diff --git a/src/shark/LZString.ts b/src/shark/LZString.ts
index e83b3e0..dc961a8 100644
--- a/src/shark/LZString.ts
+++ b/src/shark/LZString.ts
@@ -46,16 +46,16 @@ export class LZString extends StaticClass {
     let value: number;
 
     for (const contextC of uncompressed) {
-      if (!Object.keys(contextDictionary).includes(contextC)) {
+      if (!(contextC in contextDictionary)) {
         contextDictionary[contextC] = contextDictSize++;
         contextDictionaryToCreate[contextC] = true;
       }
 
       const contextWC = contextW + contextC;
-      if (Object.keys(contextDictionary).includes(contextWC)) {
+      if (contextWC in contextDictionary) {
         contextW = contextWC;
       } else {
-        if (Object.keys(contextDictionaryToCreate).includes(contextW)) {
+        if (contextW in contextDictionaryToCreate) {
           if (contextW.charCodeAt(0) < 256) {
             for (let i = 0; i < contextNumBits; i++) {
               contextDataVal <<= 1;
@@ -137,7 +137,7 @@ export class LZString extends StaticClass {
     }
 
     if (contextW !== "") {
-      if (Object.keys(contextDictionaryToCreate).includes(contextW)) {
+      if (contextW in contextDictionaryToCreate) {
         if (contextW.charCodeAt(0) < 256) {
           for (let i = 0; i < contextNumBits; i++) {
             contextDataVal = contextDataVal << 1;