diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index 8009269ad5c..cd188059de1 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -1528,12 +1528,12 @@ interface Int8Array {
       * Returns the index of the first element in the array where predicate is true, and undefined
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
-      * order, until it finds one where predicate returns true. If such an element is found, find
-      * immediately returns that element value. Otherwise, find returns undefined.
+      * order, until it finds one where predicate returns true. If such an element is found, 
+      * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
       * @param thisArg If provided, it will be used as the this value for each invocation of
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: number) => boolean, thisArg?: any): number | undefined;
+    findIndex(predicate: (value: number) => boolean, thisArg?: any): number;
 
     /**
       * Performs the specified action for each element in an array.
@@ -1801,12 +1801,12 @@ interface Uint8Array {
       * Returns the index of the first element in the array where predicate is true, and undefined
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
-      * order, until it finds one where predicate returns true. If such an element is found, find
-      * immediately returns that element value. Otherwise, find returns undefined.
+      * order, until it finds one where predicate returns true. If such an element is found, 
+      * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
       * @param thisArg If provided, it will be used as the this value for each invocation of
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: number) => boolean, thisArg?: any): number | undefined;
+    findIndex(predicate: (value: number) => boolean, thisArg?: any): number;
 
     /**
       * Performs the specified action for each element in an array.
@@ -2075,12 +2075,12 @@ interface Uint8ClampedArray {
       * Returns the index of the first element in the array where predicate is true, and undefined
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
-      * order, until it finds one where predicate returns true. If such an element is found, find
-      * immediately returns that element value. Otherwise, find returns undefined.
+      * order, until it finds one where predicate returns true. If such an element is found, 
+      * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
       * @param thisArg If provided, it will be used as the this value for each invocation of
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: number) => boolean, thisArg?: any): number | undefined;
+    findIndex(predicate: (value: number) => boolean, thisArg?: any): number;
 
     /**
       * Performs the specified action for each element in an array.
@@ -2348,12 +2348,12 @@ interface Int16Array {
       * Returns the index of the first element in the array where predicate is true, and undefined
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
-      * order, until it finds one where predicate returns true. If such an element is found, find
-      * immediately returns that element value. Otherwise, find returns undefined.
+      * order, until it finds one where predicate returns true. If such an element is found, 
+      * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
       * @param thisArg If provided, it will be used as the this value for each invocation of
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: number) => boolean, thisArg?: any): number | undefined;
+    findIndex(predicate: (value: number) => boolean, thisArg?: any): number;
 
     /**
       * Performs the specified action for each element in an array.
@@ -2622,12 +2622,12 @@ interface Uint16Array {
       * Returns the index of the first element in the array where predicate is true, and undefined
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
-      * order, until it finds one where predicate returns true. If such an element is found, find
-      * immediately returns that element value. Otherwise, find returns undefined.
+      * order, until it finds one where predicate returns true. If such an element is found, 
+      * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
       * @param thisArg If provided, it will be used as the this value for each invocation of
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: number) => boolean, thisArg?: any): number | undefined;
+    findIndex(predicate: (value: number) => boolean, thisArg?: any): number;
 
     /**
       * Performs the specified action for each element in an array.
@@ -2895,12 +2895,12 @@ interface Int32Array {
       * Returns the index of the first element in the array where predicate is true, and undefined
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
-      * order, until it finds one where predicate returns true. If such an element is found, find
-      * immediately returns that element value. Otherwise, find returns undefined.
+      * order, until it finds one where predicate returns true. If such an element is found, 
+      * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
       * @param thisArg If provided, it will be used as the this value for each invocation of
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: number) => boolean, thisArg?: any): number | undefined;
+    findIndex(predicate: (value: number) => boolean, thisArg?: any): number;
 
     /**
       * Performs the specified action for each element in an array.
@@ -3168,12 +3168,12 @@ interface Uint32Array {
       * Returns the index of the first element in the array where predicate is true, and undefined
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
-      * order, until it finds one where predicate returns true. If such an element is found, find
-      * immediately returns that element value. Otherwise, find returns undefined.
+      * order, until it finds one where predicate returns true. If such an element is found, 
+      * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
       * @param thisArg If provided, it will be used as the this value for each invocation of
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: number) => boolean, thisArg?: any): number | undefined;
+    findIndex(predicate: (value: number) => boolean, thisArg?: any): number;
 
     /**
       * Performs the specified action for each element in an array.
@@ -3441,12 +3441,12 @@ interface Float32Array {
       * Returns the index of the first element in the array where predicate is true, and undefined
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
-      * order, until it finds one where predicate returns true. If such an element is found, find
-      * immediately returns that element value. Otherwise, find returns undefined.
+      * order, until it finds one where predicate returns true. If such an element is found, 
+      * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
       * @param thisArg If provided, it will be used as the this value for each invocation of
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: number) => boolean, thisArg?: any): number | undefined;
+    findIndex(predicate: (value: number) => boolean, thisArg?: any): number;
 
     /**
       * Performs the specified action for each element in an array.
@@ -3715,12 +3715,12 @@ interface Float64Array {
       * Returns the index of the first element in the array where predicate is true, and undefined
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
-      * order, until it finds one where predicate returns true. If such an element is found, find
-      * immediately returns that element value. Otherwise, find returns undefined.
+      * order, until it finds one where predicate returns true. If such an element is found, 
+      * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
       * @param thisArg If provided, it will be used as the this value for each invocation of
       * predicate. If it is not provided, undefined is used instead.
       */
-    findIndex(predicate: (value: number) => boolean, thisArg?: any): number | undefined;
+    findIndex(predicate: (value: number) => boolean, thisArg?: any): number;
 
     /**
       * Performs the specified action for each element in an array.