Originally published by Roamin on Hashnode

In this tutorial we will create a simple smart contract in AssemblyScript and compile it to a WASM file that can be deployed to the Koinos Blockchain.

Setting up the dev environment

Install NodeJS

We will need NodeJS installed in order to develop, build and deploy the smart contract. If it is not yet installed on your system please head over to the NodeJS website and follow the installation guide.

Download the SDK CLI

In this example we will be using the Yarn package manger, but NPM will work as well.

Install the AssemblyScript CLI by running this command:

yarn global add @roamin/sdk-as-cli

The CLI should be installed globally, we can check by running this command:

koinos-sdk-as-cli -V

The output should read:


Create the smart contract

To create the smart contract boilerplate we can run the following command:

koinos-sdk-as-cli create myawesomecontract

The CLI command create takes one argument which is the name of the contract in this case the smart contract is called myawesomecontract

The output should be:

The CLI also outputs a set of commands that we can run to check if the contract was correctly generated.

From within the myawesomecontract directory run these commands:

yarn install && yarn build:debug && yarn test

Let us look at each command and break down what it does:

yarn install: installs all the dependencies that we need to compile and test a Koinos smart contract

yarn build:debug: this compiles the smart contract into WASM (the debug version of the WASM file)

yarn test: this runs the unit tests of the smart contract

If successful the output should look something like this:

The CLI generates a boilerplate smart contract which allows us to quickly setup a new contract and make sure the development environment is working properly.

The generated smart contract code

Let’s open the myawesomecontract folder in our code editor. The content should be similar to this:

We will mainly focus on the assembly folder, it’s where all the smart contract-related code lives.

assembly folder:

  • the __tests__ folder is for creating the unit tests for the contract
  • the proto folder is where your proto files live
  • index.ts file is the entry point of the contract, it’s the file that will be called by the Koinos blockchain/VM when interacting with the contract
  • Myawesomecontract.boilerplate.ts is the code that the CLI auto-generated based on the proto file description
  • Myawesomecontract.ts is the actual code of the smart contract
  • tsconfig.json is used to tell the IDE what types are available in AssemblyScript, AS is like TypeScript but with WebAssembly types

Proto folder:

The Koinos blockchain leverages Google’s Protocol Buffers (Protobuf).

Protobuf website

Protocol buffers are Google’s language-neutral, platform-neutral, extensible mechanism for serializing structured data – think XML, but smaller, faster, and simpler.

The proto folder should look like this:

These are the files:

  • myawesomecontract.proto is the file we will be working in
  • myawesomecontract.ts is the AssemblyScript code that is auto-generated from the above proto file

Let’s have a look at the myawesomecontract.proto file

syntax = "proto3";

package myawesomecontract;

// @description Says Hello!
// @read-only true
message hello_arguments {
string name = 1;

message hello_result {
string value = 1;

We will focus on the important parts of this file:

  • package myawesomecontract the package name to which this proto file belongs, in this case, it’s the name of the smart contract
  • message hello_arguments is divided into 3 parts:
  • message this indicates that we are declaring a new proto message
  • hello is the name of the smart contract function this message describes
  • arguments describes the arguments of the function
  • message hello_result the message that describes the result of the function hello

A smart contract function always contains an arguments message and a result message (even if the message is empty). Each arguments message must contain comments:

  • @description is used to describe what the function does
  • @read-only is used to indicate if the function needs to write to the blockchain state or not

Inside the hello_arguments, you can see string name = 1; this means that our hello_arguments message has one property called name and that it is a string.

To summarize, in this proto file we are describing a hello function that takes a string called name as argument and returns a string called value as result.

You can learn more about Protobuf directly on Google’s Protobuf website

After creating or modifying the proto we will need to compile it so that the new AS files get generated.

We can use the CLI:

koinos-sdk-as-cli build-all debug 0 myawesomecontract.proto

This command will re/generate the myawesomecontract.ts file, the index.ts file as well as the Myawesomecontract.boilerplate.ts file.

Let’s have a look at the generated Myawesomecontract.boilerplate.ts file. The comments describe each function.

