Coin Flip

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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:

  1. Recommended: use the app through Gitpod (a web-based interactive environment)
  2. Clone the project locally .

| Gitpod | Clone locally |
| —————————————————————————————————————————————————————– | —————————————————– |
| Open in Gitpod | 🌐 `` |

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.

cd counter
yarn deploy
yarn start

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 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:

  1. The frontend code lives in the /frontend folder.
  2. The smart contract code is in the /contract folder.


The contract presents 2 methods: flip_coin, and points_of.


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 signedInFlow() or signedOutFlow().


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 yarn test.

Integration test

Integration tests are generally written in 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 counter
in tests/integration-tests.

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.

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