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Using Local Wallet

The simplest way to sign and send transactions is using a local wallet:

Ethereum Transaction​

// First step: initialize `web3` instance
import Web3 from 'web3';
const web3 = new Web3(/* PROVIDER*/);

// Second step: add an account to wallet
const privateKeyString = '0x1f953dc9b6437fb94fcafa5dabe3faa0c34315b954dd66f41bf53273339c6d26';
const account = web3.eth.accounts.wallet.add(privateKeyString);

// Make sure the account has enough eth on balance to send the transaction

// Third step: sign and send the transaction
// Magic happens behind sendTransaction. If a transaction is sent from an account that exists in a wallet, it will be automatically signed.
try {
    const receipt = await web3.eth.sendTransaction({
        from: account.address,
        to: '0xe4beef667408b99053dc147ed19592ada0d77f59',
        value: '0x1',
        gas: '300000',
        // other transaction's params
    });
} catch (error) {
    // catch transaction error
    console.error(error);
}

List of references:

Contract Transaction​

// First step: initialize `web3` instance
import Web3 from 'web3';
const web3 = new Web3(/* PROVIDER*/);

// Second step: add an account to wallet
const privateKeyString = '0x1f953dc9b6437fb94fcafa5dabe3faa0c34315b954dd66f41bf53273339c6d26';
const account = web3.eth.accounts.wallet.add(privateKeyString);

// Make sure the account has enough eth on balance to send the transaction

// Third step: sign and send the transaction
// In any function where you can pass from the address set address of the account that exists in a wallet, it will be automatically signed.

try {
    // deploy
    const contract = new web3.eth.Contract(ContractAbi);
    const contractDeployed = await contract
        .deploy({
            input: ContractBytecode,
            arguments: ['Constructor param1', 'Constructor param2'],
        })
        .send({
            from: account.address,
            gas: '1000000',
            // other transaction's params
        });

    // call method
    await contractDeployed.methods
        .transfer('0xe2597eb05cf9a87eb1309e86750c903ec38e527e', '0x1')
        .send({
            from: account.address,
            gas: '1000000',
            // other transaction's params
        });
} catch (error) {
    // catch transaction error
    console.error(error);
}

List of references: