Coin Flip is a game where the player tries to guess the outcome of a coin flip. It is one of the simplest contracts implementing random numbers.
Starting the Game
You have two options to start the example:
- Recommended: use the app through Gitpod (a web-based interactive environment)
- Clone the project locally .
If you choose Gitpod, a new browser window will open automatically with the code. Give it a minute, and the front-end will pop up (ensure the pop-up window is not blocked).
If you are running the app locally, enter the directory where you cloned it and use
yarn to install dependencies, and
yarn start to start it.
Your contract will then be compiled and deployed to an account in the
testnet network. When done, a browser window should open.
Interacting With the Counter
Go ahead and log in with your NEAR account. If you don't have one, you can create one on the fly. Once logged in, use the
heads buttons to try to guess the next coin flip outcome.
Frontend of the Game
Structure of a dApp
Now that you understand what the dApp does, let us take a closer look to its structure:
- The frontend code lives in the
- The smart contract code is in the
The contract presents 2 methods:
The frontend is composed by a single HTML file (
/index.html). This file defines the components displayed in the screen.
The website's logic lives in
/assets/js/index.js, which communicates with the contract through a
wallet. You will notice in
/assets/js/index.js the following code:
It indicates our app, when it starts, to check if the user is already logged in and execute either
When writing smart contracts, it is very important to test all methods exhaustively. In this
project, you have two types: unit and integration tests. Before digging into them,
go ahead and perform the tests present in the dApp through the command
contract and execute methods on it. In this way, integration tests simulate interactions
from users in a realistic scenario. You will find the integration tests for the
A Note On Randomness
Randomness in the blockchain is a complex subject. We recommend you to read and investigate about it. You can start with our security page on it.