fix: ParseError message handling

src/ParseError.js

@@ -22,7 +22,14 @@ class ParseError extends Error {
     this.code = code;
     Object.defineProperty(this, 'message', {
       enumerable: true,
-      value: message,
+      value:
+        typeof message === 'string'
+          ? message
+          : typeof message === 'object' &&
+            typeof message.toString === 'function' &&
+            message.toString() !== '[object Object]'
+            ? message.toString()
+            : JSON.stringify(message),
     });
   }
 

src/__tests__/ParseError-test.js

@@ -27,4 +27,27 @@ describe('ParseError', () => {
       code: 123,
     });
   });
+
+  it('message must be a string', () => {
+    /**
+     * error as object
+     */
+    const someRandomError = { code: 420, message: 'time to chill' };
+    const error = new ParseError(1337, someRandomError);
+    expect(JSON.parse(JSON.stringify(error))).toEqual({
+      message: JSON.stringify(someRandomError),
+      code: 1337,
+    });
+
+    /**
+     * error as an Error instance
+     */
+    const someRandomError2 = new Error('time to relax');
+    const error2 = new ParseError(420, someRandomError2);
+
+    expect(JSON.parse(JSON.stringify(error2))).toEqual({
+      message: 'Error: time to relax',
+      code: 420,
+    });
+  });
 });