<div class="container">
  <h1 class="heading" data-target-resolver></h1>
</div>

* {
  box-sizing: border-box;
}

html,
body {
  height: 100%;
  margin: 0;
  font-family: 'Unica One', sans-serif;
  background: #111;
}

.container {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100%;
  padding: 5rem;
}

.heading {
  color: #eee;
  font-size: 5rem;
  font-weight: 300;
  text-transform: uppercase;
}

const resolver = {
  resolve: function resolve(options, callback) {
    // The string to resolve
    const resolveString = options.resolveString || options.element.getAttribute('data-target-resolver');
    const combinedOptions = Object.assign({}, options, {resolveString: resolveString});
    
    function getRandomInteger(min, max) {
      return Math.floor(Math.random() * (max - min + 1)) + min;
    };
    
    function randomCharacter(characters) {
      return characters[getRandomInteger(0, characters.length - 1)];
    };
    
    function doRandomiserEffect(options, callback) {
      const characters = options.characters;
      const timeout = options.timeout;
      const element = options.element;
      const partialString = options.partialString;

      let iterations = options.iterations;

      setTimeout(() => {
        if (iterations >= 0) {
          const nextOptions = Object.assign({}, options, {iterations: iterations - 1});

          // Ensures partialString without the random character as the final state.
          if (iterations === 0) {
            element.textContent = partialString;
          } else {
            // Replaces the last character of partialString with a random character
            element.textContent = partialString.substring(0, partialString.length - 1) + randomCharacter(characters);
          }

          doRandomiserEffect(nextOptions, callback)
        } else if (typeof callback === "function") {
          callback(); 
        }
      }, options.timeout);
    };
    
    function doResolverEffect(options, callback) {
      const resolveString = options.resolveString;
      const characters = options.characters;
      const offset = options.offset;
      const partialString = resolveString.substring(0, offset);
      const combinedOptions = Object.assign({}, options, {partialString: partialString});

      doRandomiserEffect(combinedOptions, () => {
        const nextOptions = Object.assign({}, options, {offset: offset + 1});

        if (offset <= resolveString.length) {
          doResolverEffect(nextOptions, callback);
        } else if (typeof callback === "function") {
          callback();
        }
      });
    };

    doResolverEffect(combinedOptions, callback);
  } 
}

/* Some GLaDOS quotes from Portal 2 chapter 9: The Part Where He Kills You
 * Source: http://theportalwiki.com/wiki/GLaDOS_voice_lines#Chapter_9:_The_Part_Where_He_Kills_You
 */
const strings = [
  'Oh thank god, you\'re alright.',
  'You know, being Caroline taught me a valuable lesson. I thought you were my greatest enemy. When all along you were my best friend.',
  'The surge of emotion that shot through me when I saved your life taught me an even more valuable lesson: where Caroline lives in my brain.',
  'Goodbye, Caroline.',
  'You know, deleting Caroline just now taught me a valuable lesson. The best solution to a problem is usually the easiest one. And I\'ll be honest.',
  'Killing you? Is hard.',
  'You know what my days used to be like? I just tested. Nobody murdered me. Or put me in a potato. Or fed me to birds. I had a pretty good life.',
  'And then you showed up. You dangerous, mute lunatic. So you know what?',
  'You win.',
  'Just go.',
  'It\'s been fun. Don\'t come back.',
  '......'
];

let counter = 0;

const options = {
  // Initial position
  offset: 0,
  // Timeout between each random character
  timeout: 5,
  // Number of random characters to show
  iterations: 10,
  // Random characters to pick from
  characters: ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'x', 'y', 'x', '#', '%', '&', '-', '+', '_', '?', '/', '\\', '='],
  // String to resolve
  resolveString: strings[counter],
  // The element
  element: document.querySelector('[data-target-resolver]')
}

// Callback function when resolve completes
function callback() {
  setTimeout(() => {
    counter ++;
    
    if (counter >= strings.length) {
      counter = 0;
    }
    
    let nextOptions = Object.assign({}, options, {resolveString: strings[counter]});
    resolver.resolve(nextOptions, callback);
  }, 1000);
}

resolver.resolve(options, callback);