discourse/app/assets/javascripts/locales/i18n.js

// https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/indexOf
if (!Array.prototype.indexOf) {
  Array.prototype.indexOf = function(searchElement /*, fromIndex */) {
    "use strict";

    if (this === void 0 || this === null) {
      throw new TypeError();
    }

    var t = Object(this);
    var len = t.length >>> 0;

    if (len === 0) {
      return -1;
    }

    var n = 0;
    if (arguments.length > 0) {
      n = Number(arguments[1]);
      if (n !== n) { // shortcut for verifying if it's NaN
        n = 0;
      } else if (n !== 0 && n !== (Infinity) && n !== -(Infinity)) {
        n = (n > 0 || -1) * Math.floor(Math.abs(n));
      }
    }

    if (n >= len) {
      return -1;
    }

    var k = n >= 0
          ? n
          : Math.max(len - Math.abs(n), 0);

    for (; k < len; k++) {
      if (k in t && t[k] === searchElement) {
        return k;
      }
    }

    return -1;
  };
}

// Instantiate the object
var I18n = I18n || {};

// Set default locale to english
I18n.defaultLocale = "en";

// Set default handling of translation fallbacks to false
I18n.fallbacks = false;

// Set default separator
I18n.defaultSeparator = ".";

// Set current locale to null
I18n.locale = null;

// Set the placeholder format. Accepts `{{placeholder}}` and `%{placeholder}`.
I18n.PLACEHOLDER = /(?:\{\{|%\{)(.*?)(?:\}\}?)/gm;

I18n.fallbackRules = {};

I18n.pluralizationRules = {
  en: function (n) {
    return n == 0 ? ["zero", "none", "other"] : n == 1 ? "one" : "other";
  }
};

I18n.getFallbacks = function(locale) {
  if (locale === I18n.defaultLocale) {
    return [];
  } else if (!I18n.fallbackRules[locale]) {
    var rules = []
      , components = locale.split("-");

    for (var l = 1; l < components.length; l++) {
      rules.push(components.slice(0, l).join("-"));
    }

    rules.push(I18n.defaultLocale);

    I18n.fallbackRules[locale] = rules;
  }

  return I18n.fallbackRules[locale];
}

I18n.isValidNode = function(obj, node, undefined) {
  return obj[node] !== null && obj[node] !== undefined;
};

I18n.lookup = function(scope, options) {
  var options = options || {}
    , lookupInitialScope = scope
    , translations = this.prepareOptions(I18n.translations)
    , locale = options.locale || I18n.currentLocale()
    , messages = translations[locale] || {}
    , options = this.prepareOptions(options)
    , currentScope
  ;

  if (typeof(scope) == "object") {
    scope = scope.join(this.defaultSeparator);
  }

  if (options.scope) {
    scope = options.scope.toString() + this.defaultSeparator + scope;
  }

  scope = scope.split(this.defaultSeparator);

  while (messages && scope.length > 0) {
    currentScope = scope.shift();
    messages = messages[currentScope];
  }

  if (!messages) {
    if (I18n.fallbacks) {
      var fallbacks = this.getFallbacks(locale);
      for (var fallback = 0; fallback < fallbacks.length; fallbacks++) {
        messages = I18n.lookup(lookupInitialScope, this.prepareOptions({locale: fallbacks[fallback]}, options));
        if (messages) {
          break;
        }
      }
    }

    if (!messages && this.isValidNode(options, "defaultValue")) {
        messages = options.defaultValue;
    }
  }

  return messages;
};

// Merge serveral hash options, checking if value is set before
// overwriting any value. The precedence is from left to right.
//
//   I18n.prepareOptions({name: "John Doe"}, {name: "Mary Doe", role: "user"});
//   #=> {name: "John Doe", role: "user"}
//
I18n.prepareOptions = function() {
  var options = {}
    , opts
    , count = arguments.length
  ;

  for (var i = 0; i < count; i++) {
    opts = arguments[i];

    if (!opts) {
      continue;
    }

    for (var key in opts) {
      if (!this.isValidNode(options, key)) {
        options[key] = opts[key];
      }
    }
  }

  return options;
};

I18n.interpolate = function(message, options) {
  options = this.prepareOptions(options);
  var matches = message.match(this.PLACEHOLDER)
    , placeholder
    , value
    , name
  ;

  if (!matches) {
    return message;
  }

  for (var i = 0; placeholder = matches[i]; i++) {
    name = placeholder.replace(this.PLACEHOLDER, "$1");

    value = options[name];

    if (!this.isValidNode(options, name)) {
      value = "[missing " + placeholder + " value]";
    }

    var regex = new RegExp(placeholder.replace(/\{/gm, "\\{").replace(/\}/gm, "\\}"));
    message = message.replace(regex, value);
  }

  return message;
};

