如何只将参数传递给需要它的中间件？

Question

我有一个Express应用程序,我正在尝试将所有中间件放在自己的文件中.一些中间件函数需要该db对象,而另一些则不需要.

这对于那些不需要的功能非常简单的db对象,但由于我下面的代码结构,我怎么可以参照db对象doesNotNeedDbParam,因为它已经拥有PARAMS req,res和next？

somefile.js:

const router = express.Router()
const doesNotNeedDbParam = require('./middleware')().doesNotNeedDbParam

function foo () {
  // Currently I have to call require and pass in the db object here b/c 
  // it's not set when requiring the function doesNotNeedDbParam
  router.use(require('./middleware')(db).needsDbParam // <-- Is there a better way to do this so that I can require the file above and pass the db object in when it's set?
}

// Setup db object here
foo()

middleware.js

function doesNotNeedDbParam (req, res, next) {
  ...
}

function needsDbParam (req, res, next) {
  // Where do I reference the db variable?
}

module.exports = (db) => {
  return {
    doesNotNeedDbParam: doesNotNeedDbParam,
    needsDbParam: needsDbParam
  }
}

Answer 1

功能方法

我认为一个好的结构,这是试图讨好你的中间件.这是一种由中间件实现的模式,例如body-parserExpress本身和内部使用的模式serve-static.这样,您只需要一次,并db在需要的地方通过,而不是在您不需要的地方:

// Instead of declaring each function directly as a middleware function,
// we declare them as a function that returns a middleware function
function doesNotNeedDbParam () {
  return function (req, res, next) {
    …
  }
}

function needsDbParam (db) {
  return function (req, res, next) {
    // You can use db here along with req, res, next
  }
}

// No need to export a function now
module.exports = {
  doesNotNeedDbParam,
  needDbParam,
};

然后,只需要:

const middleware = require('./middleware');
…
router.use(middleware.doesNotNeedDbParam()); // Since this doesn't need anything, no argument
router.use(middleware.needsDbParam(db)); // You can pass db here now

如果您对ES6 +语法感到满意,可以缩写为:

const doesNotNeedDbParam = () => (req, res, next) => {
  …
}

const needsDbParam = (db) => (req, res, next) => {
  // Use db here
}
// Export object here...

然后:

const { doesNotNeedDbParam, needsDbParam } = require('./middleware');
… 
router.use(doesNotNeedDbParam());
router.use(needsDbParam(db));

附加方法

通过将属性附加到req对象一次,还有另一种方法可以做到这一点.这样就无需db每次都需要重新进行修复.许多其他包使用此策略.它是这样的:

function attachDb (db) { // Still use curry approach here since we want db
  return function (req, res, next) {
    // Attaches the specified db to req directly
    req.db = db;
  }
}

function needsDbParam (req, res, next) { // No need for currying
  // Now use req.db here
}
// Export your other middleware…

然后,像这样使用它,确保attachDb是第一个,以便在使用它之前分配属性:

router.use(attachDb(db)); // Before all other middleware that depend on req.db
…
// No need to call because this is already the middleware function, 
// able to use req.db, which was assigned via attachDb
router.use(needDbParam);