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I'm a beginner with Web3 programming. Trying to understand how all it all works.

Managed to get some nodejs code working for buying tokens but when trying to create a script that sells tokens I get an error message related to the .buildTransaction method. I don't understand where I can get the buildTransaction method from? I guess it should go into the ABI same as for the swapExactTokensForETHSupportingFeeOnTransferTokens but I have looked everywhere and can't even find an example so it seems like I'm missing something.

Would really appreciate some help on this. Have been sitting 3 hours just trying to understand how I can get the buildTransaction to work.

My code:

exchange_txn = contract.functions.swapExactTokensForETHSupportingFeeOnTransferTokens(
    500000000, 0,
    [spending_this_token,buying_this_token],
    your_wallet_address,
    deadline
).buildTransaction({
    'from': your_wallet_address,
    'gasPrice': Web3Client.utils.toWei('20','gwei'),
    'gasLimit': estimate,
    'nonce': nonce,
})

Error message:

TypeError: contract.functions.swapExactTokensForETHSupportingFeeOnTransferTokens(...).buildTransaction is not a function

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  • Welcome, what is the library you're using, in which programming language ?
    – Franfran
    Commented Mar 9, 2022 at 12:51
  • Thank you. I am using NodeJS, including all these libraries per below: ` var Tx = require('ethereumjs-tx'); var Web3 = require('web3') var Contract = require('web3-eth-contract'); const ethers = require("ethers"); const web3utils = require("web3-utils"); `
    – Marcus
    Commented Mar 9, 2022 at 16:18
  • Alright, looks like you're using the web3.py syntax. Please refer to the web3.js library instead. This is what you wanna use to send a transaction: web3js.readthedocs.io/en/v1.7.1/… And to add an account to sign the transaction: web3js.readthedocs.io/en/v1.2.11/…
    – Franfran
    Commented Mar 9, 2022 at 18:26

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