// import of the different helpers available in the Koinos AS SDK
import { System, Protobuf, authority } from "koinos-sdk-as";

// import of the AS generated proto file myawesomecontract.ts
import { myawesomecontract } from "./proto/myawesomecontract";

// exports the contract class
export class Myawesomecontract {  

// description of the hello function
// arguments of the hello function, which are of type "hello_arguments"
args: myawesomecontract.hello_arguments

// result of the hello function, which is of type "hello_result"
myawesomecontract.hello_result {
"name" variable that's directly extracted from the "hello_arguments"
that we described in our proto file


// const name = args.name;
// this where you would describe your smart contract function's logic
// "res" variable that is of type "hello_result"
const res = new myawesomecontract.hello_result();

"value" variable that's directly extracted from the "hello_result"
that we described in our proto file
// res.value = ;
return res;

The *.boilerplate.ts is very useful when we are creating the contract or when we update the proto file. It will generate the boilerplate code of the smart contract functions.

Let’s have a look at the actual smart contract implementation, comments have been added to the code that hasn’t been explained previously:


import { System } from "koinos-sdk-as";
import { myawesomecontract } from "./proto/myawesomecontract";
export class Myawesomecontract {

hello(args: myawesomecontract.hello_arguments): myawesomecontract.hello_result {

const name = args.name;
const res = new myawesomecontract.hello_result();

  // the result of the string "Hello, " + name
  res.value = `Hello, ${name}!`;

  // The System call/function "log"
  // will log the "res.value" in the transaction's receipt
  return res;

__tests__ folder:

Let’s have a look at the unit tests.

In the __tests__ folder we should have a file called Myawesomecontract.spec.ts, this is where the unit tests for the smart contract function hello are written.

// import the smart contract ts file
import { Myawesomecontract } from '../Myawesomecontract';

// import the proto ts file
import { myawesomecontract } from '../proto/myawesomecontract';

describe('contract', () => {
it("should return 'hello, NAME!'", () => {

// instantiate the contract
const c = new Myawesomecontract();

// instantiate the arguments for the "hello" function
const args = new myawesomecontract.hello_arguments('World');

// call the "hello" function
const res = c.hello(args);

// check that the result of the "hello" function is what is expected
expect(res.value).toStrictEqual('Hello, World!');

Modify the generated smart contract code

Let’s add a function that will simply add 2 numbers together.

To add a function to our contract, we need to modify our proto file, let’s add an add function:

// @description Adds 2 numbers together
// @read-only true

message add_arguments {
// "add" accepts an argument called "a" that is of type "uint64"
uint64 a = 1;

// "add" accepts an argument called "b" that is of type "uint64"
uint64 b = 2;

message add_result {
// "add" returns a result that has a property called "value"
// "value" is of type "uint64"
uint64 value = 1;

Run the command:

koinos-sdk-as-cli build-all debug 0 myawesomecontract.proto

This will generate the new boilerplate files, we also get the following error, that is because we have not added the function add to our contract yet:

ERROR TS2339: Property ‘add’ does not exist on type ‘assembly/Myawesomecontract/Myawesomecontract’.

To remedy this problem, let’s open the Myawesomecontract.boilerplate.ts file and copy the following auto-generated code:

add(args: myawesomecontract.add_arguments): myawesomecontract.add_result {

// const a = args.a;
// const b = args.b;


const res = new myawesomecontract.add_result();

// res.value = ;
return res;

Open the Myawesomecontract.ts file and paste the new function. Let’s implement the add function as follows:

add(args: myawesomecontract.add_arguments): myawesomecontract.add_result {

const a = args.a;
const b = args.b;

// "c" is just the addition of "a" and "b"
// we use the "SafeMath" helper
// this will ensure that our result won't overflow/underflow
const c = SafeMath.add(a, b);
const res = new myawesomecontract.add_result();

// we set the "value" with the above calculated "c"
res.value = c;
return res;

Now that our new function has been implemented, let’s add a new unit test. Head over to the Myawesomecontract.spec.ts file and add the following unit test:

it("should add 2 numbers", () => {
const c = new Myawesomecontract();

const args = new myawesomecontract.add_arguments(4, 8);
const res = c.add(args);


koinos-sdk-as-cli run-tests

Let’s run our tests to make sure everything is working.

We just added a new function to the contract and also made sure that it works.

Upload to the Blockchain

We can now use the Koinos CLI to upload our smart contract to the Koinos blockchain test network Harbinger.

