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Getting Started with AssemblyScript

Casper Labs maintains the casper-contract to allow developers to create smart contracts using AssemblyScript. The package source is hosted in the main Casper Network repository.


Installing AssemblyScript

Installation of AssemblyScript requires Node.js.

After installation of Node.js, the following command will install AssemblyScript:

npm i assemblyscript

Full instructions and details for installing AssemblyScript can be found here.

Development Environment Setup

Installing the Casper Package

The casper-contract package can be installed using the following command:

npm i casper-contract

The Assemblyscript contract API documentation can be found at

Creating a Project

For each smart contract, it is necessary to create a project directory and initialize it.

The npm init process prompts for various details about the project. Answer as you see fit, but you may safely default everything except name, which needs to be specified. In this guide, we will refer to the contract name as your-contract-name.

mkdir project
cd project
npm init

Then install AssemblyScript and this package in the project directory.

npm install --save-dev assemblyscript@0.9.1
npm install --save casper-contract

Script Entries

Add script entries for AssemblyScript to your project's package.json. Note that your contract name is used for the name of the Wasm file. Replace your-contract-name with the name of your contract.

"name": "your-contract-name",
"scripts": {
"asbuild:optimized": "asc assembly/index.ts -b dist/your-contract-name.wasm --validate --optimize --use abort=",
"asbuild": "npm run asbuild:optimized",

In the project root, create an index.js file with the following contents. Replace your-contract-name with the name of your contract.

const fs = require("fs");

const compiled = new WebAssembly.Module(fs.readFileSync(__dirname + "/dist/your-contract-name.wasm"));

const imports = {
env: {
abort(_msg, _file, line, column) {
console.error("abort called at index.ts:" + line + ":" + column);

Object.defineProperty(module, "exports", {
get: () => new WebAssembly.Instance(compiled, imports).exports,

Next, create a directory called assembly, and in that directory, create a file called tsconfig.json in the following way:

"extends": "../node_modules/assemblyscript/std/assembly.json",
"include": ["./**/*.ts"]

Sample Smart Contract

In the assembly directory, also create an index.ts file, where the code for the contract needs to go.

You can use the following sample snippet, which demonstrates a simple smart contract that immediately returns an error and writes a message to a block when executed on a Casper network.

import { Error, ErrorCode } from "casper-contract/error";

// simplest possible feedback loop
export function call(): void {
Error.fromErrorCode(ErrorCode.None).revert(); // ErrorCode: 1

If you prefer a more complicated first contract, you can look at example contracts on the Casper Ecosystem GitHub repository for inspiration.

Compile to Wasm

To compile the contract to Wasm, use npm to run the asbuild script from the project root:

npm run asbuild

If the build is successful, there will be a dist folder in the root folder and in it should be your-contract-name.wasm.