node.js - 路由器级中间件 : undefined object

Question

这是符合条件的:

someRouter.get('/...',
    (req, res, next) => {
            ...
    },
    (req, res) => {...}

虽然这不符合条件:

someRouter.get('/...',
            aMiddleware,
            (req, res) => {...}

const aMiddleware = (req, res, next) => {...}

后者给出错误:

Error: Route.get() requires a callback function but got a [object Undefined]
    at Route.(anonymous function) [as get] (/Users/Leon/Documents/Projects/NodeJS/DemoJWTinNodeJS/node_modules/express/lib/router/route.js:202:15)
    at Function.proto.(anonymous function) [as get] (/Users/Leon/Documents/Projects/NodeJS/DemoJWTinNodeJS/node_modules/express/lib/router/index.js:510:19)
    at Object.get (/Users/Leon/Documents/Projects/NodeJS/DemoJWTinNodeJS/src/routes/api/userRoute.js:30:12)
    at Module._compile (internal/modules/cjs/loader.js:723:30)
    at Module._compile (/Users/Leon/Documents/Projects/NodeJS/DemoJWTinNodeJS/node_modules/pirates/lib/index.js:99:24)
    at Module._extensions..js (internal/modules/cjs/loader.js:734:10)
    at Object.newLoader [as .js] (/Users/Leon/Documents/Projects/NodeJS/DemoJWTinNodeJS/node_modules/pirates/lib/index.js:104:7)
    at Module.load (internal/modules/cjs/loader.js:620:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:560:12)
    at Function.Module._load (internal/modules/cjs/loader.js:552:3)

在我看来，它基本上提示 undefined object ，这应该是一个中间件。

中间件功能体完全相同。请问怎么了？

Answer 1

如果使用const关键字，请在router之前定义中间件，或者改为函数定义。

方法一:

const aMiddleware = (req, res, next) => {...}
someRouter.get('/...',
        aMiddleware,
        (req, res) => {...}

方法2:

someRouter.get('/...',
        aMiddleware,
        (req, res) => {...}
function aMiddleware(req, res, next){...}