From 2b77bfd8536dca8ffc3086b2386c56f21926ebe8 Mon Sep 17 00:00:00 2001
From: Robert Sayre <sayrer@gmail.com>
Date: Sat, 22 Dec 2018 13:52:49 -0800
Subject: [PATCH] [fix] improve style resolver performance

Script time in the benchmark was profiled by adding `console.profile` around
the timings for script time. The call to Array.join in the resolve function
stood out. Since the code already iterates over the array it can run slightly
faster by building the cache key in that loop instead.

Close #1213
---
 .../src/exports/StyleSheet/ReactNativeStyleResolver.js      | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/packages/react-native-web/src/exports/StyleSheet/ReactNativeStyleResolver.js b/packages/react-native-web/src/exports/StyleSheet/ReactNativeStyleResolver.js
index 4b8d593e..69e576f2 100644
--- a/packages/react-native-web/src/exports/StyleSheet/ReactNativeStyleResolver.js
+++ b/packages/react-native-web/src/exports/StyleSheet/ReactNativeStyleResolver.js
@@ -84,15 +84,19 @@ export default class ReactNativeStyleResolver {
     // otherwise fallback to resolving
     const flatArray = flattenArray(style);
     let isArrayOfNumbers = true;
+    let cacheKey = '';
     for (let i = 0; i < flatArray.length; i++) {
       const id = flatArray[i];
       if (typeof id !== 'number') {
         isArrayOfNumbers = false;
       } else {
+        if (isArrayOfNumbers) {
+          cacheKey += (id + '-');
+        }
         this._injectRegisteredStyle(id);
       }
     }
-    const key = isArrayOfNumbers ? createCacheKey(flatArray.join('-')) : null;
+    const key = isArrayOfNumbers ? createCacheKey(cacheKey) : null;
     return this._resolveStyleIfNeeded(flatArray, key);
   }