diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index 819c69a3fc4..3eced36c077 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -466,13 +466,13 @@ interface String {
     toLowerCase(): string;
 
     /** Converts all alphabetic characters to lowercase, taking into account the host environment's current locale. */
-    toLocaleLowerCase(): string;
+    toLocaleLowerCase(locales?: string | string[]): string;
 
     /** Converts all the alphabetic characters in a string to uppercase. */
     toUpperCase(): string;
 
     /** Returns a string where all alphabetic characters have been converted to uppercase, taking into account the host environment's current locale. */
-    toLocaleUpperCase(): string;
+    toLocaleUpperCase(locales?: string | string[]): string;
 
     /** Removes the leading and trailing white space and line terminator characters from a string. */
     trim(): string;
diff --git a/tests/baselines/reference/destructuringWithGenericParameter.types b/tests/baselines/reference/destructuringWithGenericParameter.types
index 7cf2b8f00ae..e5895a55deb 100644
--- a/tests/baselines/reference/destructuringWithGenericParameter.types
+++ b/tests/baselines/reference/destructuringWithGenericParameter.types
@@ -36,9 +36,9 @@ genericFunction(genericObject, ({greeting}) => {
     var s = greeting.toLocaleLowerCase();  // Greeting should be of type string
 >s : string
 >greeting.toLocaleLowerCase() : string
->greeting.toLocaleLowerCase : () => string
+>greeting.toLocaleLowerCase : (locales?: string | string[]) => string
 >greeting : string
->toLocaleLowerCase : () => string
+>toLocaleLowerCase : (locales?: string | string[]) => string
 
 });
 
diff --git a/tests/baselines/reference/typeGuardsOnClassProperty.types b/tests/baselines/reference/typeGuardsOnClassProperty.types
index 6a79412237a..0c68086ab62 100644
--- a/tests/baselines/reference/typeGuardsOnClassProperty.types
+++ b/tests/baselines/reference/typeGuardsOnClassProperty.types
@@ -106,7 +106,7 @@ if (typeof prop1 === "string" && prop1.toLocaleLowerCase()) { }
 >prop1 : string | number
 >"string" : "string"
 >prop1.toLocaleLowerCase() : string
->prop1.toLocaleLowerCase : () => string
+>prop1.toLocaleLowerCase : (locales?: string | string[]) => string
 >prop1 : string
->toLocaleLowerCase : () => string
+>toLocaleLowerCase : (locales?: string | string[]) => string