Connect the world's APIs to Web3 with Chainlink Functions. Get started

Getting Started

Use the Chainlink Functions Starter Kit with Hardhat to set up your on-chain contracts, test your requests, and send your requests to be fulfilled by the Chainlink Functions Decentralized Oracle Network (DON). Chainlink Functions is available on Sepolia and Polygon Mumbai, but this guide uses Polygon Mumbai to simplify access to testnet funds. Complete the following tasks to get started with Chainlink Functions:

  • Set up your Web3 wallet and find your private key
  • Install the required frameworks
  • Configure the starter kit with your environment variables
  • Simulate a Chainlink Functions request
  • Set up a subscription for Chainlink Functions
  • Send a Chainlink Functions request to the DON

Set up your environment

You must provide the private key from a testnet wallet to run the examples in this documentation. Install a Web3 wallet, configure Node.js, clone the Starter Kit, and configure a .env file with the required variables.

Install and configure your Web3 wallet for Polygon Mumbai:

  1. Install the MetaMask wallet or other Ethereum Web3 wallet.

  2. Set the network for your wallet to the Polygon Mumbai testnet. If you need to add Mumbai to your wallet, you can find the chain ID and the LINK token contract address on the LINK Token Contracts page.

  3. Request testnet MATIC from the Polygon Faucet.

  4. Request testnet LINK from

Install the required frameworks and dependencies:

  1. Install Node.js 18. Optionally, you can use the nvm package to switch between Node.js versions with nvm use 18.

  2. In a terminal, clone the Chainlink Functions Starter Kit repository and change directories.

    git clone && \
    cd functions-hardhat-starter-kit/
  3. Run npm install to install the dependencies.

    npm install
  4. Configure a .env file with the basic variables that you need to send your requests to the Polygon Mumbai network.

    touch .env && open .env
    • MUMBAI_RPC_URL: Set a URL for the Polygon Mumbai testnet. You can use the URL in the example config or sign up for a personal endpoint from Alchemy, Infura, or another node provider service.

    • PRIVATE_KEY: Find the private key for your testnet wallet. If you use MetaMask, follow the instructions to Export a Private Key. Set this in the .env file. Note: The Chainlink Functions hardhat starter kit uses your private key to sign any transactions you make such as deploying your consumer contract, creating subscriptions, and making requests.

  5. Compile the contracts in the repository. You will see several compile warnings, but no errors.

    npx hardhat compile

Simulate a request to test your environment and make sure everything is configured correctly. Run the npx hardhat functions-simulate command. The simulation runs on a local Hardhat network (a local Ethereum network node designed for development) and executes a request defined in the default Functions-request-config.js file. The starter kit includes the capability to simulate transactions so you can quickly test your code before you send it to the DON.

npx hardhat functions-simulate

If the simulation is successful, the output includes the simulated on-chain response:

__Simulated On-Chain Response__
Response returned to client contract represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000000f50ed
Decoded as a uint256: 1003757

Gas used by sendRequest: 360602
Gas used by client callback function: 75029

Configure your on-chain resources

After you configure your local environment, configure some on-chain resources to process your requests, receive the responses, and pay for the work done by the DON.

Deploy an on-chain FunctionsConsumer.sol contract, create a subscription, and fund the subscription with LINK to pay for requests after they are fulfilled.

  1. Deploy the consumer contract and record the contract address. For now, you can skip on-chain contract verification with the --verify false flag. If you do need to verify your contract on-chain, sign up for a free Polygonscan API key and include the key in your .env file under the POLYGONSCAN_API_KEY= variable.

    npx hardhat functions-deploy-client --network mumbai --verify false

    Find the contract address in the output:

    FunctionsConsumer contract deployed to 0x97795d62905a66f5cf0384075c64c95795d443c3 on mumbai
  2. Create a Chainlink Functions subscription and add your contract as an approved consumer contract. You can do this with a single transaction if you include your contract address with the --contract flag. For this example, 1 LINK is more than enough on the Mumbai testnet. You can always get more LINK from and later add it to the subscription. See the Subscriptions Management page for details.

    npx hardhat functions-sub-create --network mumbai --amount 1 --contract YOUR_CONSUMER_CONTRACT_ADDRESS

    Record the subscription ID from the output.

    Created subscription with ID: 7
    Owner: 0xc26881a0aF2ea1c7e8135e6ac47af756265432e8
    Balance: 1.0 LINK
    1 authorized consumer contract:
    [ '0x97795d62905a66f5cf0384075c64c95795d443c3' ]

Run the example request

After your environment and on-chain resources are configured, you can send your request to the DON to execute the request defined in the Functions-request-config.js file. Run npx hardhat functions-request. Specify your subscription ID with the --subid flag and specify your consumer contract address with the --contract flag.

npx hardhat functions-request --subid YOUR_SUBSCRIPTION_ID --contract YOUR_CONSUMER_CONTRACT_ADDRESS --network mumbai

The on-chain transaction and request will take longer to complete than the simulation. The output includes a decoded response:

Transmission cost: 0.000078762053689013 LINK
Base fee: 0.0 LINK
Total cost: 0.000078762053689013 LINK

Request 0x7294990c570bd46a24ec488f73e088be56e45a2fa43fa761f161d26b32b7b435 fulfilled!
Response returned to client contract represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000000f50ed
Decoded as a uint256: 1003757

If your request is successful, you have all the tools that you need to make your own custom requests. Edit the Functions-request-config.js file and the calculation-example.js source file with your own code. Then, run the npx hardhat functions-request command again to test your changes. You can re-use the same consumer contract and subscription for different requests.

Chainlink Functions is capable of much more than just computation. Try one of the Tutorials to see examples that can GET and POST to public APIs, securely handle API secrets, handle custom responses, and query multiple APIs.

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