for testing signing messages with metamask just using the browser console.

connect_to_metamask.js

String.prototype.hexEncode = function(){
    var hex, i;

    var result = "";
    for (i=0; i<this.length; i++) {
        hex = this.charCodeAt(i).toString(16);
        result += ("000"+hex).slice(-4);
    }

    return result
}

// paste this into your browser console
let account = await window.ethereum.request({ method: 'eth_requestAccounts' })

// for use with personal_sign
let message = "sign this please"
 
// eth_signTypedData_v4 parameters. All of these parameters affect the resulting signature.
const msgParams = JSON.stringify({
  domain: {
    // This defines the network, in this case, Mainnet.
    chainId: 1,
    // Give a user-friendly name to the specific contract you're signing for.
    name: 'Ether Mail',
    // Add a verifying contract to make sure you're establishing contracts with the proper entity.
    verifyingContract: '0xCcCCccccCCCCcCCCCCCcCcCccCcCCCcCcccccccC',
    // This identifies the latest version.
    version: '1',
  },

  // This defines the message you're proposing the user to sign, is dapp-specific, and contains
  // anything you want. There are no required fields. Be as explicit as possible when building out
  // the message schema.
  message: {
    contents: 'Hello, Bob!',
    attachedMoneyInEth: 4.2,
    from: {
      name: 'Cow',
      wallets: [
        '0xCD2a3d9F938E13CD947Ec05AbC7FE734Df8DD826',
        '0xDeaDbeefdEAdbeefdEadbEEFdeadbeEFdEaDbeeF',
      ],
    },
    to: [
      {
        name: 'Bob',
        wallets: [
          '0xbBbBBBBbbBBBbbbBbbBbbbbBBbBbbbbBbBbbBBbB',
          '0xB0BdaBea57B0BDABeA57b0bdABEA57b0BDabEa57',
          '0xB0B0b0b0b0b0B000000000000000000000000000',
        ],
      },
    ],
  },
  // This refers to the keys of the following types object.
  primaryType: 'Mail',
  types: {
    // This refers to the domain the contract is hosted on.
    EIP712Domain: [
      { name: 'name', type: 'string' },
      { name: 'version', type: 'string' },
      { name: 'chainId', type: 'uint256' },
      { name: 'verifyingContract', type: 'address' },
    ],
    // Not an EIP712Domain definition.
    Group: [
      { name: 'name', type: 'string' },
      { name: 'members', type: 'Person[]' },
    ],
    // Refer to primaryType.
    Mail: [
      { name: 'from', type: 'Person' },
      { name: 'to', type: 'Person[]' },
      { name: 'contents', type: 'string' },
    ],
    // Not an EIP712Domain definition.
    Person: [
      { name: 'name', type: 'string' },
      { name: 'wallets', type: 'address[]' },
    ],
  },
});

// personal_sign
// let sig = await window.ethereum.request({method: 'personal_sign', params: [message.hexEncode(), account[0]]})

// eth_signTypedData_v4
let sig = await window.ethereum.request({method: 'eth_signTypedData_v4', params: [account[0], msgParams]})

console.log(sig)