TypeScript: Create a MySql2 connection object inside a class

Question

I’m new to TypeScript so simple things still trip me up. I successfully created a database connection using the code below which was not in a class:

const mysql = require('mysql2');
let connection = mysql.createConnection({
    host: '127.0.0.1',
    user: 'root',
    password: 'myPW',
    database: 'myDb'
});

connection.connect(function(err: any) {
    if (err) {
      return console.error('error: ' + err.message);
    }  
    console.log('Connected to the MySQL server.');
  });

All was fine. Then I put this same code inside of a class like this:

const mysql = require('mysql2');
export class Server {
  constructor() { }

connection = mysql.createConnection({
    host: '127.0.0.1',
    user: 'root',
    password: 'myPW',
    database: 'myDb'
});

connection.connect(function(err: any) {
  if (err) {
    return console.error('error: ' + err.message);
  }    
  console.log('Connected to the MySQL server.');
});
}

and the I got these errors:

The errors below come from the red squiggly on line 13

Duplicate identifier 'connection'.

Unexpected token. A constructor, method, accessor, or property was expected.

'connect', which lacks return-type annotation, implicitly has an 'any' return type.

'function' is not allowed as a parameter name.

Duplicate function implementation.

Can someone kindly explain what I'm doing wrong here?

Thank you.

anut · Accepted Answer · 2022-07-19 19:57:35Z

1

This is happening because the connection.connect part belongs inside the constructor of the class. A better way to write this would be

import * as mysql from 'mysql2';

export class Server {
  public readonly connection: mysql.Connection;

  constructor() {
    this.connection = mysql.createConnection({
      host: '127.0.0.1',
      user: 'root',
      password: 'myPW',
      database: 'myDb'
    });

    this.connection.connect(function(err: any) {
      if (err) {
        return console.error('error: ' + err.message);
      }    
      console.log('Connected to the MySQL server.');
    });
  }
}

This moves the logic into the constructor, which is where logic should be. Putting code in the class body will raise an error.

If you wanted to use this class, you could do it as so:

const server = new Server();

server.connection.query(...);

Also note that I have converted the require call into an import, which is the recommended way in Typescript.

edited Jul 19, 2022 at 19:57

answered Jul 19, 2022 at 19:47

anut

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3 Comments

spacedog Over a year ago

Thanks @anut. That solved that problem. The next comment is the syntax for how I executed a stored procedure previously which worked. However, how ti throws an error which I will post after:

spacedog Over a year ago

server.connection.query('CALL spix_Trace_lstForReview(\"2022-1-1\", \"2022-7-7\" )', function (error: any, results: Array<any>, fields: any) { if (error) throw error; console.log(results); results.forEach(result => { console.log(result.Name); }); }); This file is being treated as an ES module because it has a '.js' file extension and 'D:\Apps\zDeleteme\TS_D2_webpack2\package.json' contains "type": "module". To treat it as a CommonJS script, rename it to use the '.cjs' file extension.

anut Over a year ago

@spacedog This is because your tsconfig.json doesn't work with your package.json. Try adding "module": "ESNext" in compilerOptions in tsconfig.json.

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