UnhandledPromiseRejectionWarning: TypeError: this.second() is not a function

Question

我在将数据附加到 csv 文件时遇到了一些问题。

首先使用 cheerio 从网页中检索数据。但是当我想执行一个函数来检查文件exists/accessible时，我不能运行class中的函数。即使我将数据传递给第三个函数writeDataIntoFile()，它仍然显示相同的错误。

代码如下：

import request = require('request');
import cheerio = require('cheerio');
import converter = require('json-2-csv');
import fs = require('fs');


export class TidalDataService {

    private readonly siteURL = "webpage.url";
    private readonly filepath = 'data.csv';
    public data = "";
    public async firstExecuted() {
        request(this.siteURL, async function (err, res, body) {
            if (!err) {
                let $ = cheerio.load(body);
                var data = [];
                var dateIndex = [];
                // Crawl data from page...
                console.log(data);
                try {
                    var isHeaderExist = this.isHeader(); // here
                }
                catch (err) {
                    console.log("ol21n31o2i3n123");
                    throw err;
                }
                try{
                    this.writeDataIntoFile(data, isHeaderExist); // and here
                }
                catch (err) {
                    throw err;
                }
            }
        });
    }

    public async isHeader(): Promise<boolean> {
        return new Promise((resolve, reject) => {
            fs.access(this.filepath, fs.constants.F_OK | fs.constants.W_OK, (err) => {
                if (err)
                    reject(err);
                resolve(true);
                // console.log(`${filepath} ${err ? 'does not exist' : 'exists'}`);
            });
        });
    }

    public async writeDataIntoFile(data: object[], isHeaderExist: boolean) {
        converter.json2csv(data, (err, csv) => {
            if (err) {
                throw err;
            }

            // print CSV string
            console.log(csv);


            // check if file is accessible/exists
            if (isHeaderExist) {
                console.log(`${this.filepath} ${err ? 'does not exist' : 'exists'}`);
                // write CSV to a file
                fs.writeFile(this.filepath, csv, (err) => {
                    if (err) throw err;
                });
            }
            else {
                // csv.shift();
                fs.appendFile(this.filepath, csv, (err) => {
                    if (err) throw err;
                });
            }
        });
    }


}

这是错误堆栈跟踪：

(node:7976) UnhandledPromiseRejectionWarning: TypeError: this.writeDataIntoFile is not a function    
[0]     at Request.<anonymous> (C:\Users\user\Projects\project\dist\modules\module123\module123.service.js:55:34)
[0]     at Generator.next (<anonymous>)
[0]     at C:\Users\user\Projects\project\dist\modules\module123\module123.service.js:8:71
[0]     at new Promise (<anonymous>)
[0]     at __awaiter (C:\Users\user\Projects\project\dist\modules\module123\module123.service.js:4:12)
[0]     at Request._callback (C:\Users\user\Projects\project\dist\modules\module123\module123.service.js:26:24)
[0]     at Request.self.callback (C:\Users\user\Projects\project\node_modules\request\request.js:185:22)
[0]     at Request.emit (events.js:400:28)
[0]     at Request.<anonymous> (C:\Users\user\Projects\project\node_modules\request\request.js:1154:10)
[0]     at Request.emit (events.js:400:28)
[0] (node:7976) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)

Answer 1

问题是您使用了 function，它遮蔽了 this。使用箭头函数。

变化自

public async firstExecuted() {
  // `this` is shadowed
  request(this.siteURL, async function (err, res, body) {

到

public async firstExecuted() {
  request(this.siteURL, async (err, res, body) => {

UnhandledPromiseRejectionWarning: TypeError: this.second() is not a function

UnhandledPromiseRejectionWarning: TypeError: this.second() is not a function

node.js

promise

fs