I18n.translate = function(scope, options) {
  options = this.prepareOptions(options);
  var translation = this.lookup(scope, options);

  try {
    if (typeof(translation) == "object") {
      if (typeof(options.count) == "number") {
        return this.pluralize(options.count, scope, options);
      } else {
        return translation;
      }
    } else {
      return this.interpolate(translation, options);
    }
  } catch (error) {
    return this.missingTranslation(scope);
  }
};

I18n.localize = function(scope, value) {
  switch (scope) {
    case "currency":
      return this.toCurrency(value);
    case "number":
      scope = this.lookup("number.format");
      return this.toNumber(value, scope);
    case "percentage":
      return this.toPercentage(value);
    default:
      if (scope.match(/^(date|time)/)) {
        return this.toTime(scope, value);
      } else {
        return value.toString();
      }
  }
};

I18n.parseDate = function(date) {
  var matches, convertedDate;

  // we have a date, so just return it.
  if (typeof(date) == "object") {
    return date;
  };

  // it matches the following formats:
  //   yyyy-mm-dd
  //   yyyy-mm-dd[ T]hh:mm::ss
  //   yyyy-mm-dd[ T]hh:mm::ss
  //   yyyy-mm-dd[ T]hh:mm::ssZ
  //   yyyy-mm-dd[ T]hh:mm::ss+0000
  //
  matches = date.toString().match(/(\d{4})-(\d{2})-(\d{2})(?:[ T](\d{2}):(\d{2}):(\d{2}))?(Z|\+0000)?/);

  if (matches) {
    for (var i = 1; i <= 6; i++) {
      matches[i] = parseInt(matches[i], 10) || 0;
    }

    // month starts on 0
    matches[2] -= 1;

    if (matches[7]) {
      convertedDate = new Date(Date.UTC(matches[1], matches[2], matches[3], matches[4], matches[5], matches[6]));
    } else {
      convertedDate = new Date(matches[1], matches[2], matches[3], matches[4], matches[5], matches[6]);
    }
  } else if (typeof(date) == "number") {
    // UNIX timestamp
    convertedDate = new Date();
    convertedDate.setTime(date);
  } else if (date.match(/\d+ \d+:\d+:\d+ [+-]\d+ \d+/)) {
    // a valid javascript format with timezone info
    convertedDate = new Date();
    convertedDate.setTime(Date.parse(date))
  } else {
    // an arbitrary javascript string
    convertedDate = new Date();
    convertedDate.setTime(Date.parse(date));
  }

  return convertedDate;
};

I18n.toTime = function(scope, d) {
  var date = this.parseDate(d)
    , format = this.lookup(scope)
  ;

  if (date.toString().match(/invalid/i)) {
    return date.toString();
  }

  if (!format) {
    return date.toString();
  }

  return this.strftime(date, format);
};

I18n.strftime = function(date, format) {
  var options = this.lookup("date");

  if (!options) {
    return date.toString();
  }

  options.meridian = options.meridian || ["AM", "PM"];

  var weekDay = date.getDay()
    , day = date.getDate()
    , year = date.getFullYear()
    , month = date.getMonth() + 1
    , hour = date.getHours()
    , hour12 = hour
    , meridian = hour > 11 ? 1 : 0
    , secs = date.getSeconds()
    , mins = date.getMinutes()
    , offset = date.getTimezoneOffset()
    , absOffsetHours = Math.floor(Math.abs(offset / 60))
    , absOffsetMinutes = Math.abs(offset) - (absOffsetHours * 60)
    , timezoneoffset = (offset > 0 ? "-" : "+") + (absOffsetHours.toString().length < 2 ? "0" + absOffsetHours : absOffsetHours) + (absOffsetMinutes.toString().length < 2 ? "0" + absOffsetMinutes : absOffsetMinutes)
  ;

  if (hour12 > 12) {
    hour12 = hour12 - 12;
  } else if (hour12 === 0) {
    hour12 = 12;
  }

  var padding = function(n) {
    var s = "0" + n.toString();
    return s.substr(s.length - 2);
  };

  var f = format;
  f = f.replace("%a", options.abbr_day_names[weekDay]);
  f = f.replace("%A", options.day_names[weekDay]);
  f = f.replace("%b", options.abbr_month_names[month]);
  f = f.replace("%B", options.month_names[month]);
  f = f.replace("%d", padding(day));
  f = f.replace("%e", day);
  f = f.replace("%-d", day);
  f = f.replace("%H", padding(hour));
  f = f.replace("%-H", hour);
  f = f.replace("%I", padding(hour12));
  f = f.replace("%-I", hour12);
  f = f.replace("%m", padding(month));
  f = f.replace("%-m", month);
  f = f.replace("%M", padding(mins));
  f = f.replace("%-M", mins);
  f = f.replace("%p", options.meridian[meridian]);
  f = f.replace("%S", padding(secs));
  f = f.replace("%-S", secs);
  f = f.replace("%w", weekDay);
  f = f.replace("%y", padding(year));
  f = f.replace("%-y", padding(year).replace(/^0+/, ""));
  f = f.replace("%Y", year);
  f = f.replace("%z", timezoneoffset);

  return f;
};

