add a workaround of V8 deoptimization which causes serious performanc…

…e degradation of `Array#concat`, #679
zloirock · Oct 26, 2019 · f0ea725 · f0ea725
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -1,5 +1,6 @@
 ## Changelog
 ##### Unreleased
+- Added a workaround of V8 deoptimization which causes serious performance degradation (~4x in my tests) of `Array#concat`, [#679](https://github.com/zloirock/core-js/issues/679)
 - Added compat data for Chromium-based Edge
 
 ##### 3.3.4 - 2019.10.25

diff --git a/packages/core-js/modules/es.array.concat.js b/packages/core-js/modules/es.array.concat.js
@@ -9,12 +9,16 @@ var createProperty = require('../internals/create-property');
 var arraySpeciesCreate = require('../internals/array-species-create');
 var arrayMethodHasSpeciesSupport = require('../internals/array-method-has-species-support');
 var wellKnownSymbol = require('../internals/well-known-symbol');
+var V8_VERSION = require('../internals/v8-version');
 
 var IS_CONCAT_SPREADABLE = wellKnownSymbol('isConcatSpreadable');
 var MAX_SAFE_INTEGER = 0x1FFFFFFFFFFFFF;
 var MAXIMUM_ALLOWED_INDEX_EXCEEDED = 'Maximum allowed index exceeded';
 
-var IS_CONCAT_SPREADABLE_SUPPORT = !fails(function () {
+// We can't use this feature detection in V8 since it causes
+// deoptimization and serious performance degradation
+// https://github.com/zloirock/core-js/issues/679
+var IS_CONCAT_SPREADABLE_SUPPORT = V8_VERSION >= 51 || !fails(function () {
   var array = [];
   array[IS_CONCAT_SPREADABLE] = false;
   return array.concat()[0] !== array;