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diff --git a/js/src/builtin/Date.js b/js/src/builtin/Date.js
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#if JS_HAS_INTL_API
+// This cache, once primed, has these properties:
+//
+//   runtimeDefaultLocale:
+//     Locale information provided by the embedding, guiding SpiderMonkey's
+//     selection of a default locale.  See intl_RuntimeDefaultLocale(), whose
+//     value controls the value returned by DefaultLocale() that's what's
+//     *actually* used.
+//   icuDefaultTimeZone:
+//     Time zone information provided by ICU. See intl_defaultTimeZone(),
+//     whose value controls the value returned by DefaultTimeZone() that's
+//     what's *actually* used.
+//   formatters:
+//     A Record storing formatters consistent with the above
+//     runtimeDefaultLocale/localTZA values, for use with the appropriate
+//     ES6 toLocale*String Date method when called with its first two
+//     arguments having the value |undefined|.
+//
+// The "formatters" Record has (some subset of) these properties, as determined
+// by all values of the first argument passed to |GetCachedFormat|:
+//
+//   dateTimeFormat: for Date's toLocaleString operation
+//   dateFormat: for Date's toLocaleDateString operation
+//   timeFormat: for Date's toLocaleTimeString operation
+//
+// Using this cache, then, requires
+// 1) verifying the current runtimeDefaultLocale/icuDefaultTimeZone are
+//    consistent with cached values, then
+// 2) seeing if the desired formatter is cached and returning it if so, or else
+// 3) create the desired formatter and store and return it.
+var dateTimeFormatCache = new_Record();
+
+/**
+ * Get a cached DateTimeFormat formatter object, created like so:
+ *
+ *   var opts = ToDateTimeOptions(undefined, required, defaults);
+ *   return new Intl.DateTimeFormat(undefined, opts);
+ *
+ * |format| must be a key from the "formatters" Record described above.
+ */
+function GetCachedFormat(format, required, defaults) {
+  assert(
+    format === "dateTimeFormat" ||
+      format === "dateFormat" ||
+      format === "timeFormat",
+    "unexpected format key: please update the comment by dateTimeFormatCache"
+  );
+
+  var formatters;
+  if (
+    !intl_IsRuntimeDefaultLocale(dateTimeFormatCache.runtimeDefaultLocale) ||
+    !intl_isDefaultTimeZone(dateTimeFormatCache.icuDefaultTimeZone)
+  ) {
+    formatters = dateTimeFormatCache.formatters = new_Record();
+    dateTimeFormatCache.runtimeDefaultLocale = intl_RuntimeDefaultLocale();
+    dateTimeFormatCache.icuDefaultTimeZone = intl_defaultTimeZone();
+  } else {
+    formatters = dateTimeFormatCache.formatters;
+  }
+
+  var fmt = formatters[format];
+  if (fmt === undefined) {
+    var options = ToDateTimeOptions(undefined, required, defaults);
+    fmt = formatters[format] = intl_DateTimeFormat(undefined, options);
+  }
+
+  return fmt;
+}
+
+/**
+ * Format this Date object into a date and time string, using the locale and
+ * formatting options provided.
+ *
+ * Spec: ECMAScript Language Specification, 5.1 edition, 15.9.5.5.
+ * Spec: ECMAScript Internationalization API Specification, 13.3.1.
+ */
+function Date_toLocaleString() {
+  // Steps 1-2.
+  var x = callFunction(ThisTimeValue, this, DATE_METHOD_LOCALE_STRING);
+  if (Number_isNaN(x)) {
+    return "Invalid Date";
+  }
+
+  // Steps 3-4.
+  var locales = ArgumentsLength() ? GetArgument(0) : undefined;
+  var options = ArgumentsLength() > 1 ? GetArgument(1) : undefined;
+
+  // Step 5-6.
+  var dateTimeFormat;
+  if (locales === undefined && options === undefined) {
+    // This cache only optimizes for the old ES5 toLocaleString without
+    // locales and options.
+    dateTimeFormat = GetCachedFormat("dateTimeFormat", "any", "all");
+  } else {
+    options = ToDateTimeOptions(options, "any", "all");
+    dateTimeFormat = intl_DateTimeFormat(locales, options);
+  }
+
+  // Step 7.
+  return intl_FormatDateTime(dateTimeFormat, x, /* formatToParts = */ false);
+}
+
+/**
+ * Format this Date object into a date string, using the locale and formatting
+ * options provided.
+ *
+ * Spec: ECMAScript Language Specification, 5.1 edition, 15.9.5.6.
+ * Spec: ECMAScript Internationalization API Specification, 13.3.2.
+ */
+function Date_toLocaleDateString() {
+  // Steps 1-2.
+  var x = callFunction(ThisTimeValue, this, DATE_METHOD_LOCALE_DATE_STRING);
+  if (Number_isNaN(x)) {
+    return "Invalid Date";
+  }
+
+  // Steps 3-4.
+  var locales = ArgumentsLength() ? GetArgument(0) : undefined;
+  var options = ArgumentsLength() > 1 ? GetArgument(1) : undefined;
+
+  // Step 5-6.
+  var dateTimeFormat;
+  if (locales === undefined && options === undefined) {
+    // This cache only optimizes for the old ES5 toLocaleDateString without
+    // locales and options.
+    dateTimeFormat = GetCachedFormat("dateFormat", "date", "date");
+  } else {
+    options = ToDateTimeOptions(options, "date", "date");
+    dateTimeFormat = intl_DateTimeFormat(locales, options);
+  }
+
+  // Step 7.
+  return intl_FormatDateTime(dateTimeFormat, x, /* formatToParts = */ false);
+}
+
+/**
+ * Format this Date object into a time string, using the locale and formatting
+ * options provided.
+ *
+ * Spec: ECMAScript Language Specification, 5.1 edition, 15.9.5.7.
+ * Spec: ECMAScript Internationalization API Specification, 13.3.3.
+ */
+function Date_toLocaleTimeString() {
+  // Steps 1-2.
+  var x = callFunction(ThisTimeValue, this, DATE_METHOD_LOCALE_TIME_STRING);
+  if (Number_isNaN(x)) {
+    return "Invalid Date";
+  }
+
+  // Steps 3-4.
+  var locales = ArgumentsLength() ? GetArgument(0) : undefined;
+  var options = ArgumentsLength() > 1 ? GetArgument(1) : undefined;
+
+  // Step 5-6.
+  var dateTimeFormat;
+  if (locales === undefined && options === undefined) {
+    // This cache only optimizes for the old ES5 toLocaleTimeString without
+    // locales and options.
+    dateTimeFormat = GetCachedFormat("timeFormat", "time", "time");
+  } else {
+    options = ToDateTimeOptions(options, "time", "time");
+    dateTimeFormat = intl_DateTimeFormat(locales, options);
+  }
+
+  // Step 7.
+  return intl_FormatDateTime(dateTimeFormat, x, /* formatToParts = */ false);
+}
+#endif  // JS_HAS_INTL_API