I18n.toNumber = function(number, options) {
  options = this.prepareOptions(
    options,
    this.lookup("number.format"),
    {precision: 3, separator: ".", delimiter: ",", strip_insignificant_zeros: false}
  );

  var negative = number < 0
    , string = Math.abs(number).toFixed(options.precision).toString()
    , parts = string.split(".")
    , precision
    , buffer = []
    , formattedNumber
  ;

  number = parts[0];
  precision = parts[1];

  while (number.length > 0) {
    buffer.unshift(number.substr(Math.max(0, number.length - 3), 3));
    number = number.substr(0, number.length -3);
  }

  formattedNumber = buffer.join(options.delimiter);

  if (options.precision > 0) {
    formattedNumber += options.separator + parts[1];
  }

  if (negative) {
    formattedNumber = "-" + formattedNumber;
  }

  if (options.strip_insignificant_zeros) {
    var regex = {
        separator: new RegExp(options.separator.replace(/\./, "\\.") + "$")
      , zeros: /0+$/
    };

    formattedNumber = formattedNumber
      .replace(regex.zeros, "")
      .replace(regex.separator, "")
    ;
  }

  return formattedNumber;
};

I18n.toCurrency = function(number, options) {
  options = this.prepareOptions(
    options,
    this.lookup("number.currency.format"),
    this.lookup("number.format"),
    {unit: "$", precision: 2, format: "%u%n", delimiter: ",", separator: "."}
  );

  number = this.toNumber(number, options);
  number = options.format
    .replace("%u", options.unit)
    .replace("%n", number)
  ;

  return number;
};

I18n.toHumanSize = function(number, options) {
  var kb = 1024
    , size = number
    , iterations = 0
    , unit
    , precision
  ;

  while (size >= kb && iterations < 4) {
    size = size / kb;
    iterations += 1;
  }

  if (iterations === 0) {
    unit = this.t("number.human.storage_units.units.byte", {count: size});
    precision = 0;
  } else {
    unit = this.t("number.human.storage_units.units." + [null, "kb", "mb", "gb", "tb"][iterations]);
    precision = (size - Math.floor(size) === 0) ? 0 : 1;
  }

  options = this.prepareOptions(
    options,
    {precision: precision, format: "%n%u", delimiter: ""}
  );

  number = this.toNumber(size, options);
  number = options.format
    .replace("%u", unit)
    .replace("%n", number)
  ;

  return number;
};

I18n.toPercentage = function(number, options) {
  options = this.prepareOptions(
    options,
    this.lookup("number.percentage.format"),
    this.lookup("number.format"),
    {precision: 3, separator: ".", delimiter: ""}
  );

  number = this.toNumber(number, options);
  return number + "%";
};

I18n.pluralizer = function(locale) {
  var pluralizer = this.pluralizationRules[locale];
  if (pluralizer !== undefined) return pluralizer;
  return this.pluralizationRules["en"];
};

I18n.findAndTranslateValidNode = function(keys, translation) {
  for (var i = 0; i < keys.length; i++) {
    var key = keys[i];
    if (this.isValidNode(translation, key)) return translation[key];
  }
  return null;
};

I18n.pluralize = function(count, scope, options) {
  var translation;

  try { translation = this.lookup(scope, options); } catch (error) {}
  if (!translation) { return this.missingTranslation(scope); }

  options = this.prepareOptions(options);
  options.count = count.toString();

  var pluralizer = this.pluralizer(this.currentLocale());
  var key = pluralizer(Math.abs(count));
  var keys = ((typeof key == "object") && (key instanceof Array)) ? key : [key];

  var message = this.findAndTranslateValidNode(keys, translation);
  if (message == null) message = this.missingTranslation(scope, keys[0]);

  return this.interpolate(message, options);
};

I18n.missingTranslation = function(scope, key) {
  var message = '[' + this.currentLocale() + "." + scope;
  if (key) { message += "." + key; }
  return message + ']';
};

I18n.currentLocale = function() {
  return (I18n.locale || I18n.defaultLocale);
};

// shortcuts
I18n.t = I18n.translate;
I18n.l = I18n.localize;
I18n.p = I18n.pluralize;