將標頭發送到客戶端后無法設置標頭-貓鼬

Question

我想編寫一個小應用程序，該應用程序通過POST接收歡迎消息並通過GET返回。 如果僅調用一種方法（GET或POST），則沒有問題，但是，一旦我調用GET和POST，就會收到以下消息：

events.js:174
      throw er; // Unhandled 'error' event
      ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:470:11)
    at ServerResponse.header (C:\Users\aph\IdeaProjects\hello-world-node\ExpressApp\node_modules\express\lib\response.js:767:10)
    at ServerResponse.send (C:\Users\aph\IdeaProjects\hello-world-node\ExpressApp\node_modules\express\lib\response.js:170:12)
    at Greeting.find (C:\Users\aph\IdeaProjects\hello-world-node\ExpressApp\routes\hello.js:16:13)
    at C:\Users\aph\IdeaProjects\hello-world-node\node_modules\mongoose\lib\model.js:4568:16
    at C:\Users\aph\IdeaProjects\hello-world-node\node_modules\mongoose\lib\query.js:4315:12
    at process.nextTick (C:\Users\aph\IdeaProjects\hello-world-node\node_modules\mongoose\lib\helpers\query\completeMany.js:35:39)
    at process._tickCallback (internal/process/next_tick.js:61:11)
Emitted 'error' event at:
    at C:\Users\aph\IdeaProjects\hello-world-node\node_modules\mongoose\lib\model.js:4570:13
    at C:\Users\aph\IdeaProjects\hello-world-node\node_modules\mongoose\lib\query.js:4315:12
    at process.nextTick (C:\Users\aph\IdeaProjects\hello-world-node\node_modules\mongoose\lib\helpers\query\completeMany.js:35:39)
    at process._tickCallback (internal/process/next_tick.js:61:11)

這是我的代碼：

const express = require("express");
const mongoose = require("mongoose");
const Schema = mongoose.Schema;

const router = express.Router();

const Greeting = mongoose.model("Greeting", new Schema({message: String}));

router.get("/", (req, res) => {
    Greeting.find({message: "Hello World!"}, (err, greetings) => {
        if (err) {
            console.log(err);
            res.status(500).send(err);
            return;
        }
        res.send(JSON.stringify(greetings));
    });

    res.send("There are no greetings!");
});

router.post('/', (req, res) => {
    mongoose.connect("mongodb://localhost:27017/test", {useNewUrlParser: true});
    new Greeting(req.body).save()
        .then(() => {
            res.send('success');
        })
        .catch(err => {
            console.log(err);
            res.status(500).send("Error: " + err)
        });
});


module.exports = router;

我掃描了這個問題，但是找不到解決我問題的方法。

Answer 1

Greeting.find是一個異步函數，因此，行res.send("There are no greetings!"); 在Greeting.find的回調運行之前運行，這意味着響應'There are no greetings!' 在回調運行之前被發送到客戶端。

然后，在Greeting.find回調中，您嘗試再次將響應發送給客戶端，從而導致錯誤。