Why is previousHash not showing when I console.log my test blockchain?

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I am trying learn more about blockchain by building a small project using JavaScript. Can anyone explain to me why the previousHash does not show when I console.log the test blockchain?

(npm install --save crypto-js if you need SHA256)

        const SHA256 = require('crypto-js/sha256');

        class Block{
            //Index: (Optional) tells us where the block is in the chain
            //Timestamp: tells us when the block was created 
            //Data: Can include any kind of data; for a currency you could store details of the transaction(transfer amount, sender id, receiver id)
            //previousHash: String which contains the hash of the block which came before it (Ensures data integrity)

            constructor(index, timestamp, data, previousHash = ''){
                this.index = index;
                this.timestamp = timestamp;
                this.data = data;
                this.previousHash = previousHash;
                //Hash of this block
                this.hash = this.calculateHash();
            }

            //Runs values from block through a hashing function to create a hash
            calculateHash(){
                return SHA256(this.index + this.previousHash + this.timestamp + JSON.stringify(this.data)).toString();
            }
        }
        //8.44

        class Blockchain{
            //Initializes blockchain
            constructor(){
                this.chain = [this.createGenesisBlock];
            }

            // first block needs to be created manually 
            //index, timeStamp, data, previousHash(there is none in this case)
            createGenesisBlock(){
                return new Block(0, "01/01/2017", "Genesis block", "0");
            }

            //Returns most recently added block
            getLatestBlock(){
            return this.chain[this.chain.length -1];
            }

            //adds a new block to the chain
            addBlock(newBlock){
                newBlock.previousHash = this.getLatestBlock().hash;
                newBlock.hash = newBlock.calculateHash
                //The chain is just an array so you can use regular array methods
                this.chain.push(newBlock);
            }
        }

        //create instance of blockchhain
        let Coin = new Blockchain();
        Coin.addBlock(new Block(1, "10/07/2017", {amount: 4}));
        Coin.addBlock(new Block(2, "12/07/2017", {amount: 10}));

        console.log(JSON.stringify(Coin, null, 4));

I expect the console.logged JSON file to contain the previousHash as well as the index, timestamp and data. It contains all except previousHash.

Thanks for the help have been scratching my head for a while trying to figure this out...

blockchain javascript
2 Answers

You also don't have hash in your console. This is the line with the error:

            newBlock.hash = newBlock.calculateHash

calculateHash is a function

            newBlock.hash = newBlock.calculateHash()

Now it is working:

{
    "chain": [
        null,
        {
            "index": 1,
            "timestamp": "10/07/2017",
            "data": {
                "amount": 4
            },
            "hash": "8f84adcf036e9aa052a4d7e689c7b8b06070b851eff535870f5cb8f7d53ab05a"
        },
        {
            "index": 2,
            "timestamp": "12/07/2017",
            "data": {
                "amount": 10
            },
            "previousHash": "8f84adcf036e9aa052a4d7e689c7b8b06070b851eff535870f5cb8f7d53ab05a",
            "hash": "a2479e7df8f2a61f97f3ae4830aff93c0d43041b4a7cbb8079c2309a915d8945"
        }
    ]
}
user_12296240

You forgot the paranthesis in the line with: this.chain = [this.createGenesisBlock()]; line, thus the first element of the chain will be a reference to a function, instead of an instance of a Block. Add the paranthesis, and it should work.

Additionally, you also forgot the paranthesis at another function call: when you are trying to call the function newBlock.hash = newBlock.calculateHash()

user_26918790
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