JavaScript管道与中间件函数解析

const toUpperCase = str => str.toUpperCase();
const addExclamation = str => str + '!';

const result = addExclamation(toUpperCase('hello')); // 'HELLO!'

const compose = (...fns) => x =>
  fns.reduceRight((acc, fn) => fn(acc), x);

const transform = compose(addExclamation, toUpperCase);
transform('hello'); // 'HELLO!'

const pipe = (...fns) => x =>
  fns.reduce((acc, fn) => fn(acc), x);

const transform = pipe(toUpperCase, addExclamation);
transform('world'); // 'WORLD!'

const cleanInput = pipe(
  str => str.trim(),
  str => str.toLowerCase(),
  str => str.replace(/\s+/g, '_')
);
cleanInput('  Hello World  '); // 'hello_world'

app.use(async (ctx, next) => {
  console.log('Before');
  await next();
  console.log('After');
});

function composeMiddleware(middlewareList) {
  return function (context) {
    const dispatch = i => {
      if (i >= middlewareList.length) return Promise.resolve();
      const fn = middlewareList[i];
      return Promise.resolve(fn(context, () => dispatch(i + 1)));
    };
    return dispatch(0);
  };
}

const middleware = [
  async (ctx, next) => {
    ctx.step1 = 'done';
    await next();
  },
  async (ctx, next) => {
    ctx.step2 = 'done';
    await next();
  }
];

const fn = composeMiddleware(middleware);
const ctx = {};
fn(ctx).then(() => console.log(ctx)); // { step1: 'done', step2: 'done' }

// 如果支持
'hello' |> toUpperCase |> addExclamation; // 'HELLO!'