Write Hello World DApp on local Ethereum blockchain

As usual, before starting on a new programming language, we need to set up the environment for development.

Here is the list of required tools:

NodeJS (https://nodejs.org/en/)
Visual Studio Code (https://code.visualstudio.com/)
- Solidity extension (https://marketplace.visualstudio.com/items?itemName=JuanBlanco.solidity)
Truffle (https://www.trufflesuite.com/truffle)

And now let start writing:

First, we need to install Truffle, this is DApp development framework.

npm install -g truffle

Let’s create the first application

Step 1: Initialize project #

Initializing the project by using truffle init command.

╭─~
╰─$ mkdir hello-dapp
╭─~
╰─$ cd hello-dapp
╭─~/hello-dapp
╰─$ truffle init

Starting init...
================

> Copying project files to /Users/mypc/hello-dapp

Init successful, sweet!

Try our scaffold commands to get started:
  $ truffle create contract YourContractName # scaffold a contract
  $ truffle create test YourTestName         # scaffold a test

http://trufflesuite.com/docs

╭─~/hello-dapp
╰─$ ls
contracts         migrations        test              truffle-config.js

At this step, Truffle already initialized the project structure for us. From here we can start write our first contract.

Step 2: Write the contract #

╭─~/hello-dapp
╰─$ cd contracts
╭─~/hello-dapp/contracts
╰─$ touch HelloWorld.sol
╭─~/hello-dapp/contracts
╰─$ ls
HelloWorld.sol Migrations.sol

And here is the content of HelloWorld.sol

Step 3: Write the migration #

We need to implement the migration to apply the contract to blockchain:

Step 4: Migrate and Run the DApp #

To run migrate please go back to your console and run the command: truffle develop then run migrate inside the Truffle Develop CLI

For example here how it looks like on my local:

╭─~/hello-dapp
╰─$ truffle develop
Truffle Develop started at http://127.0.0.1:9545/

Accounts:
(0) 0x3a69afa92f001aec989cfdb1cf7ebc7a416ca344
(1) 0x4f3d58a1f9cf37610381e08edb788016f4338167
(2) 0xc6579de9f0c0c7e5e832c58296ea23a7eecfd629
(3) 0xbc8ca3b5109d2df8121a108de7777820bb6e57ea
(4) 0xb236ab2839a0c775578da1d4c3217ad21b638e92
(5) 0x504e3f4e872aa69be39ec9eb7a361e5d6d3eb722
(6) 0x2bfcfc2a62acff4a39349c6d6eec35281c419a98
(7) 0xbe9feecfcfb9b80d7d6bb16264c35e9185775da0
(8) 0x7a77c6f8a9d6c9e947cde8e910a4a50d71872d2e
(9) 0x9c1bb3cf757154b2ca2c85d19f1cf3fdf92a424c

Private Keys:
(0) 73d77bb14d999aae350c373ae951c4155fb59ea630c83ec8a39d4e33d43290b0
(1) d2c685eca51a26cf0a50900af77a856ba38481f45657fa79120b50027f437630
(2) d8259bd6ba927cecf53e6c608014ad8be56c1172307b418165efb387e2950e6b
(3) 2d4334dc7573a4ccb47df3c954924e14fb52bca8d3c2c748b67b0cba9031efe0
(4) 15d754de5673c148f82c917fbb7fd80510dd43fe3b8e6e3b3a5c243e1737d15a
(5) 4bdf776ddcaf1444315ace616ac99e7a99acdd33d98be7d2bc2f7482104a66b0
(6) 1a340fc146089095630b4776f1f1bb22d22b20750a478a50943ed8cce8653b0c
(7) 631a7ad00f6f896689c12ca2be3a4af2f01d7c70cf70dc1e00ca91dc055795fd
(8) 331e8f5ec7c94b74d96f072a2a4b665b7314bb697d5b7e764a02ea1d9d7d98d5
(9) 3b95db2825d107460e1073c4d19f9f7b51ecca1eafb1fe4cdd1a0c6a164a45c0

