Express: Router & Middleware

前言

  1. 目的:觀察和理解 Middleware 中的:
    (1) app.use、(2) next()、(3) Router 順序,對 request 流向的影響。
  2. 方法:建立一組 Router 設定檔(範例),包含 4 組設定
    (Name代稱[Router])和最終回傳 JSON:
    (1) Kevin [app.use]
    (2) John [POST/me]
    (3) Jimmy [app.use*]
    (4) Mary [GET/me]
    (5) Final JSON:res.json({answer: req.name})

Express-Using middleware

Express is a routing and middleware web framework that has minimal functionality of its own: An Express application is essentially a series of middleware function calls.

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. The next middleware function is commonly denoted by a variable named next.

Middleware functions can perform the following tasks:

Execute any code.

Make changes to the request and the response objects.

End the request-response cycle.

Call the next middleware function in the stack.

If the current middleware function does not end the request-response cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.

An Express application can use the following types of middleware:

1. Application-level middleware

2. Router-level middleware

3. Error-handling middleware

4. Built-in middleware

5. Third-party middleware

You can load application-level and router-level middleware with an optional mount path. You can also load a series of middleware functions together, which creates a sub-stack of the middleware system at a mount point.

推理&驗證方式:

  1. 自建小專案,實際觀察結果,設置 console.log 印出:
    (1) 標記編號、(2) Name 代稱。
  2. 理解 app.use next()觀念,建立一些假設和推測 Route 可能流向。
  3. POST:透過 POSTMAN 發送 POST request 時,終端機也會印出console.log。

範例:[Folder] Node.js -> [Folder] test-route-sequelize

重點概述:

  1. app.use:
    (1) 表示 request 皆會經過 app.use 路由
    (2) 注意:不代表會優先經過,要依據上下位置順序
  2. Kevin 和 Jimmy 屬於 app.use,路由路徑皆會經過 app.use,而 Kevin位置在 Jimmy 上方,所以順序上,Kevin 先、Jimmy 後。
  3. 注意:若發送 POST/me,John 位置順序在 Jimmy 上方,所以先經過John,再經過 Jimmy。
  4. next():將 request 往下遞交給後續的 middleware 流程。

情境一、Route 呼叫 GET

  1. 印出:{“answer”: “Jimmy”}
  2. 流程:
    (1) Kevin [app.use] -> (3) Jimmy [app.use*]
    -> (5)Final JSON: 印出 Jimmy

情境二、Route 呼叫 GET /me

  1. 印出:{“answer”: “Mary”}
  2. 流程:
    (1) Kevin [app.use] -> (3) Jimmy [app.use*] -> (4) Mary [GET/me]
    -> (5) Final JSON: 印出 Mary

※ 注意:
Jimmy [app.use*] 的代表該 Route 目錄下所有字串皆符合,所以 /me包含在*條件內

情境三、Route 呼叫 POST /me

  1. 印出:{“answer”: “Jimmy”}
  2. 流程:
    (1) Kevin [app.use] -> (2) John [POST/me] -> (3) Jimmy [app.use*]
    -> (5) Final JSON: 印出 Jimmy
  3. 說明:John 順序在 Jimmy 上方,所以先經過 John,再經過 Jimmy。

※ 注意:
POST 要使用 POSTMAN API 測試工具發送 POST request,同時終端機可 console.log 印出。

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