- reduced prettier width to 100

- started work on issue #1

.prettierrc.json

@@ -1,5 +1,5 @@
 {
-  "printWidth": 120,
+  "printWidth": 100,
   "singleQuote": true,
   "tabWidth": 4,
   "trailingComma": "all"

src/index.ts

@@ -3,22 +3,79 @@ import * as yup from 'yup';
 
 // Declare a `jwt()` method on the `StringSchema` interface via module augmentation.
 declare module 'yup' {
+    export type JwtMode = 'regex' | 'decode' | 'verify';
+
+    export type JwtKey = string | Buffer;
+
     export interface StringSchema {
-        jwt(message?: string): StringSchema;
+        jwt(mode: 'regex' | 'decode', message?: string): StringSchema;
+        jwt(mode: 'verify', key: JwtKey, message?: string): StringSchema;
     }
 }
 
 // tslint:disable-next-line:no-invalid-template-strings
 export const defaultMessage: string = '${path} must be a parsable JSON web token';
 
 // Implement the `StringSchema.jwt()` method.
-yup.addMethod(yup.string, 'jwt', function(this: yup.StringSchema, message: string = defaultMessage): yup.StringSchema {
+yup.addMethod(yup.string, 'jwt', function(
+    this: yup.StringSchema,
+    mode: yup.JwtMode,
+    maybeKeyOrMessage: string | yup.JwtKey | undefined,
+    maybeMessage: string | undefined,
+): yup.StringSchema {
+    const checkMessageOrThrow = (value: string | yup.JwtKey | undefined): string => {
+        if (!(value === undefined || typeof value === 'string')) {
+            throw new TypeError(
+                `Invalid argument: 'message' must be a string or undefined but was "${value}"`,
+            );
+        }
+        return value || defaultMessage;
+    };
+
+    const checkKeyOrThrow = (value: string | yup.JwtKey | undefined): yup.JwtKey => {
+        if (!(typeof value === 'string' || Buffer.isBuffer(value))) {
+            throw new TypeError(
+                "Invalid argument: 'key' must be a string, Buffer, or callback function",
+            );
+        }
+        return value;
+    };
+
+    let message: string;
+    let key: yup.JwtKey;
+    switch (mode) {
+        case 'regex':
+        case 'decode':
+            message = checkMessageOrThrow(maybeKeyOrMessage);
+            break;
+
+        case 'verify':
+            key = checkKeyOrThrow(maybeKeyOrMessage);
+            message = checkMessageOrThrow(maybeMessage);
+            break;
+
+        default:
+            throw new TypeError(`Invalid argument: 'mode' is `);
+    }
+
     return this.test({
         exclusive: true,
         message,
         name: 'jwt',
         test(this: yup.TestContext, value: string): boolean {
-            return !!jwt.decode(value);
+            switch (mode) {
+                case 'regex':
+                    return /^[0-9a-z\-_]+?\.[0-9a-z\-_]+?\.([0-9a-z\-_]+)?$/i.test(value);
+
+                case 'decode':
+                    return !!jwt.decode(value);
+
+                case 'verify':
+                    return !!jwt.verify(value, key);
+
+                default:
+                    throw new Error('Unreachable code detected');
+            }
         },
     });
 });

test/test.ts

@@ -6,7 +6,7 @@ import { defaultMessage } from '../src/index';
 
 describe('`yup.string().jwt()`', () => {
     const nonsenseToken: string = 'foo.bar.qux';
-    const schema: yup.StringSchema = yup.string().jwt();
+    const schema: yup.StringSchema = yup.string().jwt('decode');
 
     describe('`isValidSync()`', () => {
         const stringPayload: string = 'payload';
@@ -54,7 +54,7 @@ describe('`yup.string().jwt()`', () => {
 
             it('with a custom message when one is supplied', () => {
                 const customMessage: string = 'custom message';
-                const customMessageSchema: yup.StringSchema = yup.string().jwt(customMessage);
+                const customMessageSchema: yup.StringSchema = yup.string().jwt('decode', customMessage);
                 expect(() => customMessageSchema.validateSync(nonsenseToken)).toThrowError(customMessage);
             });
         });