Mnemonic: attitude hawk adult danger solve wait bundle inject share enemy tail submit

⚠️  Important ⚠️  : This mnemonic was created for you by Truffle. It is not secure.
Ensure you do not use it on production blockchains, or else you risk losing funds.

truffle(develop)> migrate

Compiling your contracts...
===========================
> Compiling ./contracts/HelloWorld.sol
> Artifacts written to /Users/thanh/hello-dapp/build/contracts
> Compiled successfully using:
   - solc: 0.5.16+commit.9c3226ce.Emscripten.clang



Starting migrations...
======================
> Network name:    'develop'
> Network id:      5777
> Block gas limit: 6721975 (0x6691b7)


1_initial_migration.js
======================

   Replacing 'Migrations'
   ----------------------
   > transaction hash:    0xb6979cb233008ca866057cb78fc0eec0534cbabc224c09a945c187a96d63dc93
   > Blocks: 0            Seconds: 0
   > contract address:    0x9DEEE546F1F728CA2D7b25CfD2D52828009012fa
   > block number:        1
   > block timestamp:     1631962530
   > account:             0x3a69AFA92F001aec989CFDB1cf7eBc7a416ca344
   > balance:             99.999616114
   > gas used:            191943 (0x2edc7)
   > gas price:           2 gwei
   > value sent:          0 ETH
   > total cost:          0.000383886 ETH


   > Saving migration to chain.
   > Saving artifacts
   -------------------------------------
   > Total cost:         0.000383886 ETH


2_create_helloworld_contract.js
===============================

   Deploying 'HelloWorld'
   ----------------------
   > transaction hash:    0x5cd43fb9c54b6a28d41d90b755be60265f8b3d4dcbef58b99a40983a83d4270e
   > Blocks: 0            Seconds: 0
   > contract address:    0x21A7fD5d3eaae32764A4cCA0C0A62Fb1Bad2B937
   > block number:        3
   > block timestamp:     1631962530
   > account:             0x3a69AFA92F001aec989CFDB1cf7eBc7a416ca344
   > balance:             99.999301876
   > gas used:            114781 (0x1c05d)
   > gas price:           2 gwei
   > value sent:          0 ETH
   > total cost:          0.000229562 ETH


   > Saving migration to chain.
   > Saving artifacts
   -------------------------------------
   > Total cost:         0.000229562 ETH


Summary
=======
> Total deployments:   2
> Final cost:          0.000613448 ETH


- Blocks: 0            Seconds: 0
- Saving migration to chain.
- Blocks: 0            Seconds: 0
- Saving migration to chain.

truffle(develop)> const instance = await HelloWorld.deployed()
undefined
truffle(develop)> instance.SayHello.call()
'Hello World'
truffle(develop)>

To execute the program on blockchain we use some Javascript code to interact with the blockchain.

truffle(develop)> const instance = await HelloWorld.deployed()
undefined
truffle(develop)> instance.SayHello.call()
'Hello World'
truffle(develop)>

If you can see the Hello World like the screenshot below, then congratulation 🎉! You finished the very first DApp. 📦

Finally, here is the code repository: https://github.com/byhbt/dapp-helloworld

Update the Smart Contract after migrate (deployed) #

As we know that after the smart contract is deployed to the blockchain, it becomes immutable. Now if you want to change the code in the smart contract, you have to reset the migration by using the command migrate --reset within the truffle develop.

truffle develop

╭─~/hello-dapp ‹main›
╰─$ truffle develop
Truffle Develop started at http://127.0.0.1:9545/
# ...(other output from the Truffle develop)

truffle(develop)> migrate --reset

Step 1: Initialize project #

Step 2: Write the contract #

Step 3: Write the migration #

Step 4: Migrate and Run the DApp #

Update the Smart Contract after migrate (deployed) #

Further reading #