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Overview

Learn how to use Simulation API in your dapp to allow users to preview transaction outcomes before sending them on-chain. This way, you can help them avoid financial losses due to failed or incorrect transactions and increase their confidence when using your dapp. To integrate Tenderly Simulations API into your dapp to detect if a transaction would fail before you even send it on-chain, you need to:
  1. Populate raw transactions from ethers.js library.
  2. Simulate transactions before sending them.
  3. (optional) Wrap every blockchain interaction into a Tenderly simulation.
For the request and response details of an individual call, see single simulations.

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Populate raw transactions from ethers.js

The first step is to create a transaction object that’s going to be sent to our simulation endpoint. We’re going to generate it directly from the ethers.js library. In the example below, we’re going to see how we can achieve that for the transfer() function on top of a DAI contract:
example.tsx
const dai = new ethers.Contract(DAI_ADDRESS, DAI_ABI, tenderlyForkProvider);

const unsignedTx = await dai.populateTransaction.transfer(
  ZERO_ADDRESS,
  YOUR_ADDRESS,
  util.ether(1),
);

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Simulate transactions before sending

Now that we have extracted an unsigned raw transaction, let’s simulate it before sending it on-chain (to the Ethereum Mainnet):
example.tsx
...

const body = {
    "network_id": "1",
    "from": senderAddr,
    "to": contract.address,
    "input": unsignedTx.data,
    "gas": 21204,
    "gas_price": "0",
    "value": 0,
    "save_if_fails": true
}

const headers = {
    headers: {
        'content-type': 'application/JSON',
        'X-Access-Key': TENDERLY_ACCESS_KEY,
  }
}
const resp = await axios.post(apiURL, body, headers);

if (resp.data.simulation.status === false) {
	// it failed, do as you please
}

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Wrap every blockchain interaction into a Tenderly simulation (optional)

To ensure that every blockchain interaction is simulated first, write a simple wrapper around the ethers.js signer object that would always simulate transactions. Additionally, going forward, you can introduce typed errors with which your logic can interact:
example.tsx
export class TenderlySimulationSigner {
  public _provider: ethers.Provider;

  constructor(provider: ethers.Provider) {
    this._provider = provider;
  }

  public async sendTransaction(
    transaction: Deferrable<TransactionRequest>,
  ): Promise<TransactionResponse> {
    await this._simulateTx(transaction);

    return this._signer.sendTransaction(transaction);
  }

  public async getAddress(): Promise<string> {
    return this._signer.getAddress();
  }

  public async signTransaction(transaction: Deferrable<TransactionRequest>): Promise<string> {
    return this._signer.signTransaction(transaction);
  }

  _simulateTx(transaction: Deferrable<TransactionRequest>): Promise<void> {
    const unsignedTx = await contract.populateTransaction[funcName](...args);

    const apiURL = `https://api.tenderly.co/api/v1/account/me/project/project/simulate`;
    const body = {
      network_id: '1',
      from: senderAddr,
      to: contract.address,
      input: unsignedTx.data,
      gas: 21204,
      gas_price: '0',
      value: 0,
      save_if_fails: true,
    };

    const headers = {
      headers: {
        'content-type': 'application/JSON',
        'X-Access-Key': REACT_APP_TENDERLY_ACCESS_KEY as string,
      },
    };
    const resp = await axios.post(apiURL, body, headers);
    if (resp.data.simulation.status == false) {
      throw new Error('Transaction is going to fail');
    }

    return;
  }
}
Here’s a GitHub repo where you can find an example of this implementation.