DEX API

Build swap applications on EVM

Build swap applications on EVM#

In this guide, we will provide an example token swap through OKX DEX, using ETH from the Ethereum chain to purchase USDT. This process includes:

Set up your environment
Obtain swap information and build transaction data
Check allowance
Send swap transaction
Get approval and send swap transaction

1. Set up your environment#

Import the necessary Node.js libraries and set your environment variables as well as define helper functions and assembly parameters Node.js Environment Settings.

2. Obtain swap information and build transaction data#

2.1 Define your swap parameters#

Next, define the parameters for the swap you want to perform.

const swapParams = {
  chainId,
  fromTokenAddress,
  toTokenAddress,
  amount: '1000000000000000',
  slippage: '0.03',
  userWalletAddress,
  referrerAddress: '', // Optional
  feePercent: '', // Optional
};

2.2 Define helper functions#

Define helper functions that will be used to interact with the DEX swap API.

const getSwapData = async () => {
  const apiRequestUrl = getRequestUrl(apiBaseUrl, '/swap', swapParams);
  return fetch(apiRequestUrl, {
    method: 'get',
    headers: headersParams,
  })
    .then((res) => res.json())
    .then((res) => {
      return res;
    });
};

2.3 Build transaction data#

Transaction parameters are assembled to initiate transactions.

const { data } = await getSwapData();
const transaction = data[0]?.tx;
const nonce = await web3.eth.getTransactionCount(userWalletAddress, 'latest');
// You can obtain the latest nonce and process the hexadecimal numbers starting with 0x according to your needs
let signTransactionParams = {
  data: transaction.data,
  gasPrice: transaction.gasPrice,
  gas: transaction.gas,
  to: transaction.to,
  value: transaction.value,
  nonce,
};

3. Check allowance#

3.1 Please refer to the tutorial#

Get allowance

The variable allowanceAmount in the following text represents the actual allowance amount on the blockchain.

3.2 Get allowance amount#

Obtain the specific quantity. If allowanceAmount !== '0', please check step 4. If allowanceAmount === '0', please check step 5.

const { data: allowanceData } = await getAllowanceData();
const allowanceAmount = allowanceData?.[0]?.allowanceAmount;

4. Send swap transaction#

Tip

If allowanceAmount is greater than zero, it indicates that the approval operation has been performed and the transaction can be directly sent.

if (allowanceAmount !== '0') {
  const { rawTransaction } = await web3.eth.accounts.signTransaction(
    signTransactionParams,
    privateKey
  );
  const chainTxInfo = await web3.eth.sendSignedTransaction(rawTransaction);
  console.log('chainTxInfo', chainTxInfo);
}

5. Get approval and send swap transaction#

Tip

If allowanceAmount === '0', it indicates that approval has not been granted, and approval operations need to be performed at this time.

5.1 Define your transaction approval parameters#

Next, define the parameters for the transaction approval you want to perform.

const getApproveTransactionParams = {
  chainId,
  tokenContractAddress: fromTokenAddress,
  userWalletAddress,
  approveAmount: '...',
};

5.2 Define helper functions#

Define helper functions that will be used to interact with the DEX transaction approval API.

const approveTransaction = async () => {
  const apiRequestUrl = getRequestUrl(
    apiBaseUrl,
    '/approve-transaction',
    getApproveTransactionParams
  );

  return fetch(apiRequestUrl, {
    method: 'get',
    headers: headersParams,
  })
    .then((res) => res.json())
    .then((res) => {
      return res;
    });
};

5.3 Get transaction tx#

Get tx information after obtaining approval.

const { data } = await approveTransaction();

5.4 Merge transaction tx#

Update tx to approved tx.

signTransactionParams = {
  ...signTransactionParams,
  ...{ data: data[0].data },
};

5.5 Send swap transaction#

Obtain signature tx information and send transaction.

if (allowanceAmount === '0') {
  const { rawTransaction } = await web3.eth.accounts.signTransaction(
    signTransactionParams,
    privateKey
  );
  const chainTxInfo = await web3.eth.sendSignedTransaction(rawTransaction);
  console.log('chainTxInfo', chainTxInfo);
}

DEX API

Build swap applications

DEX API

Build swap applications on Solana

Table of contents

1. Set up your environment

2. Obtain swap information and build transaction data

3. Check allowance

4. Send swap transaction

5. Get approval and send swap transaction