Javascript：类中的异步函数链接，返回此

Question

I am trying to do function chaining in a Javascript class by returning this after each method, but somehow, it does not work..:

let sql = require('mssql');
let {logger} = require('../utils/logger');
let config = require('config');
//mssql_stellenanzeigen_config = config.get('stellenanzeigen');
mssql_doublettencheckui_config = config.get('doublettencheckui');


class MSSQLConnectionObject {
    constructor(configuration) {
        this.configuration = configuration; 
        this.connection = undefined;    
        this.requestObject = undefined;  
    }

    async build() {
        let pool;
        try {
            pool = await new sql.ConnectionPool(this.configuration).connect(); 
            console.log("Connection established!: ", pool);
        } catch(e) {
            logger.error("No SQL Database config or wrong config. Can't establish connection to MSSQL Server. " + e);
        }    
        this.requestObject = await new sql.Request(pool);

        return this;
    }

    static async connect(config) {        
        let pool;
        try {
            pool = await new sql.ConnectionPool(config).connect(); 
            console.log("Connection established!: ", pool);
        } catch(e) {
            logger.error("No SQL Database config or wrong config. Can't establish connection to MSSQL Server. " + e);
        }    
        this.requestObject = await new sql.Request(pool);
        return this;
    }

    async getBuchungsquelle() {
        const query = `SELECT * FROM buchungsquelle`;
        return await this.requestObject.query(query).then((result) => console.log(result)).catch(err => console.log(err));
    }
}

module.exports = {
    MSSQLConnectionObject
}

   let query= `select * from buchungsquelle`;  

   let a = new MSSQLConnectionObject(mssql_doublettencheckui_config);
   a.build().getBuchungsquelle();

I get an error:

a.build().getBuchungsquelle();            

TypeError: a.build(...).getBuchungsquelle is not a function

why does this not work? Is there an error in how to return this from my functions?

Answer 1

It doesnt work because your function is Async .

You are not actually returning a instance of MSSQLConnectionObject but a instance of Promise<MSSQLConnectionObject> .

Async Chaining:

Check out this Stackoverflow post to get a nice example of async chaining!

Answer 2

TypeError: a.build(...).getBuchungsquelle is not a function

build is an async function. When you say return this , it creates a function which returns a promise that resolves to this , not a function that returns this .

So to use it you would need to wait for the promise to resolve and then call the next function in sequence.

a.build().then( x => x.getBuchungsquelle() )