If it’s not yet installed head over to the Koinos CLI GitHub repo and install the CLI.

Generate a wallet

Let’s create a wallet so that we can interact with the Koinos blockchain.

Start the CLI


Create a new wallet

create my.wallet azerty

  • create is the command used to create a new wallet
  • my.wallet is the name of the wallet that we want to create
  • azerty is the password for the wallet, this password is used to encrypt your wallet file on your computer. (The wallet file contains the private key of the wallet)

We should get the following output:

🚫 🔐 > create my.wallet azerty

Created and opened new wallet: my.wallet

Address: 14JP7iJP1CtGvGhgczMwBE7i9igsKvNdK4

the Address 14JP7iJP1CtGvGhgczMwBE7i9igsKvNdK4 is the public key of the wallet.

We will need some test tokens tKoin to interact with the network. Follow the “Using the Discord Faucet” tutorial to get 100 tKoin.

Connect to the Harbinger Test Network

From the CLI issue this command:

connect https://harbinger-api.koinos.io

The output should be:

🔓 > connect https://harbinger-api.koinos.io

Connected to endpoint https://harbinger-api.koinos.io

Once connected we should be able to check the tKoin balance by issuing the command:


We should see the following output in the terminal:

🔓 > balance

100 tKOIN

100 mana

Deploy the Smart Contract

We can upload the smart contract to the blockchain by issuing the following command:

upload myawesomecontract/build/release/contract.wasm myawesomecontract/abi/myawesomecontract.abi

Uploading the ABI file is not mandatory but highly encouraged as this will allow users to interact with the contract in a more efficient manner.

This command should output a transaction id as well as the Mana used to upload the contract:

🔓 > upload myawesomecontract/build/release/contract.wasm myawesomecontract/abi/myawesomecontract.abi

Contract uploaded with address 14JP7iJP1CtGvGhgczMwBE7i9igsKvNdK4

Transaction with ID 0x122096042fb2e2c085eb4e78cb80a4933e6cda21ebf65722e2c0f283f39a9ba40f2a containing 1 operations submitted.

Mana cost: 0.38643078 (Disk: 16472, Network: 17562, Compute: 177948)

Important note: the wallet/address we use to upload a contract will be the address of the contract itself. This means that the address 14JP7iJP1CtGvGhgczMwBE7i9igsKvNdK4 is the address of the wallet we created earlier, but it is also the address of the contract. Smart contracts are users in Koinos. Also, right now, the block size limit on the Koinos blockchain is set to 200kb which means that the contract’s WASM files cannot exceed this size. (if it does, splitting the logic into several smaller contracts would be neccessary)

As you can see, the upload cost us 0.38643078 Mana, which means we didn’t spend any actual tKoin.

We can check the transaction on the block explorer

where 0x12205d19a5e9fc1a8d12478b7ff761c7b4619b9770404da2dff41d872cd0f0e6fdf8 is the transaction ID.

Interacting with the Smart Contract

Now that our smart contract and our abi file have been successfully uploaded onto the Koinos blockchain test network we can interact with it.

In the Koinos CLI, type the following command to register your contract:

register mycontract 14JP7iJP1CtGvGhgczMwBE7i9igsKvNdK4


  • register is the command used to register the contract with the CLI. Register tells the CLI that we want to load the ABI file of a smart contract
  • mycontract is the alias for the smart contract, this is what is used in the CLI to access the smart contract’s functions that are available in the ABI file. (this is an arbitrary name, it doesn’t have to be the real name of the contract we are trying to register)
  • 14JP7iJP1CtGvGhgczMwBE7i9igsKvNdK4 is the address of the smart contract we want to register

The output should resemble:

🔓 > register mycontract 14JP7iJP1CtGvGhgczMwBE7i9igsKvNdK4

Contract ‘mycontract’ at address 14JP7iJP1CtGvGhgczMwBE7i9igsKvNdK4 registered

Now, if you we mycontract in the CLI, we should see the functions available in the smart contract:

We should see 2 functions, add and hello.

We can use the add function by issuding following command in the CLI:

mycontract.add 40 2


🔓 > mycontract.add 40 2



In this tutorial we learned how to setup a smart contract development environment for Koinos, we also saw how to generate a smart contract and how to add functionality to it. We then uploaded the contract to the blockchain and interacted with it.