Follow-up from code review

* Ensure Intl.DateTimeFormat.length === 0
* Split out timezone lazy-init special case to a separate function
* Deep-clone inputs to prevent mutation before lazy instance creation,
  and add tests to verify that mutating inputs doesn't break anything.
* Add timing into npm test script

lib/intl.mjs

@@ -48,24 +48,40 @@ const ObjectAssign = Object.assign;
 // See https://bugs.chromium.org/p/v8/issues/detail?id=6528
 function getPropLazy(obj, prop) {
   let val = obj[prop];
-  if (prop === TZ_RESOLVED) {
-    // for TimeZone, the initializer is a string, not a function
-    if (typeof val === 'string') {
-      val = new TimeZone(val);
-      obj[prop] = val;
-    }
-  } else if (typeof val === 'function') {
+  if (typeof val === 'function') {
     val = new IntlDateTimeFormat(obj[LOCALE], val(obj[OPTIONS]));
     obj[prop] = val;
   }
   return val;
 }
+// Similarly, lazy-init TimeZone instances.
+function getResolvedTimeZoneLazy(obj) {
+  let val = obj[TZ_RESOLVED];
+  if (typeof val === 'string') {
+    val = new TimeZone(val);
+    obj[TZ_RESOLVED] = val;
+  }
+  return val;
+}
+
+function deepClone(val) {
+  if (val === null || (typeof val !== 'function' && typeof val !== 'object')) return val;
+  return JSON.parse(JSON.stringify(val));
+}
+
+export function DateTimeFormat(locale = undefined, options = undefined) {
+  // `= undefined` usage above seems unnecessary, but is actually needed because
+  // the spec requires Intl.DateTimeFormat.length === 0
+
+  // Clone inputs to protect against the caller later mutating `options` or
+  // `locale` before they're later used to (lazily) create built-in
+  // Intl.DateTimeFormat instances.
+  locale = deepClone(locale);
+  options = typeof options === 'undefined' ? {} : deepClone(options);
 
-export function DateTimeFormat(locale = undefined, options = {}) {
   if (!(this instanceof DateTimeFormat)) return new DateTimeFormat(locale, options);
   const original = new IntlDateTimeFormat(locale, options);
   const ro = original.resolvedOptions();
-  options = typeof options === 'undefined' ? {} : ObjectAssign({}, options);
 
   this[TZ_GIVEN] = options.timeZone ? options.timeZone : null;
   this[OPTIONS] = options;
@@ -353,7 +369,7 @@ function extractOverrides(temporalObj, main) {
     const nanosecond = GetSlot(temporalObj, ISO_NANOSECOND);
     const datetime = new DateTime(1970, 1, 1, hour, minute, second, millisecond, microsecond, nanosecond, main[CAL_ID]);
     return {
-      instant: ES.BuiltinTimeZoneGetInstantFor(getPropLazy(main, TZ_RESOLVED), datetime, 'compatible'),
+      instant: ES.BuiltinTimeZoneGetInstantFor(getResolvedTimeZoneLazy(main), datetime, 'compatible'),
       formatter: getPropLazy(main, TIME)
     };
   }
@@ -370,7 +386,7 @@ function extractOverrides(temporalObj, main) {
     }
     const datetime = new DateTime(isoYear, isoMonth, referenceISODay, 12, 0, 0, 0, 0, 0, calendar);
     return {
-      instant: ES.BuiltinTimeZoneGetInstantFor(getPropLazy(main, TZ_RESOLVED), datetime, 'compatible'),
+      instant: ES.BuiltinTimeZoneGetInstantFor(getResolvedTimeZoneLazy(main), datetime, 'compatible'),
       formatter: getPropLazy(main, YM)
     };
   }
@@ -387,7 +403,7 @@ function extractOverrides(temporalObj, main) {
     }
     const datetime = new DateTime(referenceISOYear, isoMonth, isoDay, 12, 0, 0, 0, 0, 0, calendar);
     return {
-      instant: ES.BuiltinTimeZoneGetInstantFor(getPropLazy(main, TZ_RESOLVED), datetime, 'compatible'),
+      instant: ES.BuiltinTimeZoneGetInstantFor(getResolvedTimeZoneLazy(main), datetime, 'compatible'),
       formatter: getPropLazy(main, MD)
     };
   }
@@ -402,7 +418,7 @@ function extractOverrides(temporalObj, main) {
     }
     const datetime = new DateTime(isoYear, isoMonth, isoDay, 12, 0, 0, 0, 0, 0, main[CAL_ID]);
     return {
-      instant: ES.BuiltinTimeZoneGetInstantFor(getPropLazy(main, TZ_RESOLVED), datetime, 'compatible'),
+      instant: ES.BuiltinTimeZoneGetInstantFor(getResolvedTimeZoneLazy(main), datetime, 'compatible'),
       formatter: getPropLazy(main, DATE)
     };
   }
@@ -439,7 +455,7 @@ function extractOverrides(temporalObj, main) {
       );
     }
     return {
-      instant: ES.BuiltinTimeZoneGetInstantFor(getPropLazy(main, TZ_RESOLVED), datetime, 'compatible'),
+      instant: ES.BuiltinTimeZoneGetInstantFor(getResolvedTimeZoneLazy(main), datetime, 'compatible'),
       formatter: getPropLazy(main, DATETIME)
     };
   }

package.json

@@ -7,7 +7,7 @@
   "browser": "dist/index.umd.js",
   "types": "index.d.ts",
   "scripts": {
-    "test": "node --no-warnings --experimental-modules --icu-data-dir node_modules/full-icu --loader ./test/resolve.source.mjs ./test/all.mjs",
+    "test": "time node --no-warnings --experimental-modules --icu-data-dir node_modules/full-icu --loader ./test/resolve.source.mjs ./test/all.mjs",
     "build": "rollup -c rollup.config.js",
     "prepublishOnly": "npm run build",
     "playground": "node --experimental-modules --no-warnings --icu-data-dir node_modules/full-icu -r ./lib/init.js",

test/intl.mjs

@@ -1050,10 +1050,25 @@ describe('Intl', () => {
       it('should return an Array', () => assert(Array.isArray(Intl.DateTimeFormat.supportedLocalesOf())));
     });
 
-    const us = new Intl.DateTimeFormat('en-US', { timeZone: 'America/New_York' });
-    const at = new Intl.DateTimeFormat('de-AT', { timeZone: 'Europe/Vienna' });
+    // Verify that inputs to DateTimeFormat constructor are immune to mutation.
+    const optionsAT = {
+      timeZone: {
+        toString: () => 'Europe/Vienna',
+        toJSON: () => 'Europe/Vienna'
+      }
+    };
+    const optionsUS = { timeZone: 'America/New_York' };
+    const localesAT = ['de-AT'];
+    const us = new Intl.DateTimeFormat('en-US', optionsUS);
+    const at = new Intl.DateTimeFormat(localesAT, optionsAT);
+    optionsAT.timeZone = {
+      toString: () => 'Bogus/Time-Zone',
+      toJSON: () => 'Bogus/Time-Zone'
+    };
+    optionsUS.timeZone = 'Bogus/Time-Zone';
     const us2 = new Intl.DateTimeFormat('en-US');
-    const at2 = new Intl.DateTimeFormat('de-AT');
+    const at2 = new Intl.DateTimeFormat(localesAT);
+    localesAT[0] = ['invalid locale'];
     const usCalendar = us.resolvedOptions().calendar;
     const atCalendar = at.resolvedOptions().calendar;
     const t1 = '1976-11-18T14:23:30+00:00[UTC]';