This converts from decimal to wei then from wei to decimal

main.js

const Web3 = require('web3')
require('dotenv').config()

const main = async () => {
    // Connect to a EVM blockchain
    const web3 = new Web3(Web3.givenProvider || process.env.INFURA_URL)

    // Convert from a decimal number to a wei number
    let amt = web3.utils.toWei('0.1', 'ether')
    console.log('Convert from a decimal number to a wei number', amt)

    // Convert from a wei number to a decimal number
    amt = web3.utils.fromWei(amt, 'ether')
    console.log('Convert from a wei number to a decimal number', amt)
}

main()
    .then(() => process.exit(0))
    .catch((err) => {
        console.error(err)
        process.exit(1)
    })