Add Yup Schemas Are Validated Asynchronously as a javascript til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_634 TILs and counting..._
+_635 TILs and counting..._
 
 ---
 
@@ -256,6 +256,7 @@ _634 TILs and counting..._
 - [Waiting On Multiple Promises](javascript/waiting-on-multiple-promises.md)
 - [Who Am I: NPM Edition](javascript/who-am-i-npm-edition.md)
 - [Yarn Commands Without The Emojis](javascript/yarn-commands-without-the-emojis.md)
+- [Yup Schemas Are Validated Asynchronously](javascript/yup-schemas-are-validated-asynchronously.md)
 
 ### Linux
 

javascript/yup-schemas-are-validated-asynchronously.md

@@ -0,0 +1,33 @@
+# Yup Schemas Are Validated Asynchronously 
+
+[Yup](https://github.com/jquense/yup) provides a flexible object schema
+validation DSL. For instance, if you want to enforce that a certain value is
+a number, you can define something like this:
+
+```javascript
+const numSchema = yup.number();
+```
+
+You can then validate anything against that schema.
+
+```javascript
+const validator = (val) => {
+  numSchema.validate(val)
+    .then(result => {
+      console.log(result); // it is the value of `val`
+      return true;
+    })
+    .catch(error => {
+      console.log(error.errors); // array of validation error messages
+      return false;
+    });
+};
+```
+
+The validation is async, so if it succeeds the `then` block is hit. If the
+validation fails, it will fall through to the `catch`.
+
+```javascript
+validator(5) // => true
+validator('what') // => false
+```