Using the New MetaMask Inpage Provider

newProvider.js

// Running on the page, in the browser
// This API will go live in early 2020
// It will be the only API available after a 6-week deprecation period

if (!ethereum || !ethereum.isMetaMask) {
  throw new Error('Please install MetaMask.')
}

/*********************************************************/
/* Handle chain (network) and chainChanged, per EIP 1193 */
/*********************************************************/

let currentChainId = null
ethereum.send('eth_chainId')
  .then(handleChainChanged)
  .catch(err => console.error(err)) // This should never happen

ethereum.on('chainChanged', handleChainChanged)

function handleChainChanged (chainId) {

  if (currentChainId !== chainId) {

    currentChainId = chainId
    // Run any other necessary logic...
  }
}

/**********************************************************/
/* Handle user accounts and accountsChanged, per EIP 1193 */
/**********************************************************/

let currentAccount = null
ethereum.send('eth_accounts')
  .then(handleAccountsChanged)
  .catch(err => {
    // In the future, maybe in 2020, this will return a 4100 error if
    // the user has yet to connect
    if (err.code === 4100) { // EIP 1193 unauthorized error
      console.log('Please connect to MetaMask.')
    } else {
      console.error(err)
    }
  })

// Note that this event is emitted on page load.
// If the array of accounts is non-empty, you're already
// connected.
ethereum.on('accountsChanged', handleAccountsChanged)

// For now, 'eth_accounts' will continue to always return an array
function handleAccountsChanged (accounts) {

  if (accounts.length === 0) {

    // MetaMask is locked or the user has not connected any accounts
    console.log('Please connect to MetaMask.')

  } else if (accounts[0] !== currentAccount) {

    currentAccount = accounts[0]
    // Run any other necessary logic...
  }
}

/***********************************/
/* Handle connecting, per EIP 1102 */
/***********************************/

// You should only attempt to connect in response to user interaction,
// such as a button click. Otherwise, you're popup-spamming the user
// like it's 1999.
// If you can't retrieve the user's account(s), you should encourage the user
// to initiate a connection attempt.
document.getElementById('connectButton', connect)

function connect () {

  // This is equivalent to ethereum.enable()
  ethereum.send('eth_requestAccounts')
    .then(handleAccountsChanged)
    .catch(err => {
      if (err.code === 4001) { // EIP 1193 userRejectedRequest error
        console.log('Please connect to MetaMask.')
      } else {
        console.error(err)
      }
    })
}