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Use Swap API Liquidity in Your Smart Contracts


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After you complete this guide, you will have a smart contract that swaps WETH for DAI, powered by Swap API.

In the example, we will:

  1. Deposit WETH into our contract.
  2. Fetch a quote to sell WETH for DAI from Swap API.
  3. Fill that quote through our contract, converting its WETH into DAI.

Set up the project

We will use our 0x API starter project, which contains the all the contracts and javascript necessary for this example. This project will use truffle for compiling and deploying our contracts and web3.js for interacting with deployed contracts.

Begin by cloning the starter project and installing dependencies:

$ git clone
$ cd 0x-api-starter-guide-code
$ npm install --dev

The forked test environment

All the examples in the project can be run without cost using a forked ganache instance against Ethereum mainnet. To start the forked instance, run the following:

# Replace ETHEREUM_RPC_URL with your mainnet node endpoint URL (e.g., infura)
$ RPC_URL=ETHEREUM_RPC_URL npm run start-fork

The swap contract

The contract that will be directly filling 0x-API quotes is SimpleTokenSwap. It implements a fillQuote() function that accepts and executes a 0x-API quote to convert some amount of its ERC20 tokens into another. For simplicity, this contract is not designed for use with plain ETH. However, WETH and ETH pairs are identical markets in 0x-API, so this will not affect pricing.

The fill function

The swap is executed when the owner calls the fillQuote() function. Prior to calling this function, the SimpleTokenSwap contract should be funded with at least sellAmount (from the API response) of sellToken to complete the swap.

In this function we:

  1. Grant the allowanceTarget an allowance to spend sellToken on this contract's behalf.
  2. Execute the ERC20->ERC20 swap.
  3. Transfer any leftover ETH (protocol fee refunds) to the sender.
// Swaps ERC20->ERC20 tokens held by this contract using a 0x-API quote.
function fillQuote(
// The `sellTokenAddress` field from the API response.
IERC20 sellToken,
// The `buyTokenAddress` field from the API response.
IERC20 buyToken,
// The `allowanceTarget` field from the API response.
address spender,
// The `to` field from the API response.
address payable swapTarget,
// The `data` field from the API response.
bytes calldata swapCallData
payable // Must attach ETH equal to the `value` field from the API response.
// ...

// Give `spender` an infinite allowance to spend this contract's `sellToken`.
// Note that for some tokens (e.g., USDT, KNC), you must first reset any existing
// allowance to 0 before being able to update it.
require(sellToken.approve(spender, uint256(-1)));
// Call the encoded swap function call on the contract at `swapTarget`,
// passing along any ETH attached to this function call to cover protocol fees.
(bool success,) ={value: msg.value}(swapCallData);
require(success, 'SWAP_CALL_FAILED');
// Refund any unspent protocol fees to the sender.

// ...

Deposit function

Since the contract will sell its own balance of a token when executing a swap, it needs to hold a balance of that token beforehand. In this example, we will be selling WETH, which can easily be minted from ETH. So the contract has a deposit function that accepts ETH and wraps it into WETH.

// Transfer ETH into this contract and wrap it into WETH.
function depositETH()
WETH.deposit{value: msg.value}();

Payable fallback

Certain quotes require a protocol fee, in ETH, to be attached to the swap call. But it's possible that by the time the transaction is mind, the swap will end up not needing to pay the protocol fee. In these cases, the protocol fee would be refunded to the taker, which is in this case is the SimpleTokenSwap contract. So it's important that your contract be able to receive this ETH through either a payable fallback() or receive() function (in solidity 0.6+).

// Payable fallback to allow this contract to receive protocol fee refunds.
receive() external payable {}

Compiling and deploying the contract

Before we can run the example, we will need to compile and deploy the contract to the forked network:

npm run deploy-fork

Using the contract

The complete interaction with the contract is in the swap-contract.js script. This script does the following:

  • Fund the deployed SimpleTokenSwap contract with WETH.
  • Fetch a swap quote from 0x-API to convert said WETH to DAI.
  • Call fillQuote() on the SimpleTokenSwap contract to execute the swap.

Fund the contract

First we call depositETH(), which accepts ETH and wraps it into WETH, to give the contract a balance (sellAmountWei) of WETH .

await waitForTxSuccess(contract.methods.depositETH().send({
value: sellAmountWei,
from: owner,

Fetch a quote from 0x API

Next we make an HTTP request to 0x-API (at for a quote to convert that WETH into DAI.

const qs = createQueryString({
sellToken: 'WETH',
buyToken: 'DAI',
sellAmount: sellAmountWei,

const headers = {'0x-api-key: [api-key]'}; // This is a placeholder. Get your live API key from the 0x Dashboard (

const quoteUrl = `${API_QUOTE_URL}?${qs}`;
const response = await fetch(quoteUrl, { headers });
const quote = await response.json();

The returned quote will have fields such as:

to: '0xdef1c0ded9bec7f1a1670819833240f027b25eff',
data: '0x415565b0000000000000000000000000...',
allowanceTarget: '0xf740b67da229f2f10bcbd38a7979992fcc71b8eb',
sellTokenAddress: '0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2',
buyTokenAddress: '0x6b175474e89094c44da98b954eedeac495271d0f',
value: '0',
gasPrice: '8000000000',
gas: 12345678,

Fill the quote through our contract

We can now pass fields from this quote into the fillQuote() function on our contract to be filled. This will cause the contract to swap its WETH balance for DAI.

const receipt = await waitForTxSuccess(contract.methods.fillQuote(
from: owner,
value: quote.value,
gasPrice: quote.gasPrice,

Running the example

We can run this example with the command:

npm run swap-contract-fork

If all goes well, you should see output similar to:

Fetching quote
Received a quote with price 371.15289757586245518
Filling the quote through the contract at 0x7382949f535C1bb4D64059b934d4A63A11D3DAa2...
✔ Successfully sold 0.1 WETH for 37.115276285540685651 DAI!

Next steps: