Error on estimating gas for swapExactTokensForTokens

Question

I'm new to blockchain and I'm experimenting with web3.js. I tried to calculate gas for swapping WBNB for DAI and I keep getting error. Here is my code:

let routerContract = new puWeb3.eth.Contract(Config.PANCAKESWAP_ROUTER_V2_ABI, Config.PANCAKESWAP_ROUTER_V2_ADDRESS);
let tokenContract = new puWeb3.eth.Contract(Config.TOKEN_ABI, Config.WBNB_ADDRESS);

let amountIn = puWeb3.utils.toWei('0.01', 'ether');
let amountOutMin = await routerContract.methods.getAmountsOut(amountIn, [Config.WBNB_ADDRESS, Config.DAI_ADDRESS]).call();
amountIn = amountOutMin[0];
amountOutMin = amountOutMin[1];

console.log(amountIn+" WBNB ----> "+amountOutMin+" DAI");

// Set deadline 1 minute from now
let now = parseInt(Date.now()/1000);
let deadline = now + 60 

// Encode data
let encodedABI = routerContract.methods.swapExactTokensForTokens(amountIn, amountOutMin, [Config.WBNB_ADDRESS, Config.DAI_ADDRESS], Config.ACCOUNT_ADDRESS, deadline).encodeABI();

//Estimate gas
let estimateGas = await puWeb3.eth.estimateGas({
    from: Config.ACCOUNT_ADDRESS,   
    to: Config.PANCAKESWAP_ROUTER_V2_ADDRESS,    
    data: encodedABI,
});

And the error is: Returned error: execution reverted: TransferHelper: TRANSFER_FROM_FAILED

Can someone tell me what am I doing wrong? Thanks.

Answer 1

I finally managed to get estimative gas and do a trade only after I approved the input token to exchange. You need 2 transactions:

1. first send a transaction to approve the input token with the input amount

   // Encode data let encodedABI = tokenContract.methods.approve(Config.PANCAKESWAP_ROUTER_V2_ADDRESS, puWeb3.utils.toHex(amountIn)).encodeABI();

2. after approve transaction, send a transaction to swap