undefined return from a class constructor -- JavaScript

Question

I have one file which is block.js:

class Block{
    constructor(timeStamp, lastBlockHash, thisBlockData, thisBlockHash){
        this.timeStamp = timeStamp;
        this.lastBlockHash = lastBlockHash;
        this.thisBlockData = thisBlockData; 
        this.thisBlockHash = thisBlockHash;
        }

    static genesis(){
        return new this(Date.now(), "---", "genesis block", "hash of the genesis");
    }
}

I have another file blockchain.js where I have the following:

const Block = require('./block');
class BlockChain{
    constructor() {
       this.chain = BlockInstance.genesis();
    }
}

and i have a test file where I am doing:

const Block = require("./block.js");
const BlockChain = require("./blockchain.js");
console.log(BlockChain.chain);

I get a "undefined" object in the print output.. this is really driving me nuts as i have already spent more than 4 hours on it.. if anyone can solve this mystery for me then a round of beers on me..

Cheers, alchemist

I think you have to still instantiate the classes and not just require them. — Daniel Gale
– Daniel Gale, Commented Sep 18, 2018 at 12:11
You don't create any instance in your code. You need to use the new keyword for creating a BlockChain instance. The class itself doesn't have chain property, it's instances have! — Ram
– Ram, Commented Sep 18, 2018 at 12:11

Daniel Gale · Accepted Answer · 2018-09-18 13:01:29Z

1

You should instantiate the classes like

const Block = require("./block.js");
const BlockChain = require("./blockchain.js");
let block = new Block(...);
let blockChain = new BlockChain();
console.log(blockChain.chain);

For an example of building a blockchain with JS, you may want to follow a site like This one from medium

The genesis() method would be more suited to be a part of the chain since that is a property of a chain not a different type of block.

edited Sep 18, 2018 at 13:01

answered Sep 18, 2018 at 12:13

Daniel Gale

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5 Comments

thealchemist Over a year ago

Hi Daniel.. it is somehow still not working.. :/ this is my dev-test.js script so far: const Block = require("./block.js"); const BlockChain= require("./blockchain.js"); let blockInstance = new Block(); let blockChainInstance = new BlockChain(); console.log(blockInstance.mineBlock(blockInstance.Genesis(), "First Block Afer Geneis")); console.log(blockChainInstance.chain);

thealchemist Over a year ago

thanks for replying.. lets say i am writing a test case to just test blockchain.js.. and i forget completely about block.js.. since when i instantiate blockchaininstance the constructor inside it with this code: this.chain= Block.genesis() should create a genesis block..

thealchemist Over a year ago

great.. thanks a million :)) on a final note.. Block.genesis() cannot be called without instantiating the Block class directly? because if i do console.log with Block.genesis() it works.. because inside genesis function i have " return new this()" which instantiates block...

Daniel Gale Over a year ago

That should be done if it was a static method, but I would strongly encourage you to move the genesis() to the BlockChain class over the Block class.

thealchemist Over a year ago

Let us continue this discussion in chat.

Moosch · Accepted Answer · 2018-09-18 12:25:52Z

0

The problem is syntactical binding. You need to instantiate your BlockChain class like this:

const myBlockChain = new BlockChain();
console.log(myBlockChain.chain);

Check out MDN for more info on this.

answered Sep 18, 2018 at 12:25

Moosch

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