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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 Gameโ€‹

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.
GitpodClone 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, you should build and deploy a contract (JavaScript or Rust version) and a client manually.

Interacting With the Counterโ€‹

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.

img Frontend of the Game

Structure of a dAppโ€‹

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 in Rust is in the /contract-rs folder.
  3. The smart contract code in JavaScript is in the /contract-ts folder.

Both Rust and JavaScript versions of the contract implement the same functionality.


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 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 ./ for the Rust version.

Integration testโ€‹

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).

A Note On Randomnessโ€‹

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.

Versioning for this article

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
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