steemLoginExample.js

import steem from 'steem';

// Get the form values
let username = document.getElementById('steem-username').value;
let privatePostingKey = document.getEmenentById('steem-posting-key').value;

// Get user details
steem.api.getAccounts([username], (err, result) => {
  if (err) {
    // Something went wrong
  }
  if (result.length === 0) {
    // No such user
  }
  // Get the public key
  let publicPostingKey = result[0].posting.key_auths[0][0];
  // Try logging in
  let loginSuccess = false;
  try {
    loginSuccess = steem.auth.wifIsValid(privatePostingKey, publicPostingKey);
  } catch (e) {
    // Failed log in
  }
  if (loginSuccess) {
    // yay!!
  } else {
    // Wrong combination
  }
})