.status).toBe(401);
expect(third).toHaveBeenCalledTimes(0);

const app = compose([
  async (ctx, next) => {
    try { await next(); } catch (e) { ctx.status = 500; ctx.error = e.message; }
  },
  async () => { throw new Error('db down'); },
]);
const ctx = {};
await app(ctx);
expect(ctx).toEqual({ status: 500, error: 'db down' });

const app = compose([async (ctx, next) => { await next(); await next(); }]);
await expect(app({})).rejects.toThrow('next() called multiple times');

const app = compose([(ctx, next) => { throw new Error('sync boom'); }]);
await expect(app({})).rejects.toThrow('sync boom');
await compose([])({});