Function not working from class

Question

I have this class:

class Blockchain {

    constructor() {

    }



    // Add new block

    async addBlock(newBlock) {

        var bla;

        this.getBlockHeightPromise().then(i => bla = i);

        console.log('bla: ' + bla);}

//Note addBlock needs to async because await db.createReadStream follows



    getBlockHeightPromise() {

        return new Promise(function (resolve, reject) {

            let i = 0;


            db.createReadStream()

                .on('data', function () {

                    i++;

                })

                .on('error', function () {

                    reject("Could not retrieve chain length");

                })

                .on('close', function () {

                    resolve(i);

                });

        })

    }

}


Now when I try to run it, it doesn’t work:

let blockchain = new Blockchain();

blockchain.addBlock(someBlock)

But it works from the command line when I try to run it like this:

let blockchain = new Blockchain();

var bla;

blockchain.getBlockHeightPromise().then(i => bla =  i);

How to make this work?

Answer 1

Try this code: 

// Add new block

    async addBlock(newBlock) {

        var bla = await this.getBlockHeightPromise();

        console.log('bla: ' + bla);

    }

The function is not working when you call it within the class is because the console statement is executed before getBlogHeightPromise is finished.

Using await to wait for the asynchronous method getBlogHeightPromise solves this problem