Coin Flip
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.
| Gitpod | Clone locally |
|---|---|
 | https://github.com/near-examples/coin-flip-examples.git |
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, you should build and deploy a contract (JavaScript or Rust version) and a client manually.
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 tails and 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
/frontendfolder. - The smart contract code in Rust is in the
/contract-rsfolder. - The smart contract code in JavaScript is in the
/contract-tsfolder.
Both Rust and JavaScript versions of the contract implement the same functionality.
Contract
The contract presents 2 methods: flip_coin, and points_of.
- 🌐 Javascript
- 🦀 Rust
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Running the Frontend
To start the frontend you will need to install the dependencies and start the server.
cd frontend
yarn
yarn dev
Understanding the Frontend
The frontend is a Next.JS project generated by create-near-app. Check _app.js and index.js to understand how components are displayed and interacting with the contract.
- _app.js
- index.js
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Testing
When writing smart contracts, it is very important to test all methods exhaustively. In this
project you have integration tests. Before digging into them, go ahead and perform the tests present in the dApp through the command yarn test for the JavaScript version, or ./test.sh for the Rust version.
Integration test
Integration tests can be written in both Rust and JavaScript. They automatically deploy a new
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 coin-flip
in contract-ts/sandbox-ts (for the JavaScript contract) and contract-rs/tests (for the Rust contract).
- 🌐 Javascript
- 🦀 Rust
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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.
At the time of this writing, this example works with the following versions:
- near-cli:
4.0.13 - node:
18.19.1 - rustc:
1.77.0
