When I sign and send raw transactions with web3 in javascipt , to call a method from Smart Contract, I always pass the transaction gas variable after getting it with estimateGas function, from Smart Contract method, as following:

mySmartContract= new web3.eth.Contract(SmartContractABI, SmartContractAddress);
mySmartContract.methods.set(inputnum).estimateGas({from: miAddress})
          tx.gas = web3.utils.toHex(gas);
        console.log("Set() - catch error");

How could I do the same to invoke the fallback function from the Smart Contract? estimateGas() is a function from "methods" object, but "fallback" is not a method.

Thanks in advance

2 Answers 2


After some research & testing I found the solution by mysef. As a fallback function is unnamed, I just need to use estimateGas with the "eth" object to get the gas stimation of the default function (fallback):

const web3eth = require('web3-eth');
const eth = new web3eth('https://ropsten.infura.io/v3/XXXXXXX');
var tx = {};

      from: miAddress
    .then(function(gas) {
      tx.gas = web3.utils.toHex(gas);
    .catch(function(error) {
      console.log("Estimategas() - catch error");

 var estimatedGas = await web3.eth.estimateGas({
  from: "0xa89a9290DD214B9341dc09Fe23eF0a6a633F9a9F",
   to: tokencontract,
   data: tk.methods.updateMinted(id).encodeABI()

var tip = await web3.eth.getMaxPriorityFeePerGas()

var block = await web3.eth.getBlock("pending")

const baseFee = Number(block.baseFeePerGas);
const max = Number(tip) + baseFee; //increse to speed up

const tx = {
  to: tokencontract,
  gas:  web3.utils.toHex(estimatedGas),
  maxPriorityFeePerGas: web3.utils.toHex(Number(tip)),
  maxFeePerGas: web3.utils.toHex(max),
  data: tk.methods.updateMinted(id).encodeABI(),


const signedTx = await web3.eth.accounts.signTransaction(tx, privateKey);

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