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Tutorial 1: Hello World

Overview

In this tutorial, we will go over how to quickly create a “Hello World” smart contract, deploy and call it.

Create a new project

Make sure all prerequisite tools are installed.

Run the following commands to create a new project:

scrypt project helloworld
cd helloworld
npm install

The resulting project will contain a sample smart contract src/contracts/helloworld.ts, along with all the scaffolding. Let's modify it to the following code:

import { assert, ByteString, method, prop, sha256, Sha256, SmartContract } from 'scrypt-ts'

export class Helloworld extends SmartContract {

    @prop()
    hash: Sha256;

    constructor(hash: Sha256){
        super(...arguments);
        this.hash = hash;
    }

    @method()
    public unlock(message: ByteString) {
        assert(sha256(message) == this.hash, 'Hash does not match')
    }
}

The Helloworld contract stores the sha256 hash of a message in the contract property hash. Only a message which hashes to the value set in this.hash will unlock the contract.

Now let’s look at what is in the smart contract.

SmartContract: all smart contracts must extend the SmartContract base class.
@prop: the @prop decorator marks a contract property.
@method: the @method decorator marks a contract method. A public method is an entry point to a contract.
assert: throws an error and makes the method call fail if its first argument is false. Here it ensures the passed message hashed to the expected digest.

Contract Deployment & Call

Before we deploy the contract, follow the instruction to fund a Bitcoin key.

To deploy a smart contract, simply call its deploy method.
To call a smart contract, call one of its public method.

Overwrite deploy.ts in the root of the project with the following code to deploy and call the Helloworld contract:

import { Helloworld } from './src/contracts/helloworld'
import { getDefaultSigner } from './tests/utils/txHelper'
import { toByteString, sha256 } from 'scrypt-ts'

(async () => {

    const message = toByteString('hello world', true)

    await Helloworld.compile()
    const instance = new Helloworld(sha256(message))

    // connect to a signer
    await instance.connect(getDefaultSigner())

    // deploy the contract and lock up 42 satoshis in it
    const deployTx = await instance.deploy(42)
    console.log('Helloworld contract deployed: ', deployTx.id)

    // contract call
    const { tx: callTx } = await instance.methods.unlock(message)
    console.log('Helloworld contract `unlock` called: ', callTx.id)

})()

Run the following command:

npx ts-node deploy.ts

You will see some output like:

You can view the deployment transaction using the WhatsOnChain blockchain explorer:

You can also view the calling transaction:

Congrats! You have deployed and called your first Bitcoin smart contract.