"Distance of Time in Words" in Vanilla JavaScript.

timeago.js

var TimeAgo = (function() {
  var self = {};
  
  // Public Methods
  self.locales = {
    prefix: '',
    sufix:  'ago',
    
    seconds: 'less than a minute',
    minute:  'about a minute',
    minutes: '%d minutes',
    hour:    'about an hour',
    hours:   'about %d hours',
    day:     'a day',
    days:    '%d days',
    month:   'about a month',
    months:  '%d months',
    year:    'about a year',
    years:   '%d years'
  };
  
  self.inWords = function(timeAgo) {
    var seconds = Math.floor((new Date() - parseInt(timeAgo)) / 1000),
        separator = this.locales.separator || ' ',
        words = this.locales.prefix + separator,
        interval = 0,
        intervals = {
          year:   seconds / 31536000,
          month:  seconds / 2592000,
          day:    seconds / 86400,
          hour:   seconds / 3600,
          minute: seconds / 60
        };
    
    var distance = this.locales.seconds;
    
    for (var key in intervals) {
      interval = Math.floor(intervals[key]);
      
      if (interval > 1) {
        distance = this.locales[key + 's'];
        break;
      } else if (interval === 1) {
        distance = this.locales[key];
        break;
      }
    }
    
    distance = distance.replace(/%d/i, interval);
    words += distance + separator + this.locales.sufix;

    return words.trim();
  };
  
  return self;
}());

Usage Example

TimeAgo.locales.seconds = 'há menos de um minuto';
alert(TimeAgo.inWords(123456789));

Thanks; here is my contribution to manage the "from now" case (seconds < 0)

let TimeAgo = (function() {
  var self = {};

  // Public Methods
  self.locales = {
    prefix: '',
    ago:  'ago',
    from_now:  '',

    seconds: 'less than a minute',
    minute:  'about a minute',
    minutes: '%d minutes',
    hour:    'about an hour',
    hours:   'about %d hours',
    day:     'a day',
    days:    '%d days',
    month:   'about a month',
    months:  '%d months',
    year:    'about a year',
    years:   '%d years'
  };

  self.inWords = function(timeAgo) {
    var seconds = Math.floor((new Date() - parseInt(timeAgo)) / 1000),
        separator = this.locales.separator || ' ',
        words = this.locales.prefix + separator,
        interval = 0,
        intervals = {
          year:   Math.abs(seconds) / 31536000,
          month:  Math.abs(seconds) / 2592000,
          day:    Math.abs(seconds) / 86400,
          hour:   Math.abs(seconds) / 3600,
          minute: Math.abs(seconds) / 60
        };

    var distance = this.locales.seconds;

    for (var key in intervals) {
      interval = Math.floor(intervals[key]);

      if (interval > 1) {
        distance = this.locales[key + 's'];
        break;
      } else if (interval === 1) {
        distance = this.locales[key];
        break;
      }
    }

    distance = distance.replace(/%d/i, interval);
    if (seconds >= 0) {
      words += distance + separator + this.locales.ago;
    } else {
      words += distance + separator + this.locales.from_now;
    }

    return words.trim();
  };

  return self;
}());