This content originally appeared on DEV Community and was authored by Amir of Ekiti
Blockchain Explained Like You’re 5 (With Code)
Imagine you and your friends are playing a game of marbles at recess. You all trust each other (kind of), but there's no teacher around to keep score.
So you come up with a plan:
Every time someone wins or loses marbles, you all write it down in your notebooks.
That’s the idea behind a blockchain.
Let me explain.
🧠 What Is a Blockchain (Like You’re 5)
You and your friends:
- Agree on the rules: If you lose, you give marbles.
- Write down every trade in your own notebook.
- Keep all your notebooks in sync.
- If someone tries to cheat and says “I didn’t lose 3 marbles,” the others can say, “Look! It’s in all our notebooks!”
That notebook?
That’s a blockchain — a shared list of events that everyone agrees on and no one can secretly change.
🧱 What’s a Block?
You don’t write down every marble trade immediately. Instead, every 10 trades, you:
- Wrap them up
- Add a date and summary
- Put it in your notebook and call it a block
Each block is like a chapter in your notebook, and it has:
- What happened
- When it happened
- A fingerprint of the previous block (so no one can remove or change a chapter)
So it’s a chain of blocks. Now we’re talking blockchain.
🧪 Let's Code a Baby Blockchain (in JavaScript)
Let’s build a tiny blockchain to understand how it works.
const crypto = require('crypto');
class Block {
constructor(index, timestamp, data, previousHash = '') {
this.index = index;
this.timestamp = timestamp;
this.data = data;
this.previousHash = previousHash;
this.hash = this.calculateHash();
}
calculateHash() {
return crypto
.createHash('sha256')
.update(this.index + this.previousHash + this.timestamp + JSON.stringify(this.data))
.digest('hex');
}
}
class Blockchain {
constructor() {
this.chain = [this.createGenesisBlock()];
}
createGenesisBlock() {
return new Block(0, Date.now(), "Genesis Block", "0");
}
getLatestBlock() {
return this.chain[this.chain.length - 1];
}
addBlock(newBlock) {
newBlock.previousHash = this.getLatestBlock().hash;
newBlock.hash = newBlock.calculateHash();
this.chain.push(newBlock);
}
isChainValid() {
for (let i = 1; i < this.chain.length; i++) {
const current = this.chain[i];
const previous = this.chain[i - 1];
if (current.hash !== current.calculateHash()) return false;
if (current.previousHash !== previous.hash) return false;
✅ Each block is linked to the one before
✅ Every change makes a new hash
✅ Tamper-proof unless someone rewrites the whole chain (and gets everyone else to agree!)
🧊 But What About Crypto?
Blockchains like Bitcoin or Ethereum use this structure to track:
- Who sent money to whom
- Who owns what
- And make sure it’s impossible to cheat (without a lot of power and math)
Instead of marbles, they’re trading coins or tokens.
👩🚀 Why Should You Care?
Because this idea:
- Makes money programmable
- Can create transparent systems (like for voting or public spending)
- Helps us build apps where no one can lie about the history
And you just built one (a tiny one, but still 😄).
🧠 TL;DR
- A block is a bundle of data + timestamp + a hash.
- A blockchain is a secure, connected list of those blocks.
- It’s like a shared notebook where everyone writes the truth—and if someone tries to lie, it breaks the chain.
💬 Got Questions?
Let me know in the comments!
Want a follow-up where we deploy this to a real blockchain? Or build a smart contract version?
Hit the ❤️ if this helped you understand blockchain just a bit better.
👋 Let’s Connect
- GitHub: https://github.com/amirofekiti/
- Say hi in the comments!
This content originally appeared on DEV Community and was authored by Amir of Ekiti
Amir of Ekiti | Sciencx (2025-07-05T02:38:45+00:00) ## Blockchain Explained Like You’re 5 (With Code). Retrieved from https://www.scien.cx/2025/07/05/blockchain-explained-like-youre-5-with-code/
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