Add definition for isSchema() method signature

types/yup/index.d.ts

@@ -26,6 +26,7 @@ export function addMethod<T extends Schema<any>>(
 export function ref(path: string, options?: { contextPrefix: string }): Ref;
 export function lazy<T>(fn: (value: T) => Schema<T>): Lazy;
 export function setLocale(customLocale: LocaleObject): void;
+export function isSchema(obj: any): obj is Schema<any>;
 
 export const mixed: MixedSchemaConstructor;
 export const string: StringSchemaConstructor;

types/yup/yup-tests.ts

@@ -3,6 +3,7 @@ import * as yup from "yup";
 // tslint:disable-next-line:no-duplicate-imports
 import {
     reach,
+    isSchema,
     date,
     Schema,
     ObjectSchema,
@@ -24,6 +25,10 @@ const schema1 = yup.object().shape({
 reach(schema1, "nested.arr.num");
 reach(schema1, "nested.arr[].num");
 
+// isSchema function
+const isSchemaResult1: boolean = isSchema(schema1);
+const isSchemaResult2: boolean = isSchema({});
+
 // addMethod function
 yup.addMethod<NumberSchema>(yup.number, "minimum", function(
     this,