# How to Swap

You will need the Stargate [router interface](https://stargateprotocol.gitbook.io/stargate/interfaces/evm-solidity-interfaces/istargaterouter.sol).

To perform a cross chain `swap()` in solidity, here is an example:

{% hint style="info" %}
For the native gas fee required for `swap()` you need to call `quoteLayerZero()` on the Router.sol contract to get the amount you should send as msg.value. 
{% endhint %}

Here is the swap() interface:

<pre><code>function swap(
<strong>  uint16 _dstChainId,
</strong>  uint256 _srcPoolId,
  uint256 _dstPoolId,
  address payable _refundAddress,
  uint256 _amountLD,
  uint256 _minAmountLD,       
  lzTxObj memory _lzTxParams,
  bytes calldata _to,
  bytes calldata _payload
) external payable override nonReentrant {
</code></pre>

First determine the swap fee, then call swap() to transfer the asset to the destination chain.

```javascript
// perform a Stargate swap() in a solidity smart contract function
// the msg.value is the "fee" that Stargate needs to pay for the cross chain message
IStargateRouter(routerAddress).swap{value:msg.value}(
    10006,                           // send to Fuji (use LayerZero chainId)
    1,                               // source pool id
    1,                               // dest pool id                 
    msg.sender,                      // refund adddress. extra gas (if any) is returned to this address
    _amountLD,                       // quantity to swap in LD, (local decimals)
    _minAmountLD,                    // the min qty you would accept in LD (local decimals)
    IStargateRouter.lzTxObj(0, 0, "0x")  // 0 additional gasLimit increase, 0 airdrop, at 0x address
    abi.encodePacked(msg.sender),    // the address to send the tokens to on the destination
    bytes("")                        // bytes param, if you wish to send additional payload you can abi.encode() them here
);
```

To perform a `swap()` using ethers via a frontend, use the abi and call `swap()` on the Router contract instance:

```javascript
let tx = await router.swap(
    2,                     // destination chainId
    1,                     // source poolId
    1,                     // destination poolId
    yourAddress,           // refund address. extra gas (if any) is returned to this address
    _amountLD,             // quantity to swap in LD (local decimals) 
    _minAmountLD,          // the min qty you would accept in LD (local decimals)
    { dstGasForCall: 0, dstNativeAmount: 0, dstNativeAddr: "0x" },
    yourAddress,           // the address to send the tokens to on the destination
    "",                    // payload
    {value: fee}           // "fee" is the native gas to pay for the cross chain message fee. see 
);
```

TESTNET ONLY: For the `fee` parameter, if you wish to simply guess the fee, you can do something like this, as Stargate will `refund` the caller the additional gas amount if they send too much.

```javascript
let messageFee = ethers.utils.parseEther('0.025');     // send 0.02 eth converter to wei
await stargateRouter.swap(..., {value: messageFee});  // swap asset across chains
```

See how to [get the cross chain message fee](https://stargateprotocol.gitbook.io/stargate/developers/cross-chain-swap-fee) to pay enough native gas when calling `swap()`